H Drouineau
email: hilaire.drouineau@irstea.fr

C Carter

M Rambonilaza

G Beaufaron

G Bouleau

A Gassiat

P Lambert

S Le Floch

S Tétard

E De Oliveira

River Continuity Restoration and diadromous fishes: much more than an ecological

Ecosystem fragmentation is a serious threat to biodiversity and one of the main challenges in ecosystem restoration. River Continuity Restoration (RCR) has often targeted diadromous fishes, a group of species supporting strong cultural and economic values and especially sensitive to river fragmentation. Yet it has frequently produced mixed results and diadromous fishes remain at very low levels of abundance. Against this background, this paper presents the main challenges for defining, evaluating and achieving effective RCR. We first identify challenges specific to disciplines. In ecology, there is a need to develop quantitative and mechanistic models to support decision making, accounting for both direct and indirect impacts of river obstacles and working at the river catchment scale. In a context of dwindling abundances and reduced market value, cultural services provided by diadromous fishes are becoming increasingly prominent. Methods for carrying out economic quantification of non-market values of diadromous fishes become ever more urgent.

Given current challenges for rivers to meet all needs sustainably, conflicts arise over the legitimate

use of water resources for human purposes. Concepts and methods from political science and geography are needed to develop understandings on how the political work of public authorities and stakeholders can influence the legitimacy of restoration projects. Finally, the most exciting challenge is to combine disciplinary outcomes to achieve a multidisciplinary approach to RCR. Accordingly, the co-construction of intermediary objects and diagrams of flows of knowledge among disciplines can be first steps towards new frameworks supporting restoration design and planning.

Keywords: diadromous fishes; river fragmentation; ecosystem goods and services; territory; policy; multidisciplinary approach 1 Context: an intensification of efforts towards RCR and diadromous fishes' At the beginning of the 21st century, the Millennium Assessment pointed out that despite a global increase in human welfare, ecosystem modifications due to anthropogenic activities were threatening ecosystems' ability to sustainably provide important goods and services, especially for future generations (Millennium Ecosystem Assessment 2005). This alarming situation applied to both terrestrial and aquatic ecosystems. For example, rivers provide a multitude of 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 are potentially threatened due to many factors including: dam and dyke construction; pollution; water extraction for human consumption or irrigation; drying out of lateral wetlands; hydromorphological modifications; fishing; and land use in the floodplain [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]. In this article, using diadromous fishes and River Continuity Restoration (RCR, i.e. restoring free movement of fishes and/or physical and ecological components continuum) in France as an example, we will illustrate the need for an interdisciplinary approach to conceptualize, achieve and evaluate effective ecosystem restoration for diadromous fishes and identify key challenges raised by such an approach. Indeed, ecological fragmentation disrupting ecological connectivity has been defined as one of the major challenges in ecosystem restoration and conservation (Tischendorf and Fahrig 2000a, b;[START_REF] Kondolf | Process-based ecological river restoration: Visualizing three-dimensional connectivity and dynamic vectors to recover lost linkages[END_REF][START_REF] Crook | A method to assess longitudinal riverine connectivity in tropical streams dominated by migratory biota[END_REF][START_REF] Humphries | Historical Impacts on River Fauna, Shifting Baselines, and Challenges for Restoration[END_REF]Sutherland et al. 2013).

The notion of "ecological connectivity/continuity" or more generally "landscape connectivity" comes from landscape ecology [START_REF] Forman | Landscape ecology[END_REF]. Ecological connectivity refers to the possibility of genes, individuals, energy or matter to move from an ecological patch (defined as a homogeneous unit of environment) to another because of the landscape structure [START_REF] Ward | Ecological connectivity in alluvial river ecosystems and its disruption by Author-produced version of the article[END_REF][START_REF] Ward | Biodiversity of floodplain river ecosystems: Ecotones and connectivity[END_REF]. Ecosystem fragmentation can arise from natural or anthropogenic obstacles that impair animal movements. Dams or mill-weirs are among the most typical examples of obstacles in the case of river connectivity. Such obstacles can either be transversal (weirs, dams…), i.e. affecting movements upstream / downstream, or lateral (dykes), i.e. impacting movements between a river and its floodplain.

In aquatic ecosystems, river fragmentation has significant impacts on fish assemblages' composition, abundance and spatial distribution [START_REF] Matthews | Influence of drainage connectivity, drainage area and regional species richness on fishes of the interior highlands in Arkansas[END_REF][START_REF] Poulet | Impact of weirs on fish communities in a Piedmont stream[END_REF][START_REF] Araújo | Longitudinal patterns of fish assemblages in a large tropical river in southeastern Brazil: Evaluating environmental influences and some concepts in river ecology[END_REF][START_REF] Nislow | Variation in local abundance and species richness of stream fishes in relation to dispersal barriers: implications for management and conservation[END_REF][START_REF] Perkin | Fragmentation alters stream fish community structure in dendritic ecological networks[END_REF][START_REF] Gardner | Distribution and abundance of stream fishes in relation to barriers: Implications for monitoring stream recovery after barrier removal[END_REF], and on the resilience of populations because of genetic isolation [START_REF] Jager | Would hydrologic climate changes in Sierra Nevada streams influence trout persistence?[END_REF][START_REF] Jager | A theoretical study of river fragmentation by dams and its effects on white sturgeon populations[END_REF][START_REF] Fagan | Connectivity, fragmentation, and extinction risk in dendritic metapopulations[END_REF][START_REF] Horreo | Impact of habitat fragmentation on the genetics of populations in dendritic landscapes[END_REF][START_REF] Webb | How does network structure and complexity in river systems affect population abundance and persistence? Limnologica[END_REF]. However, river fragmentation has other kinds of impacts on ecosystem functioning such as habitat alteration or modification, variation of water table recharge, reduced denitrification, modification of flood regimes or changes in the interdependency between the river and its floodplain [START_REF] Nilsson | Alterations of Riparian Ecosystems Caused by River Regulation Author-produced version of the article[END_REF][START_REF] Gergel | Do dams and levees impact nitrogen cycling? Simulating the effects of flood alterations on floodplain denitrification[END_REF]. RCR aims at enhancing river ecological functioning by favouring a longitudinal biotic and physical continuum [START_REF] Vannote | The river continuum concept[END_REF], as well as a lateral continuity between rivers and their floodplains [START_REF] Amoros | Interaction between water bodies within the floodplains of large rivers: function and development of connectivity[END_REF] and also a vertical continuity between the water column and river sediment.

In view of this, initiatives to restore river continuity have flourished at international, national and regional scales. In Europe for example, the Water Framework Directive (WFD -2000/60/CE) established new requirements in order to achieve good ecological status of water bodies in each European Union (EU) Member State and specifically mentioned river continuity as a key component of good ecological status. In France, the WFD resulted, inter alia, in the implementation of a water and aquatic environments' law (Loi n° 2006-177230 December, 2006 sur l'eau et les milieux aquatiques: LEMA) which implements a new river classification system to prioritize rivers in which continuity must be restored or protected. Recently, the "Blue and Green Infrastructure" policies implemented in light of both the French environmental programme 2008 (Grenelle de l'Environnement), and the EU's "Green Infrastructure Strategy" (COM(2013) 249 final), detail specific projects on terrestrial and aquatic continuity preservation and restoration, based on a network of classified patches of biodiversity interconnected along ecological corridors [START_REF] Forman | Landscape ecology[END_REF]).

