Hilaire Drouineau
email: hilaire.drouineau@irstea.fr

Caroline Durif

Martin Castonguay

Maria Mateo

Eric Rochard

Guy Verreault

Kazuki Yokouchi

Patrick Lambert

Freshwater eels: a symbol of the effects of global change

Keywords: adaptation, Anguilla spp., climate change, contamination, ecosystem fragmentation, over-exploitation

Temperate eels Anguilla anguilla (European eel), A. rostrata (American eel), and A. japonica (Japanese eel) are three catadromous species which have been declining since the 1970s/1980s despite their remarkable adaptive capacity. Because of their specific life-cycles, which share distant oceanic spawning grounds and continental growth stage, eels are affected by five components of the global change: (i) climate change affecting larval survival and drift, (ii) an increase in pollution leading to high levels of contamination exacerbated by their high lipid levels, (iii) increasing fragmentation and habitat loss that reduce dramatically the amount of available habitats and induce increased spawner mortality, (iv) the appearance of Anguillicola crassus a parasitic alien nematode that impairs spawning success and (v) the impact of commercial and recreational fisheries for all life stages of eel. In this context, the rapid increases of pressures during the "Great Acceleration" have surpassed the adaptive capacity of eels. This illustrates that cumulative effects of global change can lead to the collapse of species, even in species that have amazingly high adaptive capacities.

Table of contents

Introduction

Introduction

Global change: five components that threaten biodiversity

In 2005, the Millennium Ecosystem Assessment (2005a) pointed out that, despite an increase in human welfare during the XXth century, anthropogenic actions threaten the ability of ecosystems to sustainably provide important goods and services, especially for future generations. Such humancaused environmental changes are generally referred to as global change [START_REF] Steffen | Global Change and the Earth System[END_REF]. In this paper, we illustrate how the rate at which global change is happening can endanger even highly adaptive species such as temperate eels Anguilla anguilla (European eel), A. rostrata (American eel), and A. japonica (Japanese eel). More specifically, this example illustrates that cumulative effects of the different components of global change [START_REF] Vitousek | Introduced species: a significant component of human-caused global change[END_REF][START_REF] Western | Human-modified ecosystems and future evolution[END_REF][START_REF] Tylianakis | Global change and species interactions in terrestrial ecosystems[END_REF][START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF][START_REF] Miller | Did a "perfect storm" of oceanic changes and continental anthropogenic impacts cause northern hemisphere anguillid recruitment reductions?[END_REF] can produce rates of change that exceed a species' adaptive capacity, a central question in the debate on the effects of climate and global change [START_REF] Donner | Global assessment of coral bleaching and required rates of adaptation under climate change[END_REF][START_REF] Visser | Keeping up with a warming world; assessing the rate of adaptation to climate change[END_REF]). [START_REF] Soulé | Conservation: tactics for a constant crisis[END_REF] proposed a list of the main threats to biodiversity that include six components: habitat loss, habitat fragmentation, overexploitation, exotic species, pollution and climate change. Later, the United-States National Research Council (2000) proposed a very similar list of ongoing changes:

(1) climate, (2) land use and land cover modifications that can result in habitat loss and fragmentation, (3) biogeochemical and hydrological cycles and pollution, (4) biotic mixing including biological invasions and (5) overexploitation of natural resources especially in oceanic ecosystems. The main difference with Soulé's proposal is that habitat loss and habitat fragmentation were merged into a single component. This latter listing was then endorsed by many authors, including IPCC (2001), the Millennium Ecosystem Assessment (2005b), [START_REF] Simberloff | Sustainability of biodiversity under global changes, with particular reference to biological invasions[END_REF] and [START_REF] Pe'er | IPBES: Opportunities and Challenges for SCB and Other Learned Societies[END_REF] though [START_REF] Pe'er | IPBES: Opportunities and Challenges for SCB and Other Learned Societies[END_REF] did not mention pollution. The IGPB [START_REF] Steffen | Global Change and the Earth System[END_REF], executive summary) and the Global Change Program of the Royal Society of Canada (1992) provided more detailed lists of components of global change (IGBP: oil harvest, transformation of land surface, nitrogen waste, use of freshwater, greenhouse gas, marine habitat destruction, overexploitation of fisheries, extinction rates of species -GCPRSC: climate change, energy and considered major threats to biodiversity and represent one of the major challenges in ecosystem conservation and restoration (Tilman et al. 1994;Tischendorf and Fahrig 2000a,b;[START_REF] Sutherland | Identification of 100 fundamental ecological questions[END_REF], by impairing the ability of individuals to migrate to essential habitats [START_REF] Gros | The Impact of Global Change on the Dynamics of Marine Living Resources[END_REF], isolating populations and reducing gene flow [START_REF] Horreo | Impact of habitat fragmentation on the genetics of populations in dendritic landscapes[END_REF][START_REF] Haxton | A global perspective of fragmentation on a declining taxon the sturgeon (Acipenseriformes)[END_REF], and by modifying species community structure [START_REF] Perkin | Fragmentation alters stream fish community structure in dendritic ecological networks[END_REF]. Fragmentation and habitat loss increase the risk of extinction cascades (Terborgh et al. 2001;[START_REF] Fischer | Landscape modification and habitat fragmentation: a synthesis[END_REF][START_REF] Krauss | Habitat fragmentation causes immediate and time-delayed biodiversity loss at different trophic levels[END_REF][START_REF] Junge | Assessing the consequences of habitat fragmentation for two migratory salmonid fishes[END_REF][START_REF] Haddad | Habitat fragmentation and its lasting impact on Earth's ecosystems[END_REF]van Leeuwen et al. 2016).

Biological invasion by alien species is our fourth component of global change [START_REF] Vitousek | Introduced species: a significant component of human-caused global change[END_REF][START_REF] Occhipinti-Ambrogi | Biological invasions as a component of global change in stressed marine ecosystems[END_REF][START_REF] Ricciardi | Are Modern Biological Invasions an Unprecedented Form of Global Change?[END_REF]. Invasive species can affect native ones directly through predation, competition or parasitism, or indirectly by habitat modification or by spreading diseases [START_REF] Lymbery | Co-invaders: The effects of alien parasites on native hosts[END_REF]. Arrival of alien species due to shifts in their distribution in response to climate change, uniformisation of habitat or transport by humans, can profoundly reshape species interactions and has consequences at the species, community, and ecosystem levels but also on the provision of ecosystem services [START_REF] Alpine | Trophic interactions and direct physical effects control phytoplankton biomass and production in an estuary[END_REF][START_REF] Cloern | Phytoplankton bloom dynamics in coastal ecosystems: A review with some general lessons from sustained investigation of San Francisco Bay, California[END_REF][START_REF] Grosholz | Ecological and evolutionary consequences of coastal invasions[END_REF][START_REF] Vilà | How well do we understand the impacts of alien species on ecosystem services? A pan-European, cross-taxa assessment[END_REF]. Currently, there are about 10,000 alien species registered in Europe, while ecological impacts have been documented for 11% of them and economic impacts for 13% [START_REF] Vilà | How well do we understand the impacts of alien species on ecosystem services? A pan-European, cross-taxa assessment[END_REF].

Finally, the last component is the overexploitation of natural resources [START_REF] Brook | Synergies among extinction drivers under global change[END_REF]. For example, fisheries have had major impacts on marine ecosystems, depleting stocks with potential impacts on fishing food webs, and impacts on habitats because of destructive fishing gear [START_REF] Turner | Fishing impacts and the degradation or loss of habitat structure[END_REF][START_REF] Christensen | Hundred-year decline of North Atlantic predatory fishes[END_REF]Gascuel et al. 2011;[START_REF] Branch | Fishing Impacts on Food Webs: Multiple Working Hypotheses[END_REF]Drouineau et al. 2016b).

Fisheries have also shaped the life histories of exploited fishes by acting as a permanent and continuous selection pressure [START_REF] Jørgensen | Damned if you do, dammed if you don't: debates on dam removal in the Swedish media[END_REF][START_REF] Heino | Fisheries-induced evolution[END_REF].

Through the combined impacts of these five components, global change leads to an extremely rapid modification of ecosystems. Species can display different kinds of adaptive response to address this threat, such as local adaptation through i) a micro-evolutionary response, ii) phenotypic plasticity [START_REF] Charmantier | Adaptive Phenotypic Plasticity in Response to Climate Change in a Wild Bird Population[END_REF], or iii) modification of their distribution area [START_REF] Hughes | Biological consequences of global warming: is the signal already apparent?[END_REF]. However, adaptation is not always possible, especially in cases of synergies among components which can lead to rates of change outpacing a species adaptive capacity [START_REF] Brook | Synergies among extinction drivers under global change[END_REF]. In this context, global change threatens extinction for many species [START_REF] Spurgeon | Global warming threatens extinction for many species[END_REF]Thomas et al. 2004;[START_REF] Steffen | Global Change and the Earth System[END_REF][START_REF] Brook | Synergies among extinction drivers under global change[END_REF][START_REF] Cahill | How does climate change cause extinction[END_REF][START_REF] Urban | Accelerating extinction risk from climate change[END_REF]. The collapse of the three temperate anguillid eel populations is an excellent illustration of this phenomenon.

Temperate eels: endangered species impacted by diverse anthropogenic pressures

European eel, American eel and Japanese eel are three temperate catadromous species that share many remarkable ecological features (Figure 1). They have a large distribution area in continental waters (Figure 2 -from Norway to Morocco for the European eel (Tesch 2003), from Canada to Venezuela for American eel [START_REF] Helfman | Reproductive ecology of the American eel[END_REF], from Northern Philippines to Korea for Japanese eel [START_REF] Tsukamoto | Recruitment mechanism of the eel, Anguilla japonica, to the Japanese coast[END_REF], which are reached after a long larval drift (larvae are called leptocephali) from distant marine spawning grounds (Sargasso Sea for A. rostrata and A. anguilla [START_REF] Schmidt | Breeding places and migrations of the eel[END_REF][START_REF] Mccleave | Physical and behavioural controls on the oceanic distribution and migration of leptocephali[END_REF]) and west of Mariana Islands for A. japonica [START_REF] Tsukamoto | Discovery of the spawning area for Japanese eel[END_REF]). After this migration, the larvae metamorphose into glass eels upon reaching the continental shelf (Tesch 2003). They subsequently penetrate continental waters, turning into pigmented yellow eels, where they colonise a large range of continental habitats from brackish to freshwater (Daverat et al. 2006;[START_REF] Arai | Diverse migration strategy between freshwater and seawater habitats in the freshwater eel genus Anguilla[END_REF]. After a growth phase lasting from 3 to over 30 years, yellow eels metamorphose into silver eels and migrate back to their spawning grounds (Béguer-Pon et al. 2015b;[START_REF] Chang | Simulating the Oceanic Migration of Silver Japanese Eels[END_REF][START_REF] Righton | Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea[END_REF]. The eels mature en route and presumably die following spawning.