Diadromous fishes: high value species impacted by river fragmentation

Diadromous fishes refer to approximatively 250 species that share their life cycles between freshwater and marine habitats [START_REF] Myers | Usage of Anadromous, Catadromous and Allied Terms for Migratory Fishes[END_REF][START_REF] Mcdowall | The evolution of diadromy in fishes (revisited) and its place in phylogenetic analysis[END_REF]. Many of them are iconic food and sport fishes, and have been intensively studied [START_REF] Bloom | The evolutionary origins of diadromy inferred from a timecalibrated phylogeny for Clupeiformes (herring and allies)[END_REF]. As an illustration of the importance of diadromous fishes, in France their landings accounted for 75% of total income of the commercial inland fishery at the end of the 20 th century [START_REF] Champion | Etude socio-économique des pêches professionnelles continentales[END_REF][START_REF] Boisneau | Inland commercial fisheries management in France[END_REF]. Historically, Acipenser sturio fishing and caviar production used to be an important industry at the beginning of the 20 th century [START_REF] Castelnaud | Sturgeon Fishing, Landings, and Caviar Production During the Twentieth Century in the Garonne Basin and the Coastal Sea[END_REF]. Later, European eel (Anguilla anguilla) was the most important species landed in value in the Bay of Biscay [START_REF] Castelnaud | Localisation de la pêche, effectifs de pêcheurs et production des espèces amphihalines dans les fleuves français[END_REF] in the early 2000s, especially because of the high market value of glass eels [START_REF] Ringuet | Eels: their harvest and trade in Europe and Asia[END_REF][START_REF] Briand | An appraisal of historical glass eel fisheries and markets: landings, trade routes and future prospect for management[END_REF]). Other species have a more limited value when compared to marine species, however they may be of regional importance. Allis shad (Alosa alosa) and to a minor extent twait shads (Alosa fallax) are harvested in continental waters throughout their range though their market value has decreased because of abundance decline [START_REF] Bagliniere | The allis shad Alosa alosa: biology, ecology, range, and status of populations[END_REF].

Lampreys (Petromyzon marinus and Lampetra fluviatilis) have a long history of commercial fishing because of their gastronomic delicacy [START_REF] Kelly | A Review of the Ecology and Distribution of Three Lamprey Species, Lampetra fluviatilis (L.), Lampetra planeri (Bloch) and Petromyzon marinus (L.): A Context for Conservation and Biodiversity Considerations in Ireland[END_REF]. Atlantic salmon (Salmo salar) is targeted by a high profile recreational fishery, as all over its distribution area [START_REF] Verspoor | The atlantic salmon: Genetics, conservation and management[END_REF]).

This question of RCR is crucial for diadromous fish restoration. Most freshwater fishes carry out seasonal migrations [START_REF] Brönmark | There and back again: migration in freshwater fishes[END_REF], however, diadromous fishes must carry out migrations between fresh and marine waters to complete their life cycle [START_REF] Myers | Usage of Anadromous, Catadromous and Allied Terms for Migratory Fishes[END_REF][START_REF] Mcdowall | Diadromy in Fishes: Migrations Between Freshwater and Marine Environments[END_REF]. There are three types of diadromous species [START_REF] Mcdowall | Diadromy in Fishes: Migrations Between Freshwater and Marine Environments[END_REF]. Anadromous species, such as many salmon, spend most of their growth phase at sea and reproduce in freshwater. On the other hand, catadromous species, such as the European eel, reproduce in marine waters and spend most of their growth phase in continental waters. Finally, amphidromous fishes reproduce in freshwater and share their growth phase between freshwater and marine waters [START_REF] Mcdowall | Diadromy in Fishes: Migrations Between Freshwater and Marine Environments[END_REF]. This remarkable lifehistory behaviour has evolved in many fish groups [START_REF] Feutry | Evolution of Diadromy in Fish: Insights from a Tropical Genus ( Kuhlia Species)[END_REF]) but many of these species, as more generally most migratory species from the animal kingdom [START_REF] Sanderson | Long-term population declines in Afro-Palearctic Author-produced version of the article[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], are now in decline all over the world [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]. Many factors explain this decline, however river fragmentation due to dams, weirs, flood gates or other physical obstacles has been considered to be one of the main causes both of their decline [START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF]. By impairing free-movements from reproduction to growth habitats, obstacles can interrupt fishes life-cycle and have led to their extinction (Salmo salar in the Rhine River, in the Seine River or in the Garonne River for example), or indeed of their confinement in restricted areas of rivers basins (Salmo salar in Loire River, Alosa alosa in the Rhône River and the Garonne River) [START_REF] Porcher | Les dispositifs de franchissement : bases biologiques, limites et rappels réglementaires[END_REF][START_REF] Kondolf | Process-based ecological river restoration: Visualizing three-dimensional connectivity and dynamic vectors to recover lost linkages[END_REF][START_REF] Coutant | Fish behavior in relation to passage through hydropower turbines: A review[END_REF][START_REF] Larinier | Environmental issues, dams and fish migrations. In: Dams, fish and fisheries: Author-produced version of the article[END_REF][START_REF] Fukushima | Modelling the effects of dams on freshwater fish distributions in Hokkaido, Japan[END_REF][START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF][START_REF] Lawrence | Influence of dams on population persistence in Atlantic salmon (Salmo salar)[END_REF]. Because of their high economic and cultural values and other goods and services they support [START_REF] Dams | Poissons et contours de type pisciforme dans l'art pariétal paléolithique[END_REF][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][START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF], RCR has often focused on diadromous fishes. For example, most WFD fish indicators include a metric based on diadromous species (Breine et al. 2007;[START_REF] Coates | Efficacy of a multi-metric fish index as an analysis tool for the transitional fish component of the Water Framework Directive[END_REF][START_REF] Pont | EFI+ Project. Improvement and spatial extension of the European Fish Index Deliverable 4.1: Report on the modelling of reference conditions and on the sensitivity of candidate metrics to anthropogenic pressures. Deliverable 4.2: Report on the final development and validation of the new European Fish Index and method, including[END_REF][START_REF] Delpech | Development of a fish-based index to assess the ecological quality of transitional waters: The case of French estuaries[END_REF]Scholle and Schuchardt 2012) aiming at assessing river continuity, one of the component of ecological quality defined in this directive. Similarly, in France, river classification and the French Blue Infrastructure were mostly based on species of interest including many diadromous fishes.

Moreover, many types of fishways, the most common mitigation measure to river fragmentation, have been specifically designed for diadromous species, especially salmonids [START_REF] Larinier | Environmental issues, dams and fish migrations. In: Dams, fish and fisheries: Author-produced version of the article[END_REF][START_REF] Noonan | A quantitative assessment of fish passage efficiency[END_REF]. The predominance of diadromous fishes in RCR suggest that some diadromous fishes are good candidates to be considered as "cultural keystone species" as defined by [START_REF] Garibaldi | Cultural keystone species: implications for ecological conservation and restoration[END_REF], i.e. "culturally salient species that shape in a major way the cultural identity of a people". However, RCR for diadromous fishes implies modifications of barriers, or even obstacles removals, while many obstacles also carry strong cultural values and provide many services [START_REF] Wiley Sons | You kill the dam, you are killing a part of me": Dam removal and the environmental politics of river restoration[END_REF]. The potential reshaping of ecosystems goods and services provided by diadromous fishes, from market to non-market values, because of their decline and the emerging conflicts with ecosystem goods and services provided by obstacles, are central questions for the future of RCR for diadromous fishes.