Eels have successfully adapted to very heterogeneous growth environments at both the distribution area and catchment scale. Eels are panmictic [START_REF] Han | Population genetic structure of the Japanese eel Anguilla japonica: panmixia at spatial and temporal scales[END_REF][START_REF] Als | All roads lead to home: panmixia of European eel in the Sargasso Sea[END_REF][START_REF] Côté | Population genetics of the American eel (Anguilla rostrata): FST = 0 and North Atlantic Oscillation effects on demographic fluctuations of a panmictic species[END_REF][START_REF] Pujolar | Conclusive evidence for panmixia in the American eel[END_REF]) and their long larval drift which limit the possibility of local genetic adaptation.

However, there are correlations between environmental gradients and spatial patterns in life history traits throughout the distribution area and within river catchments (Vélez-Espino and Koops 2009;[START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF]Yokouchi et al. 2014). For example, sex ratio is generally female-biased in the northern parts of distribution areas [START_REF] Vladykov | Remarks on the American Eel (Anguilla rostrata LeSueur). Sizes of elvers entering streams; the relative abundance of adult males and females; and present economic importance of eels in North America[END_REF][START_REF] Helfman | Size, age, and sex of American eels in a Georgia river[END_REF][START_REF] Vøllestad | A 13-year study of population dynamics and growth of the European eel Anguilla anguilla in a Norwegian River: evidence for density-dependent mortality, and development of a model for predicting yield[END_REF][START_REF] Vøllestad | Geographic variation in age and length at metamorphosis of maturing European eel -Environmental effects and phenotypic plasticity[END_REF] and in upstream parts of river catchments [START_REF] Oliveira | Variation in population and life history traits of the American eel, Anguilla rostrata, in four rivers in Maine[END_REF]Tesch 2003). Length-at-silvering (onset of sexual maturation) varies between between sexes and habitats. Males follow a time-minimising strategy, leaving continental waters as soon as they reach the minimal length to achieve the spawning migration, while females follow a size-maximising strategy, adapting their length-at-silvering to local growth and mortality conditions finding a tradeoff between survival and fecundity [START_REF] Helfman | Reproductive ecology of the American eel[END_REF][START_REF] Vøllestad | Geographic variation in age and length at metamorphosis of maturing European eel -Environmental effects and phenotypic plasticity[END_REF]. Consequently, male length-at-silvering is rather set [START_REF] Vøllestad | Geographic variation in age and length at metamorphosis of maturing European eel -Environmental effects and phenotypic plasticity[END_REF][START_REF] Oliveira | Life history characteristics and strategies of the American eel, Anguilla rostrata[END_REF], while females exhibit a wider range of sizes and are often larger in the northern parts of the distribution areas [START_REF] Helfman | Reproductive ecology of the American eel[END_REF][START_REF] Davey | Sex determination in freshwater eels and management options for manipulation of sex[END_REF][START_REF] Jessop | Geographic effects on American eel (Anguilla rostrata) life history characteristics and strategies[END_REF]). These life history traits patterns are thought to be the result of adaptive phenotypic plasticity that allows individuals to adapt their life history traits to a wide range of environmental conditions [START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF]Mateo et al. 2017a) but also of genetic polyphormism leading to spatially varying selection and/or genetically based habitat selection producing genetically distinct ecotypes (Gagnaire et al. 2012;[START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Ulrik | Do North Atlantic eels show parallel patterns of spatially varying selection?[END_REF][START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF]Mateo et al. 2017a).

In addition to having similar life histories, the three anguillid species underwent a dramatic decline which started in the late 1970s [START_REF] Dekker | Québec Declaration of Concern: Worldwide decline of eel resources necessitates immediate action[END_REF][START_REF] Dekker | The 2003 Québec Declaration of Concern About Eel Declines-11 Years Later: Are Eels Climbing Back up the Slippery Slope?[END_REF]. The collapse became evident through the analyses of the recruitment indices of the three species (Figure 3). Recruitment series of glass eels are the most reliable indices to estimate trends in eel populations throughout their range, because they are less influenced by local conditions than indices using older stages. In Europe, recruitment has decreased by 90 to 99 % since the 80's (ICES 2015) while spawner abundance has also dropped (Dekker 2003a). Concerning the American eel, the recruitment in the upper Saint-Lawrence River and Lake Ontario has nearly ceased in one of the most productive areas [START_REF] Casselman | Dynamics of Resources of the American Eel, Anguilla rostrata: Declining Abundance in the 1990s[END_REF] and commercial silver eel CPUE dropped by at least 50 % in 40 years in the Saint-Lawrence River [START_REF] De Lafontaine | Abundance and individual size of American eel (Anguilla rostrata) in the St. Lawrence River over the past four decades[END_REF]. Recruitment of Japanese eel has followed a very similar decline, corresponding to 80 % in the last decades [START_REF] Dekker | Québec Declaration of Concern: Worldwide decline of eel resources necessitates immediate action[END_REF]Tanaka 2014) establishing measures for the recovery of the stock of European eel and calling for a reduction in anthropogenic mortalities. The North American eel is "endangered" since 2008 as under Ontario's Endangered Species Act, which prohibits their fishing and trading. The Canadian federal Government is currently considering whether the American eel should be listed as "Threatened" under the federal Species at Risk Act. The Japanese eel is "endangered" on the Japanese Red List published by the Ministry of Environment, Japan in 2013.

The eel population declines possibly result from oceanic changes (Castonguay et al. 1994b), overfishing [START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF]Dekker 2003c;[START_REF] Tsukamoto | Present status of the Japanese eel: resources and recent research[END_REF], contamination [START_REF] Belpaire | The European eel quality database: towards a pan-European monitoring of eel quality[END_REF][START_REF] Belpaire | Contaminants in Eels and their Role in the Collapse of the Eel Stocks[END_REF], parasitism [START_REF] Feunteun | Management and restoration of European eel population (Anguilla anguilla): An impossible bargain[END_REF][START_REF] Kirk | The impact of Anguillicola crassus on European eels[END_REF] and blockage due to dams [START_REF] Moriarty | Management of the European Eel[END_REF][START_REF] Kettle | Where once the eel and the elephant were together: decline of the European eel because of changing hydrology in southwest Europe and northwest Africa?[END_REF]. These suspected causes match the above-mentioned components of global change: oceanic modifications resulting from global warming, over-fishing corresponds to over-harvesting of natural resources, the eel swimbladder parasite is an alien species, blockage due to dams is an example of habitat loss due to land-use, and contamination is the direct result of the increased pollutant load. The respective effects of anthropogenic pressures are difficult to disentangle and probably acted synergistically in the declines of eel populations [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF][START_REF] Miller | Did a "perfect storm" of oceanic changes and continental anthropogenic impacts cause northern hemisphere anguillid recruitment reductions?[END_REF]. Here, we focus on the timelines of events and on the spatial dimension to show how some anthropogenic pressures have had a greater impact on certain habitats (and consequently eels in these habitats) than others. Finally, we discuss why the overall decline may be interpreted as the result of the combined effect of the global change components and how the cumulative pressures have had a more drastic impact, than 50 million years of evolution, by radically reducing adaptive capacities of eels. Even though there are growing concerns about the situation of tropical eels and many similarities with temperate eels [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF], we restricted our analysis to temperate eels of the genus Anguilla because they are in the worst situation according to IUCN criteria, and are the three most commercially important species [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF].

Component 1 -Global warming and ocean modification: impacts on eel migrations

Effect on leptocephalus drift and survival

The decline in eel populations is possibly linked to modifications of physical conditions in the oceans (Castonguay et al. 1994b;[START_REF] Knights | Stock assessment of European eel, Anguilla anguilla L. In: Stock assessment in inland fisheries[END_REF]Bonhommeau et al. 2008a;[START_REF] Miller | Review of ocean-atmospheric factors in the Atlantic and Pacific oceans influencing spawning and recruitment of anguillid eels. In: Challenges for Diadromous Fishes in a Dynamic Global Environment[END_REF][START_REF] Miller | Did a "perfect storm" of oceanic changes and continental anthropogenic impacts cause northern hemisphere anguillid recruitment reductions?[END_REF]. The synchronous declines of the three eel species may indicate the involvement of largescale drivers, such as changes in oceanic conditions that affect hatching and subsequent survival of larvae (Castonguay et al. 1994b;Bonhommeau et al. 2008a). Global warming has had a measurable impact on sea surface temperatures (Figure 5) and on different oceanic features (North Atlantic Oscillation, El Niño Southern Oscillation and North Equatorial Current) influencing recruitment success of temperate eels. Global warming has also had visible impacts on planktonic communities: important shifts in the diversity, abundance and spatial distribution of planktonic species in the Atlantic Ocean, where plankton is crucial for eel larval growth and survival [START_REF] Beaugrand | The North Sea regime shift: evidence, causes, mechanisms and consequences[END_REF][START_REF] Beaugrand | Rapid biogeographical plankton shifts in the North Atlantic Ocean[END_REF][START_REF] Goberville | Synchronous response of marine plankton ecosystems to climate in the Northeast Atlantic and the North Sea[END_REF].

Indeed, there are correlations between glass eel recruitment and different oceanic indicators, such as the North Oscillation Index (NAO) and sea surface temperatures (Table 1). Because most studies computed statistical correlations, the underlying mechanisms are speculative. Three main mechanisms have been proposed: a limitation in trophic conditions [START_REF] Desaunay | Seasonal and long-term changes in biometrics of eel larvae: a possible relationship between recruitment variation and North Atlantic ecosystem productivity[END_REF]ICES 2001;[START_REF] Knights | A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere[END_REF][START_REF] Kettle | How does the European eel (Anguilla anguilla) retain its population structure during its larval migration across the North Atlantic Ocean?[END_REF]Friedland et al. 2007;Bonhommeau et al. 2008b[START_REF] Bonhommeau | Estimates of the mortality and the duration of the trans-Atlantic migration of European eel Anguilla anguilla leptocephali using a particle tracking model[END_REF][START_REF] Munk | Oceanic fronts in the Sargasso Sea control the early life and drift of Atlantic eels[END_REF], changes in oceanic currents modifying larval transport (Castonguay et al. 1994b;ICES 2001;[START_REF] Knights | A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere[END_REF]Friedland et al. 2007;Zenimoto et al. 2009), and/or spatial oscillations of a salinity front used by adult eels to detect the spawning grounds which then lead to oscillations in the success of larval transport [START_REF] Kimura | Fluctuation in the distribution of low-salinity water in the North Equatorial Current and its effect on the larval transport of the Japanese eel[END_REF][START_REF] Kimura | The salinity front in the North Equatorial Current: A landmark for the spawning migration of the Japanese eel (Anguilla japonica) related to the stock recruitment[END_REF]). In addition to statistical correlations, Lagrangian simulations of larval drift have also been carried out to explore some mechanisms [START_REF] Kettle | How does the European eel (Anguilla anguilla) retain its population structure during its larval migration across the North Atlantic Ocean?[END_REF][START_REF] Kim | Effect of El Niño on migration and larval transport of the Japanese eel (Anguilla japonica)[END_REF][START_REF] Bonhommeau | Estimates of the mortality and the duration of the trans-Atlantic migration of European eel Anguilla anguilla leptocephali using a particle tracking model[END_REF][START_REF] Bonhommeau | The duration of migration of Atlantic Anguilla larvae[END_REF]Zenimoto et al. 2009;[START_REF] Melià | Integrating field data into individual-based models of the migration of European eel larvae[END_REF][START_REF] Pacariz | Climate change and passive transport of European eel larvae[END_REF]. These simulations suggested that, while for European eel the correlation between NAO and recruitment more likely reflects an indirect effect of trophic conditions in the Sargasso Sea [START_REF] Bonhommeau | Estimates of the mortality and the duration of the trans-Atlantic migration of European eel Anguilla anguilla leptocephali using a particle tracking model[END_REF][START_REF] Pacariz | Climate change and passive transport of European eel larvae[END_REF], changes in oceanic currents directly affect Japanese eel larval drift [START_REF] Kim | Effect of El Niño on migration and larval transport of the Japanese eel (Anguilla japonica)[END_REF]Zenimoto et al. 2009).