RCR for diadromous fishes: inconsistent results and upcoming challenges 2.1 Restoration of aquatic ecosystems and RCR for diadromous fishes: inconsistent results

Though aquatic ecosystem restoration has a long history in Europe and all over the world [START_REF] Palmer | Ecological restoration of streams and rivers: shifting strategies and shifting goals[END_REF][START_REF] Morandi | How is success or failure in river restoration projects evaluated? Feedback from French restoration projects[END_REF], its impacts are often considered mixed [START_REF] Palmer | River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice?[END_REF]Suding 2011;[START_REF] Jähnig | River restoration success: a question of perception[END_REF][START_REF] Palmer | Ecological restoration of streams and rivers: shifting strategies and shifting goals[END_REF]). Several reasons have been proposed to explain these disappointing results. The first reason is the complexity of aquatic ecosystems functioning based on biological, physical and chemical processes and anthropogenic pressures that work at different temporal and spatial scales. This complexity impairs our ability to understand ecosystem functioning and challenges our ability to develop appropriate tools to predict and validate the effects of restoration actions [START_REF] Arthington | Preserving the biodiversity and ecological services of rivers: new challenges and research opportunities[END_REF][START_REF] Olden | Are large-scale flow experiments informing the science and management of freshwater ecosystems?[END_REF][START_REF] Lamouroux | The ecological restoration of large rivers needs science-based, predictive tools meeting public expectations: an overview of the Rhône project[END_REF].

Additionally, it makes it difficult to implement restoration actions at appropriate temporal and spatial scales [START_REF] Palmer | River restoration, habitat heterogeneity and biodiversity: a failure of theory or practice?[END_REF][START_REF] Hermoso | Systematic planning for river rehabilitation: integrating multiple ecological and economic objectives in complex decisions[END_REF][START_REF] Perring | Advances in restoration ecology: rising to the challenges of the coming decades[END_REF]. Over and above this complexity, the mixed results of aquatic ecosystem restoration may also be the result of a poor linkage between science and management. This results in gaps in scientific knowledge that would be crucial for managers, in scientific results that are of poor practical use for managers, and conversely, in the under-use of scientific results by managers [START_REF] Cabin | Science-Driven Restoration: A Square Grid on a Round Earth?[END_REF][START_REF] Palmer | Reforming Watershed Restoration: Science in Need of Application and Author-produced version of the article[END_REF]Suding 2011).

Another barrier to effective restoration is the lack of consideration of the societal context in which restoration programs take place [START_REF] Hermoso | Systematic planning for river rehabilitation: integrating multiple ecological and economic objectives in complex decisions[END_REF][START_REF] Wortley | Evaluating ecological restoration success: a review of the literature[END_REF]. In a review, [START_REF] Wortley | Evaluating ecological restoration success: a review of the literature[END_REF] highlighted that socio-economic dimensions had been considered in only a few restoration programs even though [START_REF] Pahl-Wostl | Environmental flows and water governance: managing sustainable water uses[END_REF] had underlined that the main barriers to effective restoration often arise from the socio-economic context because of diverging and fluctuating objectives among stakeholders [START_REF] Barthélémy | La restauration écologique du fleuve Rhône sous le double regard du sociologue et de l'écologue[END_REF][START_REF] Jørgensen | Damned if you do, dammed if you don't: debates on dam removal in the Swedish media[END_REF][START_REF] Perring | Advances in restoration ecology: rising to the challenges of the coming decades[END_REF] and unshared spatial and temporal scales at which they consider restoration [START_REF] Hermoso | Systematic planning for river rehabilitation: integrating multiple ecological and economic objectives in complex decisions[END_REF].

Despite long-terms efforts to restore diadromous fishes (the first laws were adopted in the 1700s for salmon: [START_REF] Brown | Fish and hydropower on the U.S. Atlantic coast: failed fisheries policies from half-way technologies[END_REF])), these restoration programs have also had mixed success [START_REF] Lichatowich | Salmon Without Rivers: A History Of The Pacific Salmon Crisis[END_REF][START_REF] Lichatowich | Failures to incorporate science into fishery management and recovery programs: lessons from the Columbia River[END_REF]. A famous example is the failure of the recovery program of Pacific salmon in the Columbia river, which has been called the world's largest attempt at ecosystem restoration, but resulted in a failure [START_REF] Lichatowich | Failures to incorporate science into fishery management and recovery programs: lessons from the Columbia River[END_REF].

Regulation on fishing activities, construction of fishways and restocking from hatcheries are among the main measures implemented to conserve and restore diadromous fishes. Regarding more specifically RCR for diadromous fishes, the construction of fishways to mitigate the impact of obstacles to migration is the most common mitigation measure. But, fishways can be considered as half-way measures [START_REF] Brown | Fish and hydropower on the U.S. Atlantic coast: failed fisheries policies from half-way technologies[END_REF], i.e. measures that do not prevent or solve the problem but only mitigate the symptoms, and often have limited efficiency [START_REF] Noonan | A quantitative assessment of fish passage efficiency[END_REF]. Obstacles removal appears to be much more ecologically efficient [START_REF] De Leaniz | Weir removal in salmonid streams: Implications, challenges and practicalities[END_REF][START_REF] Hitt | Dam Removal Increases American Eel Abundance in Distant Headwater Streams[END_REF] and is more and more perceived as a critical tool in river restoration in general and migratory fishes in particular [START_REF] Doyle | Toward Policies and Decision-Making for Dam Removal[END_REF][START_REF] Magilligan | The Social, Historical, and Institutional Contingencies of Dam Removal[END_REF]). However, it raises many more socioeconomic questions than half-way measures [START_REF] Jørgensen | Damned if you do, dammed if you don't: debates on dam removal in the Swedish media[END_REF][START_REF] Magilligan | The Social, Historical, and Institutional Contingencies of Dam Removal[END_REF] because of the potential loss of recreational benefits or cultural, aesthetic and historical values provided by the obstacle (e.g. historical heritage of mills, artificial reservoirs creates by the dams used for fishing, sailing or canoeing).

In light of this, the next section will address the ecological challenges raised by the RCR for diadromous fishes. The following sections will then focus on why RCR for diadromous fishes should not just be treated as an ecological issue and why, on the contrary, we need integrated multidimensional approaches [START_REF] Barthélémy | La restauration écologique du fleuve Rhône sous le double regard du sociologue et de l'écologue[END_REF] to achieve legitimate and comprehensive RCR.

Three upcoming ecological challenges: a need for tools to support decision making at the appropriate scale

Scientific tools to support decision making are crucial for managers [START_REF] Palmer | Reforming Watershed Restoration: Science in Need of Application and Author-produced version of the article[END_REF]Suding 2011).