Analysing simultaneously the declines of the three species, Bonhommeau et al. (2008a) highlighted the synchrony between regime shifts in the Atlantic and Pacific sea surface temperature, primary production and recruitment of the three species. They postulated that an increase in sea surface temperature due to climate change, led to a higher stratification of the Sargasso Sea and consequently to a lower primary production, which could translate into lower food availability for the leptocephali. Recently, [START_REF] Miller | Did a "perfect storm" of oceanic changes and continental anthropogenic impacts cause northern hemisphere anguillid recruitment reductions?[END_REF] proposed a more precise mechanism: the regime shift resulted in a lower abundance of diatoms and a higher abundance of cyanobacteria, which may have resulted in a lower production of carbohydrates which are crucial for the production of "marine snow", the main food of eel larvae [START_REF] Riemann | Qualitative assessment of the diet of European eel larvae in the Sargasso Sea resolved by DNA barcoding[END_REF].

Effect on silver eel spawning migration

Climate change can also affect the later stages of eel. Oceanic conditions and climate change can, indirectly, influence river discharge [START_REF] Arnell | The effect of climate change on hydrological regimes in Europe: a continental perspective[END_REF][START_REF] Milly | Global pattern of trends in streamflow and water availability in a changing climate[END_REF] through modifications of precipitation regimes [START_REF] Kettle | Where once the eel and the elephant were together: decline of the European eel because of changing hydrology in southwest Europe and northwest Africa?[END_REF]. The discharge regime is also modified by water extraction for human use, agriculture and other industrial processes [START_REF] Postel | Rivers for life: managing water for people and nature[END_REF][START_REF] Verreault | Spawning migration of American eel Anguilla rostrata from pristine (1843-1872) to contemporary (1963-1990) periods in the St Lawrence Estuary, Canada[END_REF]. River discharge and rainfall are important triggers (direct or indirect) of silver eel migration [START_REF] Boubée | Factors regulating the downstream migration of mature eels (Anguilla spp.) at Aniwhenua Dam, Bay of Plenty, New Zealand[END_REF][START_REF] Acou | Migration pattern of silver eel (Anguilla anguilla, L.) in an obstructed river system[END_REF]Durif and Elie 2008;[START_REF] Bruijs | Silver Eel Migration and Behaviour[END_REF][START_REF] Trancart | Forecasting animal migration using SARIMAX: an efficient means of reducing silver eel mortality caused by turbines[END_REF][START_REF] Reckordt | Downstream migration dynamics of female and male silver eels (Anguilla anguilla L.) in the regulated German lowland Warnow River[END_REF]Drouineau et al. 2017). Higher river discharge increases migration speed [START_REF] Vøllestad | Environmental factors regulating the seaward migration of European silver eels (Anguilla anguilla)[END_REF]Tesch 2003). Reduced discharge delays migration and eels may even be stopped until the following year if environmental conditions are not favourable (Durif et al. 2003;Drouineau et al. 2017). Finally, reduced discharge can lead to higher proportions of eels going through turbines, since at low flow a higher proportion of water is guided through the turbines, leading to higher mortalities [START_REF] Jansen | Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge[END_REF][START_REF] Bau | Anguille et ouvrages : migration de dévalaison. Suivi par radiopistage de la dévalaison de l'anguille argentée sur le Gave de Pau au niveau des ouvrages hydroélectriques d[END_REF].

Component 2 -Increased contamination load: contamination of eels and consequences on its physiology Eels are vulnerable to contamination because of their high trophic level and high level of lipid storage [START_REF] Geeraerts | The effects of contaminants in European eel: a review[END_REF][START_REF] Belpaire | The European eel quality database: towards a pan-European monitoring of eel quality[END_REF][START_REF] Ices | Report of the Workshop of the Working Group on Eel and the Working Group on Biological Effects of Contaminants (WKBECEEL)[END_REF]. Contaminants that have been found in eels include: organic contaminants [START_REF] Hodson | Spatial and Temporal Variations in Chemical Contamination of American Eels, Anguilla rostrata, Captured in the Estuary of the St, Lawrence River[END_REF][START_REF] Ohji | Differences in organotin accumulation among ecological migratory types of the Japanese eel Anguilla japonica[END_REF][START_REF] Bilau | Probabilistic approach to polychlorinated biphenyl (PCB) exposure through eel consumption in recreational fishermen vs. the general population[END_REF][START_REF] Blanchet-Letrouvé | Dioxin-like, non-dioxin like PCB and PCDD/F contamination in European eel (Anguilla anguilla) from the Loire estuarine continuum: Spatial and biological variabilities[END_REF][START_REF] Kammann | PAH metabolites, GST and EROD in European eel (Anguilla anguilla) as possible indicators for eel habitat quality in German rivers[END_REF][START_REF] Guhl | Contaminant levels in the European eel (Anguilla anguilla) in North Rhine-Westphalian rivers[END_REF]; heavy metals [START_REF] Yang | Uptake and elimination of cadmium by Japanese eel, Anguilla japonica, at various temperatures[END_REF]Maes et al. 2005;Pierron et al. 2008a,b;[START_REF] Nunes | Effects of chronic exposure to lead, copper, zinc, and cadmium on biomarkers of the European eel, Anguilla anguilla[END_REF][START_REF] Pannetier | A comparison of metal concentrations in the tissues of yellow American eel (Anguilla rostrata) and European eel (Anguilla anguilla)[END_REF] and pesticides [START_REF] Hodson | Spatial and Temporal Variations in Chemical Contamination of American Eels, Anguilla rostrata, Captured in the Estuary of the St, Lawrence River[END_REF][START_REF] Gimeno | Pesticide effects on eel metabolism[END_REF][START_REF] Couillard | Correlations between pathological changes and chemical contamination in American eels, Anguilla rostrata, from the St. Lawrence River[END_REF][START_REF] Byer | Spatial trends of organochlorinated pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers in Atlantic Anguillid eels[END_REF][START_REF] Privitera | Impact of a short-term exposure to tributyl phosphate on morphology, physiology and migratory behaviour of European eels during the transition from freshwater to the marine environment[END_REF].

Consequently, eels are sometimes used as bioindicators of contamination [START_REF] Amiard-Triquet | The eel (Anguilla anguilla L.) as a bioindicator of metal pollution: factors limiting its use[END_REF][START_REF] Linde | Eel (Anguilla anguilla) and brown trout (Salmo trutta) target species to assess the biological impact of trace metal pollution in freshwater ecosystems[END_REF][START_REF] Belpaire | The European eel Anguilla anguilla, a rapporteur of the chemical status for the water framework directive?[END_REF][START_REF] Mchugh | The occurrence of persistent chlorinated and brominated organic contaminants in the European eel (Anguilla anguilla) in Irish waters[END_REF]. Contamination levels are often above human consumption standards [START_REF] Bilau | Probabilistic approach to polychlorinated biphenyl (PCB) exposure through eel consumption in recreational fishermen vs. the general population[END_REF][START_REF] Geeraerts | The effects of contaminants in European eel: a review[END_REF][START_REF] Byer | Spatial trends of organochlorinated pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers in Atlantic Anguillid eels[END_REF][START_REF] Ices | Report of the Joint EIFAAC/ICES Working Group on Eels (WGEEL)[END_REF][START_REF] Ices | Report of the Workshop of the Working Group on Eel and the Working Group on Biological Effects of Contaminants (WKBECEEL)[END_REF] and have led to fishing prohibitions in various sites in European countries (Germany, Belgium, Netherlands, France, Italy) [START_REF] Belpaire | Contaminants in Eels and their Role in the Collapse of the Eel Stocks[END_REF].

These contaminants are widely found in freshwater fishes [START_REF] Streit | Bioaccumulation of contaminants in fish[END_REF] and their effects on fish biology [START_REF] Gilliers | Are growth and density quantitative indicators of essential fish habitat quality? An application to the common sole Solea solea nursery grounds[END_REF][START_REF] Kerambrun | Growth and condition indices of juvenile turbot, Scophthalmus maximus[END_REF][START_REF] Fonseca | Modeling fish biological responses to contaminants and natural variability in estuaries[END_REF]) and the danger for human consumption have been demonstrated [START_REF] Schuhmacher | Mercury concentrations in marine species from the coastal area of Tarragona Province, Spain. Dietary intake of mercury through fish and seafood consumption[END_REF][START_REF] Järup | Hazards of heavy metal contamination[END_REF][START_REF] Halldorsson | Is high consumption of fatty fish during pregnancy a risk factor for fetal growth retardation? A study of 44,824 Danish pregnant women[END_REF]. While metallic contaminants have a long history in countries with extraction activities, organic contamination, pesticides, and nutrients loads are much more recent (Malmqvist and Rundle 2002; [START_REF] Morée | Exploring global nitrogen and phosphorus flows in urban wastes during the twentieth century[END_REF]. Many of them appeared in the second half of the XXth century in relation to agriculture intensification, urbanisation and industrial activities. During this period, fertiliser utilisation grew exponentially (Figure 6). PCBs and DDT production peaked around the 1960's [START_REF] Harrad | Polychlorinated biphenyls (PCBs) in the British environment: sinks, sources and temporal trends[END_REF]Van Metre et al. 1998), i.e. about 20 years before eels started to decline, and concentrations remain high in river sediments explaining why levels are still high in eels [START_REF] Ices | Report of the Workshop of the Working Group on Eel and the Working Group on Biological Effects of Contaminants (WKBECEEL)[END_REF]. Persistent Organic Pollutants (POPs) had a dramatic effect on lake trout (Salvelinus namaycush) in Lake Ontario [START_REF] Cook | Effects of Aryl Hydrocarbon Receptor-Mediated Early Life Stage Toxicity on Lake Trout Populations in Lake Ontario during the 20th Century[END_REF] and concentrations peaked in American eel in Lake Ontario in the late 1960s, about 20 years before the American eel recruitment collapse [START_REF] Byer | Trends of persistent organic pollutants in American eel (Anguilla rostrata) from eastern Lake Ontario, Canada, and their potential effects on recruitment[END_REF]. The increased nutrient load to water bodies has caused detrimental impacts on humans and aquatic ecosystem health [START_REF] Grizzetti | Nitrogen as a threat to European water quality[END_REF][START_REF] Grizzetti | Changes of nitrogen and phosphorus loads to European seas[END_REF] and continued to increase until the mid-90s before declining in many rivers [START_REF] Minaudo | Eutrophication mitigation in rivers: 30 years of trends in spatial and seasonal patterns of biogeochemistry of the Loire River (1980-2012)[END_REF]. Fertilisers are still used at a very high level (Figure 6). Moreover, new contaminants are appearing in the water and in eels, such as Perfluorooctanesulfonic acid (PFOS), textile dyes, musk compounds, perfluorinated substances, organophosphorus flame retardants and plasticisers (ICES 2016).

There have been a few cases of direct eel mortalities due to contaminants [START_REF] Dutil | Electrolyte changes of serum and muscle, and related mortalities in maturing Anguilla rostrata migrating down the St. Lawrence Estuary (Canada)[END_REF][START_REF] Dutil | Osmoregulatory and ionoregulatory changes and associated mortalities during the transition of maturing American eels to a marine environment[END_REF] but in the majority of cases the impact is at the sublethal level ranging from tissue damage, stress, effects on osmoregulation, behaviour alteration, hormonal perturbation and genotoxic effects [START_REF] Couillard | Correlations between pathological changes and chemical contamination in American eels, Anguilla rostrata, from the St. Lawrence River[END_REF][START_REF] Geeraerts | The effects of contaminants in European eel: a review[END_REF]. Contaminants may also be transferred to the offspring resulting in larval malformation [START_REF] Robinet | Sublethal effects of exposure to chemical compounds: a cause for the decline in Atlantic eels?[END_REF][START_REF] Byer | Spatial trends of organochlorinated pesticides, polychlorinated biphenyls, and polybrominated diphenyl ethers in Atlantic Anguillid eels[END_REF][START_REF] Rigaud | Temporal variations in embryotoxicity of Lake Ontario American eel (Anguilla rostrata) extracts to developing Fundulus heteroclitus[END_REF][START_REF] Foekema | Maternally transferred dioxinlike compounds can affect the reproductive success of European eel[END_REF]. As a fatty fish, eels are particularly sensitive to contamination. Most contaminants are highly concentrated in the lipid stores [START_REF] Robinet | Sublethal effects of exposure to chemical compounds: a cause for the decline in Atlantic eels?[END_REF] and affect lipid metabolism [START_REF] Fernández-Vega | Thiobencarb toxicity and plasma AChE inhibition in the European eel[END_REF][START_REF] Corsi | Contamination and sub-lethal toxicological effects of persistent organic pollutants in the European eel (Anguilla anguilla) in the Orbetello lagoon (Tuscany, Italy)[END_REF][START_REF] Pierron | Impairment of lipid storage by cadmium in the European eel (Anguilla anguilla)[END_REF]. This is especially critical at the silver eel stage when lipid levels are highest (over 13%) to achieve their transoceanic migration to the spawning grounds (van Ginneken and van den Thillart 2000; Van Den Thillart et al. 2004, 2007;[START_REF] Belpaire | Decreasing eel stocks: survival of the fattest?[END_REF]. For female eels, 67% of their fat store is spent on the spawning migration and oocyte maturation (Palstra and van den Thillart 2010). As lipids are mobilised during spawning migration, contaminants are more likely to be released into the blood at high concentrations, thus negatively affecting gonad maturation and oocyte production, as they do in other fish species [START_REF] Pierron | Abnormal Ovarian DNA Methylation Programming during Gonad Maturation in Wild Contaminated Fish[END_REF][START_REF] Baillon | Gonadal transcriptome analysis of wild contaminated female European eels during artificial gonad maturation[END_REF][START_REF] Ices | Report of the Workshop of the Working Group on Eel and the Working Group on Biological Effects of Contaminants (WKBECEEL)[END_REF], and also impairing migration success [START_REF] Robinet | Sublethal effects of exposure to chemical compounds: a cause for the decline in Atlantic eels?[END_REF]Pierron et al. 2008a;[START_REF] Geeraerts | The effects of contaminants in European eel: a review[END_REF].

As a summary, contaminants can act as a classical stressor during the continental stage of eel, but then have the potential to dramatically impair maturation and migration success, i.e. the whole reproduction success.

Component 3 -Fragmentation and habitat loss: fragmentation by weirs and dams and consequences on upstream and downstream migration

Movements, habitats and fragmentation

Movement is a key feature of living organisms to find food, mates and avoid predation [START_REF] Nathan | A movement ecology paradigm for unifying organismal movement research[END_REF]. Several types of movements can be distinguished. The first type, called "station-keeping" [START_REF] Dingle | Migration : the biology of life on the move[END_REF] takes place within the home range of the animal and corresponds to simple movements for foraging and predation avoidance. The two other types of movement, ranging and migration, occur outside the home range [START_REF] Dingle | What is migration?[END_REF]. Ranging is dedicated to the search of a specific resource (mate, food, etc.) and stops when the resource is found [START_REF] Jeltsch | Integrating movement ecology with biodiversity research -exploring new avenues to address spatiotemporal biodiversity dynamics[END_REF]. Migration is generally triggered by physiological and environmental cues and not by the search for a specific resource such as food or mates. It affects most individuals in the population, occurs over a long timescale, requires orientation, and suggests a return journey [START_REF] Dingle | Migration : the biology of life on the move[END_REF][START_REF] Dingle | What is migration?[END_REF].

Diadromous fish, such as eels undergo two long migrations (Tesch 2003): the first migration, from the spawning grounds to their growth habitat, includes a phase of active upstream migration in river catchments during the early years of their continental life-stage (Castonguay et al. 1994a;[START_REF] Imbert | Resident and migratory tactics in freshwater European eels are size-dependent[END_REF]Fukuda et al. 2016). During the second migration, eels return to the oceanic spawning grounds from their growth habitats in rivers or coastal waters. Eels may also move between different habitats during their continental stage [START_REF] Daverat | Tracking continental habitat shifts of eels using otolith Sr/Ca ratios: Validation and application to the coastal, estuarine and riverine eels of the Gironde-Garonne-Dordogne watershed[END_REF][START_REF] Kaifu | Dispersal of yellow phase Japanese eels Anguilla japonica after recruitment in the Kojima Bay-Asahi River system, Japan[END_REF][START_REF] Arai | Diverse migration strategy between freshwater and seawater habitats in the freshwater eel genus Anguilla[END_REF][START_REF] Yokouchi | Influences of early habitat use on the migratory plasticity and demography of Japanese eels in central Japan[END_REF]Béguer-Pon et al. 2015a), movements which correspond to station keeping and ranging.

The construction of dams accelerated worldwide during the 1950/60s [START_REF] Dynesius | Fragmentation and flow regulation of river systems in the northern third of the world[END_REF][START_REF] Postel | Rivers for life: managing water for people and nature[END_REF]MacGregor et al. 2009) (Figure 7), about 20 years before the eel population declined. This massive construction of dams has restrained eel movements and available habitats. The construction of hydropower dams during the XXth century in the St. Lawrence catchment caused a 40% habitat loss for the North American eel in this basin [START_REF] Verreault | Habitat losses and anthropogenic barriers as a cause of population decline for American eel (Anguilla rostrata) in the St. Lawrence watershed, Canada[END_REF]).

The situation is similar or worse in the United States [START_REF] Busch | Distribution and availability of Atlantic Coast freshwater habitats for American eel (Anguilla rostrata)[END_REF], especially since most dams lack fishways (MacGregor et al. 2009). In Europe, 50-90% of habitats were lost by the end of XXth century [START_REF] Feunteun | Management and restoration of European eel population (Anguilla anguilla): An impossible bargain[END_REF]. For the Japanese eel, approximately 75% of effective habitats were lost between 1970 and 2010 in Japan, Korea, Taiwan and China, with a maximum in China (>80%) and Taiwan (~50%) [START_REF] Chen | Impact of long-term habitat loss on the Japanese eel Anguilla japonica[END_REF].

Intensive dam constructions in Spain, Morocco and Portugal, have had drastic consequences on European eel distribution [START_REF] Nicola | Dams and fish passage facilities in the large rivers of Spain: effects on migratory species[END_REF][START_REF] Lobon-Cervia | The decline of eel Anguilla anguilla (L.) in a river catchment of northern Spain 1986-1997. Further evidence for a critical status of eel in Iberian waters[END_REF][START_REF] Clavero | Historical data to plan the recovery of the European eel[END_REF], possibly affecting the sex ratio since this area yields mainly male eels and is closest to the spawning area [START_REF] Kettle | Where once the eel and the elephant were together: decline of the European eel because of changing hydrology in southwest Europe and northwest Africa?[END_REF].

Dams and weirs are not the only factors affecting eel habitats: rivers provide multiple goods and services to society [START_REF] Postel | Rivers for life: managing water for people and nature[END_REF][START_REF] Wolanski | Ecological Economics of Estuaries and Coasts[END_REF][START_REF] Elliott | Challenging paradigms in estuarine ecology and management[END_REF] that have led to river channelization, hydro-morphological modifications, drying out of lateral wetland, wetland drainage, water extraction, modification of land use in the floodplain that can lead to higher erosion and sedimentation [START_REF] Elliott | Fishes in Estuaries[END_REF][START_REF] Postel | Rivers for life: managing water for people and nature[END_REF][START_REF] Basset | A unifying approach to understanding transitional waters: Fundamental properties emerging from ecotone ecosystems[END_REF]. As an example, typical eel habitats, such as estuarine marshes and intertidal zones have been lost because of flood protection walls, agriculture activities and navigation [START_REF] Gros | The Impact of Global Change on the Dynamics of Marine Living Resources[END_REF]. In Japan, catch reduction rates in several rivers and lakes were positively correlated with the rate of revetment along rivers and around lakes (Itakura et al. 2015b), also the condition factor of eels and prey diversity were significantly lower in these modified habitats (Itakura et al. 2015a).

Blockage during upstream migrations

During their first year in continental waters, eels display an active migratory behaviour and then shift to a resident behaviour [START_REF] Imbert | Resident and migratory tactics in freshwater European eels are size-dependent[END_REF][START_REF] Benchetrit | Using otolith microchemistry to reconstruct habitat use of American eels Anguilla rostrata in the St. Lawrence River-Lake Ontario system[END_REF]. Resident behaviour does not exclude habitat shifts [START_REF] Daverat | Tactics and demographic attributes in the European eel Anguilla anguilla in the Gironde watershed, SW France[END_REF] though these types of movement correspond more to ranging than strict migration [START_REF] Dingle | What is migration?[END_REF]. Upstream migration has a cost and its evolutionary benefit is still unclear since eels can settle in a wide range of habitats (Tsukamoto et al. 1998;Daverat et al. 2006;[START_REF] Yokouchi | Influences of early habitat use on the migratory plasticity and demography of Japanese eels in central Japan[END_REF]Marohn et al. 2013). Glass eels with high feeding rate and fast weight gain have a higher propensity to migrate (Bureau du Colombier et al.

2008

). These glass eels also display a more gregarious and less aggressive behaviour [START_REF] Geffroy | Differential effects of behaviour, propensity to migrate and recruitment season on glass eels and elvers' growing performance[END_REF]. Habitat selection could be a trade-off between growth (generally higher in downstream habitats), survival (generally higher in upstream habitats), competition avoidance (higher competition in downstream habitats) and energetic cost of migration (Mateo et al. 2017a;[START_REF] Edeline | Adaptive phenotypic plasticity of eel diadromy[END_REF][START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF]. Habitat selection is also partly related to genetic or epigenetic polymorphism (Gagnaire et al. 2012;[START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF]Podgorniak et al. 2015b;Mateo et al. 2017a). In such a scheme, habitat selection would be the result of a fitness optimisation process in which fitness in a habitat would depend on habitat characteristics, competition in the habitat, but also individual variability of growth rates due to the existence of genetically distinct clusters of individuals [START_REF] Côté | Growth, Female Size, and Sex Ratio Variability in American Eel of Different Origins in Both Controlled Conditions and the Wild: Implications for Stocking Programs[END_REF]Mateo et al. 2017a).