Consequently, there is a need to develop tools to predict the ecological outcomes of management options. For RCR, this raises three different challenges: (i) the need to work at an appropriate spatial scale, (ii) the need for comprehensive quantification of the impact of obstacles on diadromous fishes, (iii) the need to take these quantifications into account in predictive models.

2.2.1

First challenge: shifting from 'site'-based approaches to 'population scale'-based approaches

The question of scale is a key one in ecology [START_REF] Levin | The Problem of Pattern and Scale in Ecology: The Robert H. MacArthur Award Lecture[END_REF][START_REF] Chave | The problem of pattern and scale in ecology: what have we learned in 20 years?[END_REF]) and a key issue for our

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686

The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3 complete understanding of diadromous fish ecology and conservation [START_REF] Mcdowall | Diadromy, history and ecology: a question of scale[END_REF]. The impact of river fragmentation is generally studied at three biological scales: individual scale, the population/meta-population scale and the ichthyofaunistic assemblage scale. Though the catchment scale has sometimes been explored for anadromous species [START_REF] Buchanan | A migratory life-cycle release-recapture model for Salmonid PIT-tag investigations[END_REF][START_REF] Crane | Setting the river free": The removal of the Edwards dam and the restoration of the Kennebec River[END_REF]; Susquehanna River Anadromous Fish Restoration Cooperative 2010), surprisingly, the impact of obstacles on diadromous fishes has generally been assessed either at the individual or at the population scale but focusing on a specific life stage (upstream or downstream migration) and at the obstacle or river section spatial scale. Since population dynamics processes operate at these scales, working at smaller scale can impair the ability to assess the effect of obstacles on diadromous population dynamics and on their viability. This is even more crucial as the dendritic nature of rivers implies that obstacles and restoration actions in specific sites of a catchment interact in complex ways [START_REF] Fagan | Connectivity, fragmentation, and extinction risk in dendritic metapopulations[END_REF][START_REF] Labonne | Linking dendritic network structures to population demogenetics: The downside of connectivity[END_REF][START_REF] Kemp | Procedures for evaluating and prioritising the removal of fish passage barriers: A synthesis[END_REF], and consequently, the effect of several RCR actions is not necessarily equal to the sum of individual restoration effects [START_REF] Kuby | A multiobjective optimization model for dam removal: An example trading off salmon passage with hydropower and water storage in the Willamette basin[END_REF][START_REF] O'hanley | Optimizing the removal of small fish passage barriers[END_REF][START_REF] Palmer | Hydroecology and river restoration: Ripe for research and synthesis[END_REF]. In the light of these kinds of findings, there is clearly a need to upscale RCR for diadromous fishes from a local approach to a catchment scale thereby bringing this form of restoration in line with calls to upscale aquatic ecosystem restoration more generally [START_REF] Friberg | Effective restoration of aquatic ecosystems: scaling the barriers[END_REF]).

Second challenge: considering indirect impacts of obstacles on diadromous fishes

There is an abundant literature about the impact of obstacles on diadromous fishes. Two types of impacts have mainly been studied: direct mortality due to downstream passage through hydropower facilities, either at the obstacle scale [START_REF] Travade | Passage of Fish Downstream and through Turbines and Hydroelectric Plants: EDF Experience[END_REF][START_REF] Čada | Efforts to reduce mortality to hydroelectric turbine-passed fish: Locating and quantifying damaging shear stresses[END_REF][START_REF] Dedual | Survival of juvenile rainbow trout passing through a francis turbine[END_REF]Svendsen et al. 2011) or at the river section scale (Blackwell et al. 1998a;[START_REF] Mccleave | Simulation of the impact of dams and fishing weirs on reproductive potential of silver-phase American eels in the Kennebec River basin[END_REF][START_REF] Buchanan | A migratory life-cycle release-recapture model for Salmonid PIT-tag investigations[END_REF][START_REF] Welch | Survival of migrating salmon smolts in large rivers with and without dams[END_REF][START_REF] Rechisky | Experimental measurement of hydrosystem-induced delayed mortality in juvenile Snake River spring Chinook salmon (Oncorhynchus tshawytscha) using a large-scale acoustic array[END_REF][START_REF] Holbrook | Survival of Migrating Atlantic Salmon Smolts through the Penobscot River, Maine: a Prerestoration Assessment[END_REF][START_REF] Pedersen | Loss of European silver eel passing a hydropower station[END_REF],

and fish blockage at upstream migration that impairs or delays the completion of the migration.

However, beyond turbine mortality or blockage at upstream migration, obstacles can have other more indirect impacts such as over-predation, [START_REF] Agostinho | Fish ladders: safe fish passage or hotspot for predation?[END_REF][START_REF] Drouineau | Assessing the efficiency of an elver ladder using a multi-state mark-recapture model[END_REF], overfishing [START_REF] Briand | Estuarine and fluvial recruitment of the European glass eel, Anguilla anguilla, in an exploited Atlantic estuary[END_REF][START_REF] De Leaniz | Weir removal in salmonid streams: Implications, challenges and practicalities[END_REF], stress, diseases, energetic costs [START_REF] Budy | Evidence linking delayed mortality of Snake River salmon to their earlier hydrosystem experience[END_REF] or selective pressure [START_REF] Caudill | Slow dam passage in adult Columbia River salmonids associated with unsuccessful migration: Delayed negative effects of passage obstacles or condition-dependent mortality?[END_REF][START_REF] Podgorniak | Differences in brain gene transcription profiles advocate for an important role of cognitive function in upstream migration and water obstacles crossing in European eel[END_REF].

These indirect impacts can be significant. For example, energetic costs can impair reproduction success for eels, salmon or shads, which stop feeding during reproduction migration [START_REF] Bracken | A fish-based assessment tool for the ecological quality of the brackish Schelde estuary in Flanders (Belgium)[END_REF][START_REF] Quignard | Alosa alosa. In: The Freshwater Fishes of Europe[END_REF][START_REF] Kadri | What Controls the Onset of Anorexia in Maturing Adult Female Atlantic Salmon?[END_REF][START_REF] Bruijs | Silver Eel Migration and Behaviour[END_REF].

Recently, [START_REF] Mateo | Impacts that cause the highest direct mortality of individuals do not necessarily have the greatest influence on temperate eel escapement[END_REF] showed that obstacles to eel upstream migration can indirectly severely impact spawning stock biomass even without direct mortality. Regarding eel downstream migration, [START_REF] Drouineau | Silver eel downstream migration in fragmented rivers: use of a Bayesian model to track movements triggering and duration[END_REF] pointed out that indirect impacts are potentially as important as direct impacts and, that they too, should be taken into account in the future when assessing the impact of obstacles. Restricting the impacts of mortality to direct mortality can lead to an inadequate quantification of those impacts and therefore to an inappropriate prioritization of management actions.