Given this plasticity in habitat use, the consequences of obstacles on upstream migrations are difficult to assess. Methods have been proposed to assess the passability of obstacles [START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF]Drouineau et al. 2015;[START_REF] Tremblay | Assessment of upstream and downstream passability for eel at dams[END_REF]. Densities of eels are higher downstream of obstacles. This leads to (i) increased competition between individuals, which can subsequently result in lower survival [START_REF] Vøllestad | A 13-year study of population dynamics and growth of the European eel Anguilla anguilla in a Norwegian River: evidence for density-dependent mortality, and development of a model for predicting yield[END_REF][START_REF] Bevacqua | Intra-specific scaling of natural mortality in fish: The paradigmatic case of the European eel[END_REF]), (ii) increased susceptibility to predation (Drouineau et al. 2015;Larinier 2001;Garcia De Leaniz 2008;[START_REF] Agostinho | Fish ladders: safe fish passage or hotspot for predation?[END_REF], and overfishing (Dekker 2003c;[START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF]) and, (iii) possible modification to the sex ratio, since sex determination is density-dependent [START_REF] Poole | Observations on the silver eel migrations of the Burrishoole River system, Ireland, 1959 to 1988[END_REF][START_REF] De Leo | Trends in vital rates of the European eel: evidence for density dependence?[END_REF][START_REF] Roncarati | Influence of stocking density of European eel (Anguilla anguilla, L.) elvers on sex differentiation and zootechnical performances[END_REF]Tesch 2003;[START_REF] Davey | Sex determination in freshwater eels and management options for manipulation of sex[END_REF].

Finally, obstacles to upstream migration can act as a permanent selection pressure (Podgorniak et al. 2015a,b;Mateo et al. 2017b). [START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF] demonstrated the existence of two clusters of individual eels with differing genetic basis: a cluster of slow growers and a cluster of fast growers, while [START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF] demonstrated the existence of genetically distinct ecotypes, with different growth rates and different sex ratios. By impairing migration within catchments, obstacles can decrease the fitness of some types of individuals; those individuals who genetically belong to the "freshwater habitat" may not be able to reach suitable habitats, or will suffer damage during their downstream migration (Mateo et al. 2017b).

Impaired downstream migrations

Most studies dealing with downstream migration have focused on mortality due to passage through hydropower turbines [START_REF] Coutant | Assessing supply chain and market impacts of a CITES listing on Anguilla species[END_REF][START_REF] Gosset | Tests of two types of bypass for downstream migration of eels at a small hydroelectric power plant[END_REF][START_REF] Boubée | Downstream passage of silver eels at a small hydroelectric facility[END_REF][START_REF] Williams | Survival estimates for downstream migrant yearling juvenile salmonids through the Snake and Columbia rivers hydropower system, 1966-1980 and 1993-1999[END_REF][START_REF] Carr | Migration of silver American eels past a hydroelectric dam and through a coastal zone[END_REF][START_REF] Calles | Size-dependent mortality of migratory silver eels at a hydropower plant, and implications for escapement to the sea[END_REF][START_REF] Pedersen | Loss of European silver eel passing a hydropower station[END_REF]. Several factors influence the mortality induced by hydropower plants:

 Turbine characteristics: The mortality due to strikes by Kaplan turbines are generally greater than 15% and sometimes as high as 100% depending on fish length, wheel diameter, nominal discharge flow and speed of rotation [START_REF] Gomes | Dommages subis par les anguilles lors de leur passage au travers des turbines Kaplan[END_REF]. For Francis turbines, [START_REF] Calles | Size-dependent mortality of migratory silver eels at a hydropower plant, and implications for escapement to the sea[END_REF] estimated a mortality rate of 60% at a Swedish site while a mortality rate of about 16% was found at an American site [START_REF] Richkus | Review of research and technologies on passage and protection of downstream migrating catadromous eels at hydroelectric facilities[END_REF]. Even if they survive passage through the turbines, eels can be wounded and have a reduced chance of reaching the spawning area.

 Site configuration can greatly influence the probability that a fish will pass through or bypass the turbines. Since silver eels follow the main flow [START_REF] Jansen | Just go with the flow? Route selection and mortality during downstream migration of silver eels in relation to river discharge[END_REF], the orientation of the water intake with respect to the main channel influences the probability of turbine passage [START_REF] Bau | Anguille et ouvrages : migration de dévalaison. Suivi par radiopistage de la dévalaison de l'anguille argentée sur le Gave de Pau au niveau des ouvrages hydroélectriques d[END_REF]. Different types of barriers have been proposed to divert eels from turbine passage, such as fish-friendly trashracks [START_REF] Raynal | An experimental study on fish-friendly trashracks -Part 1. Inclined trashracks[END_REF][START_REF] Raynal | Streamwise bars in fishfriendly angled trashracks[END_REF], flow field manipulation [START_REF] Piper | Response of seaward-migrating European eel (Anguilla anguilla) to manipulated flow fields[END_REF], light [START_REF] Patrick | Effectiveness of a strobe light eel exclusion scheme[END_REF][START_REF] Hadderingh | Deflecting eels from water inlets of power stations with light[END_REF]) and infrasound barriers [START_REF] Sand | Avoidance responses to infrasound in downstream migrating European silver eels, Anguilla anguilla[END_REF][START_REF] Sand | Detection of infrasound in fish and behavioral responses to intense infrasound in juvenile salmonids and European silver eels: a minireview[END_REF]. The installation of bypasses is also a mitigation measure to prevent passages through turbines (Durif et al. 2003;[START_REF] Gosset | Tests of two types of bypass for downstream migration of eels at a small hydroelectric power plant[END_REF][START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF].

 Environmental conditions: In a period of low discharge, when the flow through the turbine is high compared to the flow over weir, more eels will pass through the turbines than at high discharge, when the turbine flow is small compared to the weir flow.

 Obstacle location within the catchment: since eels are not uniformly distributed within a river catchment [START_REF] Ibbotson | Colonisation of freshwater habitats by the European eel Anguilla anguilla[END_REF], the number of eels impacted by a given facility depends on the number of eels that settle upstream the facility. Therefore, it is necessary to estimate the distribution of fish within catchments to assess the effect of hydropower plants at the catchment scale. In the SEAHOPE model, the total mortality induced by hydropower plants in a given catchment was estimated by coupling a model that predicts the proportion of fish killed when passing each individual plant with a model that predicts the spatial distribution of eels within the catchment [START_REF] Jouanin | Evaluation des mortalités d'anguilles induites par les ouvrages hydroélectriques en France-Projet SEA HOPE Silver Eels escApement from HydrOPowEr[END_REF]).

However, direct mortality is not the only impact obstacles can have on downstream migrants. First, sub-lethal injuries can occur during obstacle passages because of impingements on hard structures (even in the absence of turbins), which can then impair spawning migration success [START_REF] Bruijs | Silver Eel Migration and Behaviour[END_REF]. Predation during downstream passage has also been recorded for many fish species [START_REF] Williams | Survival estimates for downstream migrant yearling juvenile salmonids through the Snake and Columbia rivers hydropower system, 1966-1980 and 1993-1999[END_REF][START_REF] Muir | Post-hydropower system delayed mortality of transported snake river stream-type Chinook salmon: Unraveling the mystery[END_REF]Garcia De Leaniz 2008). Moreover, increased energy costs induced by obstacle passage may have a delayed impact on migration success and fecundity: silver eels stop feeding during the spawning migration and their lipid stores are crucial to achieve the oceanic migration and produce oocytes (van Ginneken and van den Thillart 2000). Delays induced by obstacles can impair escapement, especially when the environmental migration suitability window is limited (Verbiest et al. 2012;Drouineau et al. 2017). Finally, similarly to obstacles to upstream migration, obstacles to downstream migration affect specific types of individual: individuals that settle upstream of the obstacle (i.e. individuals that settle in upstream habitats and individuals that were able to pass the obstacle), as such, obstacles may have the potential to exert a selection pressure on the population (Mateo et al. 2017b).

Component 4 -Alien species: effects of alien parasitoid Anguillicola crassus

Though competition is possible with some alien species such as the European catfish [START_REF] Bevacqua | Intra-specific scaling of natural mortality in fish: The paradigmatic case of the European eel[END_REF], or even with introduced American [START_REF] Han | The exotic American eel in Taiwan: ecological implications[END_REF] and European eels in East Asia [START_REF] Aoyama | The European eel, Anguilla anguilla (L.), in Japanese waters[END_REF], Anguillicola crassus is the alien species that has the most documented and widespread impact on eels, at least for European and American eels. Anguillicola crassus is a natural parasite of Japanese eel which was introduced into Europe in the mid 70's, early 80's, probably through the aquaculture trade [START_REF] Koops | Anguillicola-infestations in Germany and in German eel imports[END_REF]. It is now widespread in Europe [START_REF] Kennedy | Colonization, larval survival and epidemiology of the nematode Anguillicola crassus, parasitic in the eel, Anguilla anguilla, in Britain[END_REF][START_REF] Evans | Anguillicola crassus (Nematoda, Dracunculoidea); first documented record of this swimbladder parasite of eels in Ireland[END_REF]Lefebvre et al. 2002;[START_REF] Kirk | The impact of Anguillicola crassus on European eels[END_REF][START_REF] Norton | Epidemiology of Anguillicola crassus in the European eel (Anguilla anguilla) from two rivers in southern England[END_REF][START_REF] Neto | Epidemiology and pathology of Anguillicoloides crassus in European eel Anguilla anguilla from the Tagus estuary (Portugal)[END_REF][START_REF] Becerra-Jurado | Distribution, prevalence and intensity of Anguillicola crassus (Nematoda) infection in Anguilla anguilla in the Republic of Ireland[END_REF], and Northern Africa [START_REF] Koops | Anguillicola-infestations in Germany and in German eel imports[END_REF][START_REF] El Hilali | Premières données épidémiologiques sur l'anguillicolose au Maroc[END_REF][START_REF] Maamouri | Occurrence of Anguillicola crassux (Nematode, Anguillicolidae) in the Ichkeul Lake (Northern Tunisia)[END_REF][START_REF] Hizem Habbechi | Etude de la contamination de l'anguille européenne (Anguilla Anguilla L., 1758) par Anguillicoloides crassus dans quelques hydrosystèmes de la Tunisie septentrionale: analyse de son impact sur les paramètres de croissance[END_REF][START_REF] Dhaouadi | Etude de l'infestation de l'anguille européenne, Anguilla anguilla, par le nématode Anguillicoloides crassus dans la lagune de Ghar El Melh (Nord de la Tunisie)[END_REF]. Systematic monitoring of eel diseases is still limited to a few countries, impairing our ability to assess the overall prevalence (ICES 2015). However, many studies have reported a significant prevalence at sites in both North America and Europe [START_REF] Aieta | Distribution, prevalence, and intensity of the swim bladder parasite Anguillicola crassus in New England and eastern Canada[END_REF][START_REF] Denny | Distribution, prevalence and intensity of Anguillicoloides crassus in the American eel, Anguilla rostrata, in the Bras d'Or Lakes[END_REF][START_REF] Becerra-Jurado | Distribution, prevalence and intensity of Anguillicola crassus (Nematoda) infection in Anguilla anguilla in the Republic of Ireland[END_REF]) and an analysis of the European Eel Quality Database confirmed the prevalence of the infection in Europe [START_REF] Belpaire | The European eel quality database: towards a pan-European monitoring of eel quality[END_REF].

The invasion in North America started for the same reason, a few years after its introduction into Europe. The first record occurred in the second half of the 90's in Texas (Fries et al. 1996) and then in Chesapeake Bay and the Hudson River [START_REF] Barse | An exotic nematode parasite of the American eel[END_REF]. The invasion then quickly spread in the United States and in Canada (Machut and Limburg 2008;[START_REF] Aieta | Distribution, prevalence, and intensity of the swim bladder parasite Anguillicola crassus in New England and eastern Canada[END_REF][START_REF] Rockwell | First record of Anguillicoloides crassus (Nematoda) in American eels (Anguilla rostrata) in Canadian estuaries, Cape Breton, Nova Scotia[END_REF][START_REF] Denny | Distribution, prevalence and intensity of Anguillicoloides crassus in the American eel, Anguilla rostrata, in the Bras d'Or Lakes[END_REF][START_REF] Hein | Invasive swimbladder parasite Anguillicoloides crassus: infection status 15 years after discovery in wild populations of American eel Anguilla rostrata[END_REF]. Though transmission is possible in brackish waters [START_REF] Reimer | Anguillicola crassus in the Baltic Sea: Field data supporting transmission in brackish waters[END_REF]Kirk et al. 2000a,b;Lefebvre et al. 2002), the level of infection is lower than in freshwater (Kirk et al. 2000a,b;[START_REF] Kirk | The impact of Anguillicola crassus on European eels[END_REF].

This swimbladder parasite has multiple impacts on its host. The parasite causes inflammation of the swimbladder leading to multiple bacterial infections, stress and loss of appetite [START_REF] Kirk | The impact of Anguillicola crassus on European eels[END_REF]Lefebvre et al. 2013). However, the most serious damage is on the swimbladder itself. The infection may alter the gas composition of the swimbladder, block the pneumatic duct, impairing the organ's function (Kirk et al. 2000b;Lefebvre et al. 2013) leading to necrosis in the most extreme cases [START_REF] Molnár | Dynamics of Anguillicola crassus (Nematoda: Dracunculoidea) infection in eels of Lake Balaton, Hungary[END_REF][START_REF] Würtz | Histopathological changes in the swimbladder wall of the European eel Anguilla anguilla due to infections with Anguillicola crassus[END_REF]. The alteration of the swimbladder has a direct impact on swimming capacity [START_REF] Sprengel | Infection by endoparasites reduces maximum swimming speed of European smelt Osmerus eperlanus and European eel Anguilla anguilla[END_REF][START_REF] Székely | Impact of the swim-bladder Parasite on the health and performance of European eels. In: Spawning Migration of the Tischendorf[END_REF]. It may imperil the transoceanic spawning migration [START_REF] Palstra | Swimming performance of silver eels is severely impaired by the swim-bladder parasite Anguillicola crassus[END_REF][START_REF] Clevestam | Too short to spawn? Implications of small body size and swimming distance on successful migration and maturation of the European eel Anguilla anguilla[END_REF], especially because migrant eels display important diurnal vertical migrations [START_REF] Chow | Light-sensitive vertical migration of the Japanese eel Anguilla japonica revealed by real-time tracking and its utilization for geolocation[END_REF]Béguer-Pon et al. 2015b;[START_REF] Righton | Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea[END_REF]) that require buoyancy control. This higher energetic cost of migration, due to a malfunctioning swimbladder, will affect individuals which may already have reduced lipid storage available, due to the infection (Marohn et al. 2013).

Component 5 -Exploitation of natural resources: an intensive exploitation of eels at all their stages Eels are targeted by recreational and commercial fisheries at all continental life stages (glass eels, yellow eels and silver eels) with a great variety of active and passive gears [START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF]Tesch 2003). Yellow and silver eels have been exploited for a long time as attested by representations of eels in prehistoric pictographs [START_REF] Citerne | Le thème du poisson dans l'art paléolithique des Pyrénées[END_REF][START_REF] Citerne | Présence de l'esturgeon dans le bestiaire figuré paléolithique: conditions et limites de l'analyse morphologique[END_REF]. Eel was an important food resource for Native Americans (MacGregor et al. 2009) and is a traditional food in Japan and East Asia (Tatsukawa 2003). The first official record of European eel fisheries dates back to 1086 [START_REF] Dekker | Climbing back up what slippery slope? Dynamics of the European eel stock and its management in historical perspective[END_REF]. In contrast to the situation for many commercial species, the culture of eel is not a closed system in that it is still dependent on wild caught glass eels. Artificial reproduction and rearing of glass eels has only been achieved for the Japanese eel (Tanaka et al. 2001(Tanaka et al. , 2003;;[START_REF] Kagawa | The first success of glass eel production in the world: basic biology on fish reproduction advances new applied technology in aquaculture[END_REF]) although these operations are still not commercially viable [START_REF] Okamura | Recent advances in artificial production of glass eels for conservation of anguillid eel populations[END_REF]. Artificial reproduction has been achieved in European [START_REF] Palstra | Artificial Maturation and Reproduction of the European Eel[END_REF] and American eels [START_REF] Oliveira | Artificial maturation, fertilization, and early development of the American eel (Anguilla rostrata)[END_REF] but not rearing of glass eels.

The main shift in the traditional artisanal eel fisheries occurred as a result of the demand from ongrowing aquaculture [START_REF] Moriarty | Management of the European Eel[END_REF][START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF]. According to FAO statistics, eel farming is now responsible for 90% of total eel production (versus wild-caught eels) and Japan is thought to consume 70% of total freshwater eel production [START_REF] Shiraishi | Eel market dynamics: an analysis of Anguilla production[END_REF]. While eel aquaculture started in the late XIXth century and early XXth century in eastern Asia, it turned into a stable industry after World War II [START_REF] Ringuet | Eels: their harvest and trade in Europe and Asia[END_REF]. The high value of eel in Eastern Asia food markets led to the development of highly competitive aquaculture farms [START_REF] Langenfelds | Atmospheric methane, carbon dioxide, hydrogen, carbon monoxide, and nitrous oxide from Cape Grim flask air samples analysed by gas chromatography, Baseline 2007-2008[END_REF]Lee et al. 2003). The development of intensive farming explains why despite the decline in the wild population, the production of Anguilla spp. increased nearly 20-fold between 1950and 2007[START_REF] Coutant | Assessing supply chain and market impacts of a CITES listing on Anguilla species[END_REF]. Since these farms depend on wild-caught animals, the demand for glass eel increased considerably and prices climbed to very high levels, completely transforming the industry.

The shortage of Japanese glass eels since the early 1970s lead aquaculture farms to import European and American glass eels [START_REF] Moriarty | Management of the European Eel[END_REF][START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF][START_REF] Ringuet | Eels: their harvest and trade in Europe and Asia[END_REF]Lee et al. 2003), leading to an increase in fishing effort in Europe and a peak in landings in 1976 [START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF] and to the development of a large fishery targeting glass eels from North America [START_REF] Meister | Recent developments in the American eel fisheries of eastern North America[END_REF]. After a period of less favourable market conditions, the prices soared again during the early 1990s [START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF]. A threefold increase in prices of European glass eel was observed between 1993 and 1997 [START_REF] Ringuet | Eels: their harvest and trade in Europe and Asia[END_REF], resulting in a "gold rush" for entry into the North-American fishery [START_REF] Haro | Passage of downstream migrant American eels through an airlift-assisted deep bypass[END_REF]. Because of these incredibly high prices, eel became the most valuable species landed in France in the early 2000s [START_REF] Castelnaud | Localisation de la pêche, effectifs de pêcheurs et production des espèces amphihalines dans les fleuves français[END_REF] and Europe exported half of its production to Asia in the mid-2000s [START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF]. The increase in fishing effort led to very high exploitation rates in certain French and Spanish catchments [START_REF] Prouzet | Historique des captures de civelles, intensité actuelle de leur exploitation, variation de leur capturabilité par la pêche professionnelle maritime et indice de colonisation sur le bassin versant de l'Adour. Rapport final[END_REF][START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade route and future prospect for management[END_REF][START_REF] Aranburu | Glass eel recruitment and exploitation in a South European estuary (Oria Bay of Biscay)[END_REF] since Spain and France recruit the highest proportion of European eel (Dekker 2000a). Similarly, high exploitation rates were observed in catchments on Canadian Atlantic seaboard [START_REF] Jessop | Size, and exploitation rate by dip net fishery, of the run of American eel, Anguilla rostrata (LeSueur), elvers in the East River, Nova Scotia[END_REF]2000) or in Taiwan [START_REF] Tzeng | An estimate of the exploitation rate of Anguilla japonica elvers immigrating into the coastal waters off Shuang-Chi River, Taiwan[END_REF]. In France, about 25% of the arriving glass eels were harvested by commercial fisheries, and this estimate did not include the catch from illegal fisheries (Figure 8 -(Drouineau et al. 2016a)). In Japan, these proportions rose from about 25% in the early 1950s to approximately 40% in the 1980s (Tanaka 2014). The Eel European Regulation has limited the fishing effort and required that 60% of caught eels be dedicated for restocking. Moreover, European eel exports have been restricted after its inclusion on Appendix II of the Convention on Trade of Endangered Species in 2009 and a ban of all imports and exports from and to the European Union implemented in 2010 (Nijman 2010). In Japan, glass eel fisheries are forbidden and a special licence is required to capture seed for aquaculture and research. Specific permission is now required for aquaculture, and restrictions have also been implemented in China and Taiwan. In 2014, China, Japan, the Republic of Korea and Taiwan agreed to restrict "initial input" into farms of glass eel of A. japonica.

The catch of silver eels has decreased throughout the world (Dekker 2003a;Tatsukawa 2003;[START_REF] Tsukamoto | Present status of the Japanese eel: resources and recent research[END_REF][START_REF] Cairns | Recovery potential assessment for the American Eel (Anguilla rostrata) for eastern Canada: life history, distribution, reported landings, status indicators, and demographic parameters[END_REF]) because of a reduction in abundance of the stock and because of a decrease in fishing effort, accelerated by recent management measures. Silver eel fisheries have for example completely disappeared in Taiwan [START_REF] Tzeng | Fisheries, Stocks Decline and Conservation of Anguillid Eel[END_REF] and are restricted in 11 prefectures in Japan [START_REF] Jacoby | Anguilla japonica. In: The IUCN Red List of Threatened Species[END_REF]. In Europe, the decline in the silver eel catches has preceded the decline in recruitment (Dekker 2003a). Silver eel fisheries used to predominate at the northern edge of their distribution area and in the western Mediterranean (Dekker 2003b,c;[START_REF] Amilhat | Silver eel population size and escapement in a Mediterranean lagoon: Bages-Sigean, France[END_REF][START_REF] Aalto | Quantifying 60 years of declining European eel (Anguilla anguilla L., 1758) fishery yields in Mediterranean coastal lagoons[END_REF] and in some catchments exploitation rates can still be high. Regarding the American eel, silver eel landings used to be dominated by catches in the Saint-Lawrence River (Castonguay et al. 1994a), but they have also severely declined and a large-scale licence buyout in Quebec has recently accelerated this trend [START_REF] Cairns | Recovery potential assessment for the American Eel (Anguilla rostrata) for eastern Canada: life history, distribution, reported landings, status indicators, and demographic parameters[END_REF].