2.2.3

Third challenge: a need for tools to support decision-making

We have seen that RCR for diadromous fishes should be considered at the river basin scale and that indirect impacts of obstacles should not be neglected. At such a large spatial scale, managers need tools to support and prioritize decisions [START_REF] Doyle | Toward Policies and Decision-Making for Dam Removal[END_REF][START_REF] Kemp | Procedures for evaluating and prioritising the removal of fish passage barriers: A synthesis[END_REF]. [START_REF] Kemp | Procedures for evaluating and prioritising the removal of fish passage barriers: A synthesis[END_REF] made a review of existing tools to prioritize management actions. They distinguished three main kinds of approaches. Criterion-based approaches used a set of indicators to evaluate the impact of each obstacle independently and, to some extent, to their cumulative impacts [START_REF] Nunn | Restoring River Connectivity: Prioritizing Passage Improvements for Diadromous Fishes and Lampreys[END_REF]. While these methods are generally easy to implement, they tend to neglect the complexity of river networks and population dynamics. A second family of approaches relies on GIS [START_REF] Brevé | Supporting decision-making for improving longitudinal connectivity for diadromous and potamodromous fishes in complex catchments[END_REF]) and more-recently graph theory (Segurado et al. 2013). While these approaches can be applied at a large scale and produce appealing maps to support decision-making, they generally suffer from an improper consideration of fish movements and population dynamics, and from a limited ability to describe the impact of obstacles on fishes. The optimisation method [START_REF] Kuby | A multiobjective optimization model for dam removal: An example trading off salmon passage with hydropower and water storage in the Willamette basin[END_REF]; O'Hanley and Tomberlin 2005) is a third method which appears very relevant despite a higher computation cost. This is based on a model that quantifies the benefits of different restoration actions on one of several criteria such as available habitats (O'Hanley and Tomberlin 2005), but also economic criteria such as minimal loss of hydropower and storage capacity [START_REF] Kuby | A multiobjective optimization model for dam removal: An example trading off salmon passage with hydropower and water storage in the Willamette basin[END_REF]. A numerical optimizer can then be used to objectively to find the sets of restorations actions that maximize outcomes [START_REF] Zheng | Optimizing multiple dam removals under multiple objectives: Linking tributary habitat and the Lake Erie ecosystem[END_REF].

Different models can be used in conjunction with the optimizers. A common approach is to use a statistical species distribution model (SDM) that assesses the impact of obstacles on the distribution of diadromous fishes in a river catchment (Segurado et al. 2014;[START_REF] Clavero | Historical data to plan the recovery of the European eel[END_REF]. This approach is suitable when historical data are available and can be used to prioritize management actions. However, similarly to the call for the development of predictive mechanistic SDM [START_REF] Keith | Predicting extinction risks under climate change: Author-produced version of the article[END_REF][START_REF] Keith | Predicting extinction risks under climate change: Author-produced version of the article[END_REF][START_REF] Franklin | Moving beyond static species distribution models in support of conservation biogeography[END_REF], the development of mechanistic models that combine river fragmentation and diadromous fishes population dynamics is a challenge to predict ecological effectiveness of management measures and support decision-making at the river basin scale. Such models are required to account for population dynamics and fish movement [START_REF] Letcher | Population response to habitat fragmentation in a stream-dwelling brook trout population[END_REF]).

Similarly to SDMs, mechanistic models can provide complementary results to correlative approaches, and enhance predictive ability [START_REF] Kearney | Correlative and mechanistic models of species distribution provide congruent forecasts under climate change[END_REF][START_REF] Rougier | The Combined Use of Correlative and Mechanistic Species Distribution Models Benefits Low Conservation Status Species[END_REF].

In view of this, there is an obvious need for mechanistic model that accounts for fish movements and population dynamics, dendritic structure of fragmented river networks, direct and indirect impact of obstacles to (i) assess the impact of obstacles at the obstacle scale and at the population scale and to (ii) predict the effect of restoration actions. These models can then be coupled with an optimization tool to support decision-making.

RCR and diadromous fishes much more than an ecological issue

RCR and diadromous fishes: interactions of regulations and regulators

In addition to the increasing numbers of regulation on ecological continuity that target diadromous fishes, many kinds of regulation, implemented at different spatial scales, also target diadromous Regulations have also been implemented at lower scales. In France, six water agencies manage water in regional hydrographic districts. They coordinate the district master plan for water management (SDAGE, "schéma d'aménagement et de gestion des eaux") -a planned and concerted water management tool at the basin scale-and river basin water management plans (SAGE, Schémas d'Aménagement et de Gestion des Eaux) at the river basin scale [START_REF] Richard | Water governance in France: institutional framework, stakeholders, arrangements and process[END_REF]. At this scale, committees for the management of migratory fish (COGEPOMI COmité de GEstion des POissons MIgrateurs) are in charge of the management of diadromous fishes through the implementation of migratory fish management plans (PLAGEPOMI/ PLAn de GEstion des POissons MIgrateurs).

Dekker (2016) illustrates the difficulties of this piling up of overlapping regulations and organisations with the example of the European eel. He suggested that the lack of coordination and control between these different regulatory scales is potentially a key explanation for failure to achieve efficient restoration of this species. This type of hypothesis thus merits detailed analysis mobilising social science discipline research techniques and methods. This is particularly so because challenges emerging from interdependent regulations operating at different scales managing diadromous fishes are neither specific to France nor to Europe. For example, the American eel (Anguilla rostrata) is both managed at the state and the federal scales in Canada and in the United States of America and efforts are made to implement a bi-national management [START_REF] Haro | Population decline of the American eel: implications for research and management[END_REF][START_REF] Macgregor | Declines of American eel in North America: complexities associated with bi-national management[END_REF][START_REF] Macgregor | Natural heritage, anthropogenic impacts, and biopolitical issues related to the status and sustainable management of American eel: a Author-produced version of the article[END_REF]. Regarding Atlantic salmon (Salmo salar), management is based on shared stewardship between federal and states governments, First Nations and other

Aboriginal organizations, volunteers, other stakeholders and other federal agencies, and Canada is also member of the North Atlantic Salmon Conservation Organization, an international organization aiming at restoring and protecting the Atlantic salmon.

All these conservation and restoration regulations potentially interact with one another and, depending on how they are interpreted by policy actors, can cause policy conflicts. For example, some wetlands or lakes created by the construction of a dam are classified by Natura 2000 because of interest for birds or other animals or plants, though they alter fish and sediment free-circulation.

But this regulation may conflict with others, for example on water usage. For example, at the European scale, Directive 2009/28/CE promotes the use of renewable energy, including hydropower, though hydropower facilities are often an obstacle to fish free-movement and a source of mortality to migrant species (Blackwell et al. 1998b;[START_REF] Muir | Post-hydropower system delayed mortality of transported snake river stream-type Chinook salmon: Unraveling the mystery[END_REF][START_REF] Larinier | Fish passage experience at small-scale hydro-electric power plants in France[END_REF][START_REF] Pedersen | Loss of European silver eel passing a hydropower station[END_REF]. RCR also potentially comes into conflict with regulations on the spreading of alien species and diseases [START_REF] Rahel | Intentional Fragmentation as a Management Strategy in Aquatic Systems[END_REF][START_REF] Mclaughlin | Unintended consequences and tradeoffs of fish passage[END_REF][START_REF] Tullos | Synthesis of Common Management Concerns Associated with Dam Removal[END_REF]. [START_REF] Rahel | Intentional Fragmentation as a Management Strategy in Aquatic Systems[END_REF] provides many interesting examples on how fragmentation was used by managers to prevent the spread of diseases in Norway, Czech Republic or United States. In Europe, this issue is especially important for fish farming aquaculture: the European Directive 2006/88/CE defines the condition that should be fulfilled for a zone to be considered as "disease-free", granting specific facilities for aquaculture. Such a zone may either be one or several entire catchments, or a sub-part of the catchment delimited by "a natural or artificial barrier that prevents the upward migration of aquatic animals". Consequently, restoring connectivity and migration may question the disease-free status granted to Finnish, Swedish, Irish, Danish or British areas (European Commission Decision of 15 April 2010). Finally, RCR may interfere with other water usage regulation, especially rules about minimum discharge and water extraction.

Grabosky (1995) mentioned 7 types of potential interactions amongst regulations ranging from synergy to neutralisation. Understanding interactions between conservation regulation and other public policy rules is therefore crucial for offering a comprehensive understanding of why RCR measures can be ineffective. This point has been recently recognised by the European Commission in its encouragement of the integration of ecosystem restoration agenda within the delivery of major policy objectives, especially regional development (rural or urban). For example, the 2020 Strategy for biodiversity and the "Green Infrastructure" both call for a better integration of environment preservation policy within other public policies on territorial development to achieve a better reconciliation of environmental and socio-economic targets. Yet, this question of interdependent regulation goes far beyond a simple ignorance of potential interactions: depending on territories, stakeholders might choose to favour a policy to the detriment of another one for political reasons.

For these reasons too RCR should not be analysed in a vacuum but integrated within broader social science evaluations of tensions between public policies and local territorial projects (Carter et al. submitted;[START_REF] Friberg | Effective restoration of aquatic ecosystems: scaling the barriers[END_REF]). This also explains why 'one size fits all' strategies do not work to achieve effective RCR for diadromous fishes. goods and services provided by rivers. In a context of climate change and increasing tensions about water use, obstacles have become a cornerstone of competitions between ecosystem goods and services (Figure 1). These species used to have high market value [START_REF] Mcdowall | Diadromy in Fishes: Migrations Between Freshwater and Marine Environments[END_REF][START_REF] Mcdowall | Different kinds of diadromy: Different kinds of conservation problems[END_REF], but the total income of the commercial inland fishery has strongly declined because of the collapse of populations [START_REF] Champion | Etude socio-économique des pêches professionnelles continentales[END_REF][START_REF] Boisneau | Inland commercial fisheries management in France[END_REF]. Meanwhile, other goods and services provided by diadromous fishes, such as cultural services, are potentially becoming more prominent and the value of diadromous fishes should not be restricted to their commercial value. case of alewives restoration was only a small portion of the overall social benefits for stakeholders.

While the turnover generated by the fishery used to be more or less proportional to catches, and therefore to fish abundance, the relationship between fish abundance and the quantity of cultural or recreational value they provide is less obvious. The dramatic decline in the market values of diadromous fishes urges (i) updating the ecosystem goods and services directly associated with diadromous fishes and obstacles (ii) based on traditional methods used to quantify non-market ecosystem goods and services, developing a method to assess ecosystem goods and services provided by diadromous fishes and RCR. This framework could be developed around well-honed methods to elicit non-market values, such as choice experiment analysis (Adamowicz et al. 1998;[START_REF] Cameron | Alternative non-market value-elicitation methods: are the underlying preferences the same?[END_REF][START_REF] List | Using choice experiments to value non-market goods and services: evidence from field experiments[END_REF]) and could take into account both their uses evolution including economic/productive and environmental, and stakeholders' needs and expectations. Then, indirect goods and services impacted by RCR could be listed and quantified and the relationship between fish abundance and quantity of services elicited.

This challenge is crucial to carry out costs-benefit analysis of RCR for diadromous fishes, and consequently to support decision-making.

 Understanding the social construction of RCR to identify political opportunities and points of tension

Whereas, as we have argued, public policies and regulation play a critical role in structuring RCR actions, in fact knowledge gaps exist concerning political processes leading to their implementation which are often invisible in practice and little studied in the literature. Indeed, few studies have focused expressly on the link between ecosystem restoration and the politics of implementation of public policy [START_REF] Baker | A policy analysis perspective on ecological restoration[END_REF]. Even less have focused on ecosystem restoration as a territorial project [START_REF] Germaine | Les rivières de l'ouest de la France sont-elles seulement des infrastructures naturelles? Les modèles de gestion à l'épreuve de la directive-cadre sur l'eau[END_REF][START_REF] Germaine | Le démantèlement des barrages de la Sélune (Manche). Des réseaux d'acteurs au projet de territoire ? Dév Durable[END_REF]. A critical challenge is grasping RCR analytically as a social construction. On the one hand, scholarship has shown the limits of functionalist accounts linking actors' incentives to policy contents: it is not possible to

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3

explain the origins and contents of policies based purely on their functions [START_REF] Bartley | Institutional emergence in an era of globalization: the rise of transnational private Author-produced version of the article[END_REF]).

To illustrate this point, we can argue that from a functionalist perspective, the EU WFD and Blue and Green infrastructure are the results of the functional imperatives of European and French authorities to restore ecological continuity, therefore the efficiency of this measure will be directly assessed regarding economical or ecological outcomes, as in a DPSIR framework (OECD 1993;[START_REF] Gari | A review of the application and evolution of the DPSIR framework with an emphasis on coastal social-ecological systems[END_REF]. By comparison, from a constructivist approach, both the formulation and the local implementation of the WFD and the Green and Blue Infrastructure result from complex political action of many actors (European and French public authorities, local stakeholders, scientists...), and their contents must be explained as contingent 'settlements of conflicts' [START_REF] Bartley | Institutional emergence in an era of globalization: the rise of transnational private Author-produced version of the article[END_REF]) over both ecological continuity and potentially also other regulations. This is because actors' interests are neither fixed nor can they be assumed: rather they are built in interaction with others' and are socially constructed [START_REF] Eckersley | The green state: Rethinking democracy and sovereignty[END_REF]). In the case of RCR, through their collective interaction over policy implementation, actors' interests and understandings of fish, obstacles and rivers can transform and change.

On the other hand, RCR is not disconnected from other political projects but takes place in relation to other territorial projects and river management plans. Conflicts and tensions can emerge between industrial actors and public policies (e.g. when RCR causes tensions with sectoral policy objectives)

and over the relative importance of RCR for a community (e.g. in some places RCR has been viewed favourably, [START_REF] Mcclenachan | Social benefits of restoring historical ecosystems and fisheries: alewives in Maine[END_REF]in others not, Fox et al, 2010). For example, the opposition of fish farmers to RCR does not necessarily relate to the direct issue of ecological continuity but to a connected issue of disease dispersal in river networks. Therefore, constructivist approaches postulate that the link between a public authority, an official objective and a regulation is not a linear one, and must be analysed in a more comprehensive way, and may change through time. This understanding is necessary to identify tensions and and incentives to achieve efficient reviews.