When forty years of global change has had a greater impact than the ice ages or continental drift

The genus Anguilla appeared more than 50 million years ago during the Eocene [START_REF] Tsukamoto | Evolution of freshwater eels of the genus Anguilla: a probable scenario[END_REF]. A. japonica is thought to have appeared about 15 million years ago [START_REF] Lin | A Phylogeny of Freshwater Eels Inferred from Mitochondrial Genes[END_REF] and A. rostrata and A. anguilla separated about 3 million years ago during the emergence of the isle of Panama [START_REF] Jacobsen | Speciation and demographic history of Atlantic eels (Anguilla anguilla and A. rostrata) revealed by mitogenome sequencing[END_REF]. Those species have survived enormous changes: a succession of ice ages (the last ice age maximum occurred approximately 22,000 years ago) and continental drift that has progressively increased the distance between the spawning grounds and growth habitats [START_REF] Knights | A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere[END_REF]. This demonstrates their evolutionary robustness [START_REF] Knights | A review of the possible impacts of long-term oceanic and climate changes and fishing mortality on recruitment of anguillid eels of the Northern Hemisphere[END_REF]) and remarkable adaptive capacity (Mateo et al. 2017a) based on adaptive phenotypic plasticity (Daverat et al. 2006;[START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF]genetic polymorphism (Gagnaire et al. 2012;Pujolar et al. 2014;[START_REF] Ulrik | Do North Atlantic eels show parallel patterns of spatially varying selection?[END_REF][START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF]. Despite millions of years of adaptation, these three eel species have undergone a dramatic decline in only a few decades.

Identifying the main drivers of the eel decline is still in debate. The main arguments to support the importance of specific stressors are based on the synchrony between the time of the collapse in eel and the stressor. However, many factors impair our ability to disentangle their respective effects.

First, the simultaneous decline of the three species strongly suggests the influence of large-scale factors and therefore of a possible oceanic influence. However, other stressors display very similar increasing trends at the global scale before the beginning of the decline (Figures 4,6,[START_REF] Lin | A Phylogeny of Freshwater Eels Inferred from Mitochondrial Genes[END_REF]. Moreover, the beginning of the eel decline is very difficult to identify because of the complex life cycles of the species (Figure 2) and their long life expectancy (up to 30 years). It would be interesting to compare with tropical species that also show signs of decline but comparative data with Southern hemisphere tropical species are scarce [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF][START_REF] Jellyman | Management and fisheries of Australasian eels (Anguilla australis, Anguilla dieffenbachii, Anguilla reinhardtii)[END_REF]. Secondly, robust quantitative historical data on eel and the anthropogenic pressures are lacking for this period. Third, where these data do exist they mainly come from specific river catchments and it is not possible to extrapolate these data to the whole distribution area because the anthropogenic pressure do not have the same effect everywhere and eels display a great diversity in life history traits. Each stressor probably played a role in the collapse and the combination of stressors in the second half of the XXth century probably had a cumulative effect that heightened the overall effect of the individual stressors [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF][START_REF] Miller | Did a "perfect storm" of oceanic changes and continental anthropogenic impacts cause northern hemisphere anguillid recruitment reductions?[END_REF]. The decline occurred about 30 years after the Second World War, i.e. approximately 1-3 eel generations. This period corresponds to a period of high economic development "Les Trentes Glorieuses", in which agricultural production process, industrial process and energy consumption quickly increased. This can be seen through the acceleration of many indicators since the 1950s / 1960s listed in the study of [START_REF] Steffen | Global Change and the Earth System[END_REF], for example world population, Gross Domestic Product (increased by a factor of 15 since 1950), world petroleum consumption which has increased by 3.5x since 1960, motor vehicles by a factor of16 since the early 50s and increased water use for human consumption and agriculture (Figure 9). This acceleration of human activity and consumption has been referred to as the "Great Acceleration" [START_REF] Steffen | Global Change and the Earth System[END_REF][START_REF] Steffen | The trajectory of the Anthropocene: the great acceleration[END_REF], and occurred about 20 years before the first signs of the decline in eel populations, i.e. 1-2 eel generations. As mentioned earlier, river, estuaries and ecosystems have suffered intense modifications over this period [START_REF] Elliott | Fishes in Estuaries[END_REF][START_REF] Postel | Rivers for life: managing water for people and nature[END_REF]Basset et al. 2013, Postel and[START_REF] Postel | Rivers for life: managing water for people and nature[END_REF][START_REF] Wolanski | Ecological Economics of Estuaries and Coasts[END_REF][START_REF] Elliott | Challenging paradigms in estuarine ecology and management[END_REF].

Eel populations are likely affected by global change as a whole, rather than by one specific anthropogenic pressure, explaining why Castonguay et al. (1994a) could not identify a primary cause for the decline of the American eel.

The resilience of eels severely impaired by global change

Several factors contribute to the resilience of eel populations. First, the presence of a brackish / marine contingent (which skip the freshwater phase) can buffer the pressures specific to the catadromous contingent such as dams, contamination, fishing or the parasite (ICES 2009).

Additionally, their very large diet spectrum [START_REF] Sinha | On the food of the freshwater eels and their feeding relationship with the salmonids[END_REF]Tesch 2003), their resistance to fluctuations in temperature, salinity, oxygen, food availability and temporary emersion [START_REF] Brusle | The Eel (Anguilla sp) and organic chemical pollutants[END_REF]Tesch 2003) allow them to grow in a very large range of habitats. This plasticity in growth habitat can generate a "storage effect" and a "portfolio effect" that mitigate against environmental variability (ICES 2009). In a complex life cycle, a storage effect corresponds to a situation where a specific stage of long duration and of limited sensitivity to environmental conditions, buffers the effects of environmental conditions on other stages. For eels, the long duration of the continental growth phase and its variability across habitats with generational overlaps allows the species to buffer the faster cyclic variations of oceanic conditions affecting recruitment (even in a single cohort, some individuals are likely to face unfavourable oceanic conditions while others will face more favourable oceanic conditions during their spawning migration, reproduction and larval drift of their offspring) (Secor 2015a). A portfolio effect corresponds to the expression "don't put all your eggs in the same basket". For eels, their large adaptive capacity allows them to settle in a wide range of habitats, smoothing out environmental fluctuations in each habitat: if one habitat is temporarily unsuitable, it is compensated by other habitats that remain suitable (Secor 2015a). More generally, the large diversity of tactics during the continental phase and presumably during the spawning migration may correspond to remarkable bet-hedging well suited to address environmental variability (Daverat et al. 2006;[START_REF] Righton | Empirical observations of the spawning migration of European eels: The long and dangerous road to the Sargasso Sea[END_REF]. The environmental sex determination may also be a compensatory mechanism: the higher production of females in a context of depleted population may mitigate the reduction in eggs production that would resulted from the decline in silver eel abundance (Geffroy and Bardonnet 2015;Mateo et al. 2017b), especially since eels have a high fecundity.

Then, how might have global change led to such a fast collapse despite eel adaptive capacities and those compensatory mechanisms? Eels are panmictic and thus have long been considered genetically homogeneous, however, recently a genetic polymorphism in eel populations was found to be correlated with environmental gradients (Gagnaire et al. 2012;[START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Pujolar | Genome-wide singlegeneration signatures of local selection in the panmictic European eel[END_REF][START_REF] Ulrik | Do North Atlantic eels show parallel patterns of spatially varying selection?[END_REF][START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF]. These correlations are thought to result from spatially variable selection (some individuals are genetically more adapted than others to survive in some habitats) or of genetically based habitat selection (some types of individuals tend to settle preferentially in some habitats to maximise their fitness). The existence of genetically distinct types of individuals which are more or less adapted to the different types of habitats available within their distribution area (northern vs southern habitats, marine vs brackish vs freshwater habitats), i.e. ecotypes [START_REF] Pavey | RAD Sequencing Highlights Polygenic Discrimination of Habitat Ecotypes in the Panmictic American Eel[END_REF], combined with a large phenotypic plasticity are assumed to play the main role in eel adaptive capacity, enabling the species to address the wide environmental heterogeneity at both the distribution and catchment scale [START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF]Mateo et al. 2017a). In such a scheme, individuals are able to grow and survive in a wide range of habitats thanks to phenotypic plasticity but some individuals are more adapted to some habitats than others (ecotypes), and all individuals reproduce together (panmixia) ensuring that ecotypes are reshuffled in each generation. The synergy of phenotypic plasticity and genetic polymorphism could explain how a panmictic population can survive in such a wide and varied distribution area and be the basis for the adaptive capacities of eels.

In this review, we have highlighted that not all pressures affect all habitats and individuals evenly. Indeed, obstacles affect mostly individuals that settle preferentially in upstream habitats and habitat loss mainly affects males located in the south-western part of the range of the European eel.

Anguillicola crassus has a greater impact on individuals that settle in freshwater habitats as opposed to estuarine or marine [START_REF] Kirk | The impact of Anguillicola crassus on European eels[END_REF]. Finally, fisheries are not uniformly distributed, with European silver eel fisheries mainly occurring at the edge of the distribution area, especially the northern edge, though fisheries are also important along the Mediterranean coast (Castonguay et al. 1994a;[START_REF] Moriarty | Management of the European Eel[END_REF]Dekker 2003b,c), and glass eel fisheries in the core (Dekker 2003c). By affecting different habitats, anthropogenic pressures affect life history traits and ecotypes in different ways (Figure 10). Climate change and glass eel fisheries probably affect all ecotypes: climate change affects recruitment success. Glass eel fisheries, though not evenly distributed in the distribution area, generally operate downstream of river catchments and consequently harvest evenly all incoming glass eels. On the other hand, all the other anthropogenic pressures tend to affect ecotypes corresponding to more upstream habitats. As such, these anthropogenic pressures reduce the fitness of those individuals and can become an important selective pressure (Mateo et al. 2017). For example, half the American silver eels migrating down the St. Lawrence River, one of the most productive areas for American eel [START_REF] Casselman | Dynamics of Resources of the American Eel, Anguilla rostrata: Declining Abundance in the 1990s[END_REF], have been killed by hydropower dams and fisheries [START_REF] Verreault | An estimation of American eel escapement from the upper St. Lawrence River and Lake Ontario in 1996 and[END_REF]. Such selection pressure over 30 years or more (one to two eel generations) may have reduced the prevalence of individuals adapted to such types of habitats (northernmost area, longest migration from spawning grounds) in the panmictic eel population and explain why recruitment to the St. Lawrence River has been so much more reduced than elsewhere in their distribution range. Reducing this genetic polymorphism as a result of anthropogenic-induced selection may irrevocably alter the species capacity to adapt and modify its sex ratio.