To provide critical understandings about the social construction of interests towards RCR and their

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3

outcomes, including how actors mange potentially conflicting policy processes, research can usefully draw upon the well-honed concept of 'political work' [START_REF] Jullien | The EU's government of industries: Markets, institutions and politics[END_REF] used in public policy analysis. Political work is defined as action by public and collective private actors to translate their immediate local concerns into public problems and which demand public regulatory responses (e.g. public policies). Political work involves a number of activities including the making of arguments to put issues on the political agenda ; the forging of alliances with other actors ; the establishment of new decisional structures ; strategies of mediatisation and public speeches advancing and justifying actor positions held. Actors' political work can be aimed at putting in place new public policies, implementing public policies already in place, changing public policies, countering public policies. As an illustration, in the context of the RCR, the political work of some NGOs over RCR can consist in strategies to promote the restoration of continuity in opposition to other stakeholders, while at the same time promoting the development of renewable sources of energy in the context of energy transition. These links are crucial, especially in the current period of increasing involvement of local stakeholders in river management after decades of distant relationships [START_REF] Zylberblat | La logique de réhabilitation physique appliquée à un grand fleuve : le Rhône /~~ A Methodology for Physical Restoration applied to a Major River : the Rhône~~[END_REF][START_REF] Blackstock | Evaluating stakeholder involvement in river basin planning: a Scottish case study[END_REF][START_REF] Pahl-Wostl | Social learning and water resources management[END_REF][START_REF] Germaine | Les rivières de l'ouest de la France sont-elles seulement des infrastructures naturelles? Les modèles de gestion à l'épreuve de la directive-cadre sur l'eau[END_REF]. Studying political work can therefore enable research to generate understandings about how successful RCR projects came to be successful (i.e. going beyond the question of social benefits of projects once implemented) and/or about lines of conflict and tension which require to be addressed to bring about future change.

In a regulatory context, when the EU biodiversity strategy for 2020 (EU Commission 2011) calls for a better integration of biodiversity protection into major public policies, social science can and should provide valuable insights on how RCR impacts on-going territorial construction processes led by different or competing actors at different scales and whether RCR is altering peoples' relations to rivers and diadromous fish. This is a critical point, especially responding to climate change and increasing tensions around water use that may modify choices and priorities. For example, the decline of the abundance of diadromous species may either lead to a reinforcement of management actions [START_REF] Mcclenachan | Social benefits of restoring historical ecosystems and fisheries: alewives in Maine[END_REF] or to a loss of societal interest for these fish leading to loss of oversight of their decline. This last threat was defined as "ecosocial anomie" by [START_REF] Limburg | Dramatic Declines in North Atlantic Diadromous Fishes[END_REF]. But, we contend, to assess whether migratory fish hold any cultural value will require social science analysis of political work of actors on RCR in different places compared. The example of the Scottish fishery [START_REF] Carter | The transformation of Scottish fisheries: Sustainable interdependence from 'net to plate[END_REF]) is a very instructive example of a largely unpredictable mobilization of actors through political work that lead to deep modification of management and sustainable fisheries. Also within the example of Scottish-EU fisheries, Carter (2013) demonstrated how insights from constructivist approaches can identify lines of tension blocking implementation as well as opportunities for major change. Building on such approaches, a challenge to achieve effective RCR is to map political work socially constructing RCR for diadromous fishes, by describing actors networks, arguments, expectations and preferences over time. This task is required to elicit formal and informal links between stakeholders and how they manage interdependencies between possible divergent policy objectives, between territories and between knowledge forms (Carter et al. submitted). By doing this, research can identify the 'type' of political work around RCR and diadromous fishes; 'how' and 'why' incentives and conceptions of restoration and ecosystem services' values may vary and the consequences for effective integration in particular territories. In so doing, it can point out the for legitimate RCR, conflicts blocking effective implementation, and potentially new types of incentives to achieve effective RCR.

2.4

Proposing tools to move beyond traditional disciplinary frontiers and achieve a multidimensional approach to RCR

Organising knowledge flows between disciplines

We have listed three types of challenges in three different fields and which more or less correspond to three groups of disciplines: ecology, environmental economics and geography/political science around the question of RCR and diadromous fishes (Figure 2). Each discipline provides unique insights on RCR. However, a comprehensive analysis requires that these insights are collectively organised in a coherent way. This is especially true since each discipline needs the results provided by the others for effective RCR evaluation. Consequently, it is important to acknowledge that disciplines are interdependent. Ecology is required to quantify and predict the effect of RCR on the abundance of diadromous fishes. Through the economical quantification of market and non-market values of diadromous fishes, their abundance can then be converted into goods and ecosystem services that can be used to manage trade-off between conflicting services. To elicit and understand these trade-offs and their shifts through time, geographers and political scientists can provide insights on the social construction of the restoration program and more specifically of the objectives of the different stakeholders and the temporal and spatial scales of political activity in territories. In turn, this analysis of the social construction of RCR requires spatial information on ecological factors (such as the spatial zones of ecological importance, the ecological importance of species or their interactions with other species) and economical ones (for example, economic valuation of services of services).

To organise the necessary exchange of knowledge and moreover, the sharing of assumptions among disciplines, we propose using cross-disciplinary knowledge flows (Figure 2). These are flows of questions and results and visions of the problems encountered by each discipline and which are communicated to the others. This does not necessarily happen spontaneously. To facilitate the process, we propose working with "intermediary objects" [START_REF] Vinck | Les objets intermédiaires dans les réseaux de coopération scientifique. Contribution à la prise en compte des objets dans les dynamiques sociales[END_REF]. In collaborative networks, intermediary objects refer to objects that are meaningful for all actors of the network, though their meaning can vary among actors [START_REF] Vinck | Mediating and commissioning objects in the sociotechnical process of product design: a conceptual approach[END_REF][START_REF] Vinck | Les objets intermédiaires dans les réseaux de coopération scientifique. Contribution à la prise en compte des objets dans les dynamiques sociales[END_REF][START_REF] Boujut | Intermediary objects as a means to foster co-operation in engineering design[END_REF][START_REF] Eckert | The role of objects in design co-operation: communication through physical or virtual objects[END_REF][START_REF] Vinck | L'instrumentation du travail interdisciplinaire: cadrage des échanges et médiation par les objets intermédiaires[END_REF][START_REF] El-Kechai | Understanding the collective design process by analyzing intermediary objects[END_REF] Maps of actors are also useful tool for identifying the role and importance of different stakeholders and public authorities form a multidimensional perspective. Building a multidimensional timeline is a way to establish relationship between events from different fields. Listing and co-constructing intermediary objects can therefore be useful to achieve a comprehensive view of RCR in territories.