Additionally, by decreasing the diversity of ecotypes and consequently, decreasing the capacity of eels to live in a wide range of habitats, anthropogenic pressures may have reduced the porfolio and storage effects which, as we said before, are crucial to address environmental variability and to improve resilience. In view of this, diversity is crucial for temperate eels (Secor 2015b) and management should preserve this diversity to ensure population resilience. Moreover, it is crucial to improve our knowledge of the mechanisms involved in eel adaptation and of the effects of anthropogenic pressures on their capacity to adapt to the global change. A recent analysis outlines that even pressures that do not kill any eels can have impacts on eel populations by penalising some ecotypes more than others (Mateo et al. 2017b).

Other implications for eel management and research

The eel decline due to global change has several implications for management. First, global causes means global solutions are warranted. By global solutions, we do not mean that there should be a unique set of management measures across all distribution areas, but rather coordinated international management acting on each source of anthropogenic pressure. This was proposed in the Quebec declaration of concern [START_REF] Dekker | Québec Declaration of Concern: Worldwide decline of eel resources necessitates immediate action[END_REF][START_REF] Dekker | The 2003 Québec Declaration of Concern About Eel Declines-11 Years Later: Are Eels Climbing Back up the Slippery Slope?[END_REF]) that called for immediate action and coordination at all scales. Though some progress has been made since the first declaration, there is clear need to improve management coordination among regional, national and international authorities. Dekker (2016) pointed out the difficulties in the implementation of the Eel Management Plan in Europe. International coordination has not yet started for the American eel (MacGregor et al. 2008(MacGregor et al. , 2009;;Jacoby et al. 2014;[START_REF] Castonguay | Understanding the decline in anguillid eels[END_REF]. The East Asia Eel Resource Consortium does not yet have any official support [START_REF] Jacoby | Anguilla japonica. In: The IUCN Red List of Threatened Species[END_REF], and the first attempt at international coordination took place in 2014 between South Korea, China, Taiwan and Japan with an agreement on the amount of glass eel that can be used for aquaculture.

Second, though it is difficult to disentangle the relative effects of various anthropogenic pressures implicated in the decline, it is important to develop tools and methods to monitor and quantify their effects in the future. Eels grow in very small and almost independent units corresponding to river catchments (Dekker 2000b) with specific anthropogenic pressures, within which eels have different life history traits. Consequently, it is difficult to assess the stock and extrapolate the overall effect of anthropogenic pressure at the population scale, from observations collected at the river catchment scale (Dekker 2000a). However, the improvement in data quality and the recent development of a generic model that can be used at a larger geographic scale is a first step. For example, the GEREM model provides estimates of glass eel recruitment that can be used to assess glass eel fishery exploitation rates (Drouineau et al. 2016a;[START_REF] Bornarel | Modelling the recruitment of European eel (Anguilla anguilla) throughout its European range[END_REF]. The models EDA [START_REF] Briand | Eel density analysis (EDA 2.2) Estimation de l'échappemement en anguilles argentées (Anguilla anguilla) en France -Rapport 2015. Rapport Onema-Irstea-EPTB-Vilaine[END_REF] or SMEP [START_REF] Aprahamian | On the application of models of European eel (Anguilla anguilla) production and escapement to the development of Eel Management Plans: The River Severn[END_REF]) can be used to assess the abundance of yellow eels in river catchments. These can then be coupled with other models to assess spawner escapement and the effect of different anthropogenic pressures such as hydropower production or fisheries [START_REF] Jouanin | Evaluation des mortalités d'anguilles induites par les ouvrages hydroélectriques en France-Projet SEA HOPE Silver Eels escApement from HydrOPowEr[END_REF]. Stock assessment models have also been proposed to support management (Dekker 2000a;[START_REF] Bevacqua | A length and age structured demographic model for European eel populations (DemCam)[END_REF][START_REF] Oeberst | German Eel Model (GEM II) for describing eel, Anguilla anguilla (L.), stock dynamics in the river Elbe system[END_REF][START_REF] Bevacqua | A global viability assessment of the European eel[END_REF]. However, few tools are currently available to assess the impact of contaminants on eel populations. Similarly, there is a lack of tools to quantify the effect of lost habitats on population dynamics, though some methodologies are available which can quantify the amount of habitat lost due to fragmentation.

Though it is not possible to quantify the historical effects of anthropogenic pressures, quantifying and predicting pressures in the future would provide valuable information to prioritise management actions. Quantification is even more important since (1) it is not possible to mitigate some of the pressures affecting eels (parasitism, climate change) so it is necessary to compensate their effects with mitigation measures on the other pressures (fishery, fragmentation, contamination), (2) management practices cannot mitigate anthropogenic pressures at similar temporal scales: reduction in fishing efforts are recent but are thought to operate quickly, whereas effort to mitigate contamination or fragmentation are older but are much more complex and longer to implement.

Of course, temperate eels are not the only species endangered by global change and most diadromous fishes have undergone severe declines [START_REF] Mcdowall | Different kinds of diadromy: Different kinds of conservation problems[END_REF][START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF][START_REF] Mota | New insights into biology and ecology of the Minho River Allis shad (Alosa alosa L.): contribution to the conservation of one of the last European shad population[END_REF]. The effects of fragmentation (Larinier 2001;[START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF][START_REF] Haxton | A global perspective of fragmentation on a declining taxon the sturgeon (Acipenseriformes)[END_REF]Cano 2016), global warming (Friedland 1998;Friedland et al. 2000;Lassalle et al. 2008Lassalle et al. , 2009;;[START_REF] Jonsson | A review of the likely effects of climate change on anadromous Atlantic salmon Salmo salar and brown trout Salmo trutta, with particular reference to water temperature and flow[END_REF][START_REF] Elliott | Temperature requirements of Atlantic salmon Salmo salar, brown trout Salmo trutta and Arctic charr Salvelinus alpinus: predicting the effects of climate change[END_REF][START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF], fisheries and pollution [START_REF] Mcdowall | Different kinds of diadromy: Different kinds of conservation problems[END_REF][START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF] have been documented for most of these species. More generally, most migratory animals regardless of taxa have undergone similar declines [START_REF] Sanderson | Long-term population declines in Afro-Palearctic migrant birds[END_REF][START_REF] Berger | Protecting Migration Corridors: Challenges and Optimism for Mongolian Saiga[END_REF][START_REF] Wilcove | Going, Going, Gone: Is Animal Migration Disappearing[END_REF] [START_REF] Tsukamoto | Oceanic spawning ecology of freshwater eels in the western North Pacific[END_REF][START_REF] Miller | A century of research on the larval distributions of the Atlantic eels: a re-examination of the data[END_REF] (open circles) and continental distribution of yellow eels (filled shapes) [START_REF] Jacoby | Synergistic patterns of threat and the challenges facing global anguillid eel conservation[END_REF] for A. rostrata (red), A. anguilla (blue) and A. japonica (green). Figure 5 Ocean temperature anomalies (left panel). Source [START_REF] Morice | Quantifying uncertainties in global and regional temperature change using an ensemble of observational estimates: The HadCRUT4 data set[END_REF][START_REF] Steffen | The trajectory of the Anthropocene: the great acceleration[END_REF]

  Global change: five components that threaten biodiversity Temperate eels: endangered species impacted by diverse anthropogenic pressures Component 1 -Global warming and ocean modification: impacts on eel migrations Effect on leptocephalus drift and survival Effect on silver eel spawning migration Component 2 -Increased contamination load: contamination of eels and consequences on its physiology Component 3 -Fragmentation and habitat loss: fragmentation by weirs and dams and consequences on upstream and downstream migration Movements, habitats and fragmentation Blockage during upstream migrations Impaired downstream migrations Component 4 -Alien species: effects of alien parasitoid Anguillicola crassus Component 5 -Exploitation of natural resources: an intensive exploitation of eels at all their stages When forty years of global change has had a greater impact than the ice ages or continental drift The resilience of eels severely impaired by global change Other implications for eel management and research

  . As a result, IUCN classified A. anguilla as critically endangered in 2008 (confirmed in 2010 and 2014) (Jacoby and Gollock 2014a) while A. rostrata and A. japonica have been classified as endangered since 2014 (Jacoby and Gollock 2014b; Jacoby et al. 2014). Meanwhile, conservation regulations have flourished for the three species (Figure 4). The European Commission implemented a regulation in 2007 (Council Regulation (EC) No 1100/2007)

  raising the question of sustainability of migratory tactics in the face of global change. In this context, why should eels be considered as a symbol of the effect of global change? Because the original life cycle of eels make them vulnerable to all five components of global change, and the cumulated impacts of those five components has outpaced the adaptive capacities of these species acquired through million years of evolution. The rate of change during the Great Acceleration in the second half of the XXth century, was too fast for the adaptive capacity of the eel, especially since the five components of global change acted simultaneously. It explains how a species that was considered a vermin species in French salmonid rivers until the 1980s has become critically endangered in only 25 years, after millions of years of existence.

Figure 2

 2 Figure 2Spawning grounds[START_REF] Tsukamoto | Oceanic spawning ecology of freshwater eels in the western North Pacific[END_REF][START_REF] Miller | A century of research on the larval distributions of the Atlantic eels: a re-examination of the data[END_REF]) (open circles) and

Author produced version of the article published inFish and Fisheries, 2018, 19 (5), 903-930 The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

l a r v a l d r i f t

Acknowledgements

Acknowledgements

The total number of eels passing Moses-Saunders Hydroelectric Dam are monitored and reported by Ontario Power Generation and the New York Power Authority since 2006 and were provided by Alastair Mathers. Hilaire Drouineau and Kazuki Yokouchi were partially supported by the fund for international exchange and collaboration of the Japan Fisheries Research and Education Agency.

We thank Miran Aprahamian for his help and the improvements he made to the manuscript. We also would like to thank the editor and two anonymous referees for their suggestions and comments.

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Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

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Author produced version of the article published in Fish and Fisheries, 2018, 19 (5), 903-930 The original publication is available at https://onlinelibrary.wiley.com/doi/full/10.1111/faf.12300 Doi : 10.1111/faf.12300

Figure 10

Adaptation mechanisms to environmental heterogeneity as proposed in (Mateo et al. 2017a;b ;Gagnaire et al. 2012;[START_REF] Côté | In absence of local adaptation, plasticity and spatially varying selection rule: a view from genomic reaction norms in a panmictic species (Anguilla rostrata)[END_REF][START_REF] Rougier | The GR3D model, a tool to explore the Global Repositioning Dynamics of Diadromous fish Distribution[END_REF][START_REF] Boivin | How does salinity influence habitat selection and growth in juvenile American eels Anguilla rostrata?[END_REF]. A red arrow stands for "unfavourable", a green arrow stands for "favourable". A blue arrow stands for a relationship which is either favourable" or "unfavourable" depending on situations. There is a double arrow between genotypes and "settlement in upstream habitats" because it represents "spatially varying selection" and "genetic-dependent habitat selection". Regarding phenotypes, female is considered as opposite to male and "settlement in upstream habitats" as opposite to "settlement in downstream habitats".