Addressing the complexity of spatial and temporal scalar interactions

We outlined earlier the major importance of time and space for restoration programs. Consequently scale integration is a major challenge in a multidisciplinary approach of RCR. Regarding diadromous fishes and RCR, the river basin scale seems to be a central integrative scale. This is both an ecological (population or sub-population scale) and physical/hydrological scale. The river catchment scale also corresponds to water administrative units. Hydrographic districts were introduced in the context of the Water Framework Directive, and correspond to one or several adjacent river basins in which MSs were required to implement River Basin Management Plans (presently, the plans cover the period 2009-2015). Finally, river basins tend also to be a political unit. Water and its governance is a cornerstone issue in the territorialisation process [START_REF] Ghiotti | Les Territoires de l'eau et la décentralisation. La gouvernance de bassin versant ou les limites d'une évidence[END_REF] and the notion of basin common principle is a key argument in many political constructions. For example, each Water Agency in France has articulated a specific understanding of the river basin common interest [START_REF] Meublat | Les agences de bassin: un modèle français de décentralisation pour les pays émergents? La rénovation des institutions de l'eau en Indonésie, au Brésil et au Mexique[END_REF][START_REF] Bouleau | The co-production of science and waterscapes: The case of the Seine and the Rhône Rivers, France[END_REF]. In this context, the basin scale appears to be the landscape scale advocated by [START_REF] Perring | Advances in restoration ecology: rising to the challenges of the coming decades[END_REF] when calling for a scaling-up of restoration activities. It appears to be a unit scale common to the multiple dimensions involved in RCR. However, it will be necessary to understand how this central scale interacts with other spatio- temporal scales to achieve effective RCR for diadromous fishes. For example, while a river catchment is a water administrative unit, it does not match the geographical scale of administrative regions, nor other issues which are relevant for territorial development such as the governing of agriculture or fish farming. Moreover, the definition of basin might vary depending on questions and mobilizations [START_REF] Fernandez | Bringing politics back into water planning scenarios in Europe[END_REF][START_REF] Bréthaut | The shifting territorialities of the Rhone River's transboundary governance: a historical analysis of the evolution of the functions, uses and spatiality of river basin governance[END_REF]. In this context of interacting scales in multiple dimensions, scale should not be seen only as an "euclidean distance" or "temporal-distance", but also as a relational concept which sees space as the result of sociological, ecological or biospherical relationships [START_REF] Healey | The Treatment of Space and Place in the New Strategic Spatial Planning in Europe[END_REF]). Places, sites and territories are not seen as geographical units but as a social-construct with meanings specific to the social context. The coconstruction of intermediary objects such as maps, networks of actors and timelines should favour the understanding of scalar interactions around the river catchment scale. This is particularly important as the challenges of scalar interactions are not, as is sometimes implied, merely a matter of coordination, and being legitimate at one scale does not necessarily mean being legitimate at another (Carter et al. submitted).

2.4.3

Reconciling constructivism and functionalism to achieve efficient restoration A specific challenge emerges bringing about knowledge flows in the case of RCR. This is because whereas ecological and economic valuations are frequently based on functionalist approaches, this is not necessarily the case for political science and geography. As we have argued, it has been well documented that river restoration is a social construction. Recognising this is especially important for building multidisciplinary evaluations of effectiveness. Since RCR is a social construction, an example of success or failure in one territory does not mean that the same approach would necessarily have had the same results in an other context: "one size does not fit all". For example, [START_REF] Blackstock | Linking process to outcomes -Internal and external criteria for a stakeholder involvement in River Basin Management Planning[END_REF] have shown that the process of implementing a program can be just as important for stakeholders as its outputs and, moreover, that effectiveness of outputs is directly associated by stakeholders with effectiveness of inputs.

Although all disciplines recognize the need for multiscalar and multidimensional approaches, the integration between functionalist and constructivist approaches is not straightforward because of ontological and methodological differences. Traditional frameworks support restoration planning, such as DPSIR and related frameworks (OECD 1993;[START_REF] Gari | A review of the application and evolution of the DPSIR framework with an emphasis on coastal social-ecological systems[END_REF] or PCDA (Plan-Do-Check-Act) are generally based on linear causal relationships from an anthropogenic pressure to the implementation of a restoration measure. These functionalist approaches, consistent with some ecological and economical ontologies, have proved to be useful and efficient to describe systems and provide tools to support restoration planning. However, they loose their usefulness as soon as they confuse their simplified and static version of the world with an accurate description of reality. This is even more critical when evaluating the effectiveness of RCR since initial objectives and causal links may have changed through time.

Consequently, there is a need for new frameworks that aim to conciliate the constructivist point of view as upheld in some strands of political science and geography with the more functionalist approach of natural and economic science. It is not only a methodological challenge that would consist in lining up deliverables of a different nature (especially quantitative and qualitative results)

but the development of a new framework based on two antagonistic ontologies, to achieve effective RCR for diadromous fishes. Once again, the co-construction of cross-disciplinary knowledge flows and intermediary objects can be critical elements of such frameworks to elicit, and hence make visible, complementarities between different ontologies.

Acknowledgments

We would like to thank Christian Rigaud, Clarisse Cazals and Guy Verreault for fruitful discussions, and three anonymous referees for their constructive comments.

Literature Cited

Adamowicz W, Boxall P, Williams M, Louviere J (1998) Stated Preference Approaches for

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3

protection 1 . 1

 11 Ecosystem restoration and the specific case of River Continuity Restoration in the light of river fragmentation

  fishes. For example, the Natural Habitats (Natura 2000) Directive (92/43/EEC), a directive that aims at protecting biodiversity, imposes on each MS of the EU to draw up a list of sites hosting natural habitats and wild fauna and flora of interests. Many diadromous fishes are part of the list of species of interest: for example, sturgeon (Acipenser sturio), shads (Alosa alosa and Alosa fallax) and lampreys (Petromyzon marinus and Lampetra fluviatilis). Diadromous fishes conservation is also targeted by international measures or recommendations: for example, the North Atlantic Salmon Conservation Organization recommendations, the Bern Convention on the conservation of European wildlife and natural habitats, the Bonn Convention on the Conservation of Migratory Species of Wild Animals) or Life projects (LIFE Project on shad 2007-2010 ; Life+ project on shad 2011-2014). Regarding eels, the European Eel Regulation (1100/2007 CE) imposes a new set of measures on MSs designed to reverse the decline of the European eel (Anguilla anguilla) population by decreasing all sources of anthropogenic mortality, including impacts of obstacles on migration.

Figure 1 .

 1 Figure 1. Context of river continuity restoration. Both diadromous fishes (dark grey box) and river obstacles (black box) potentially provide ecosystem goods and services (white boxes). The hierarchy given by actors to these competing ecosystem goods and services -and which may change over time -results from political work between stakeholders and public authorities over knowledge, territory and regulation interdependencies.

Figure 2 .

 2 Figure 2. Conceptualising, evaluating and achieving RCR for diadromous fishes require interdisciplinary exchanges. Each discipline can provide different results (ecology: quantification of the ecological outcome of restoration, environmental economics: valuation in terms of ecosystem

  Author-produced version of the article published in EnvironmentalManagement, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3goods and services, geography and political science: regulation, territories, actors and political work, specifying actors and arguments either 'in favour', 'against' or 'indifferent' to RCR).

  Author-produced version of the article published in EnvironmentalManagement, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3 construction of maps is a way to share visions of territories: each discipline can show on a same tool where and why specific zones are of interest (spawning grounds, administrative delimitation, implantation of an enterprise, conflicts…). Regarding a map of obstacles, while the geographical position is common to any discipline, the attributes of interest for the different disciplines are different (for example, passability in ecology, turnover in economy, cultural value in geography).

	. They have proved to be
	valuable mediation tools among disciplines or domains, enhancing communication among them by
	facilitating translation and a support to share a representation of problems (Boujut and Blanco 2003;

[START_REF] Buller | The lively process of interdisciplinarity[END_REF]

. Typically, in the case of the RCR, since spatial and temporal scales are especially important, maps and timelines are among the most relevant intermediary objects. The co-

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3

Author-produced version of the article published in Environmental Management, 2018, 61(4), 671-686 The original publication is available at https://link.springer.com/ doi : 10.1007/s00267-017-0992-3