Luigi Piazzi

José Luis

Sánchez Lizaso

Enric Ballesteros

Franck Lagarde

Charles-François Boudouresque

Aurélie Blanfuné

Gérard Pergent

Christine Pergent-Martini

Michèle Perret-Boudouresque

Thierry Thibaut

Fr Lagoons

Cas

Cy Al

Impacts of Marine and Lagoon Aquaculture on Macrophytes in Mediterranean Benthic Ecosystems

Keywords: aquaculture, fish farms, macrophytes, mediterranean, Paracentrotus lividus, shellfish culture

The direct and indirect impact of fish farms, shellfish aquaculture, and extensive forms of aquaculture such as seeding of juvenile sea urchins, on macrophytes (seaweeds and seagrasses), is reviewed in Mediterranean benthic ecosystems. Fish farms constitute a source of organic matter and nutrients (food and fecal pellets) that causes the extirpation of Posidonia oceanica seagrass meadows beneath and near to farm facilities. In addition to direct effects, the nitrogen enrichment of macrophytes tissues increases the grazing pressure by herbivorous fishes and sea urchins. In some cases, the impact can continue to increase several years after the cessation of farming activities. Natural restoration of extirpated seagrass meadows is generally unlikely at the human time scale. Shellfish aquaculture is the cause of the main flow of introduced macrophytes in the Mediterranean; the main vector is the importation of oyster spat from Japan and Korea. North-eastern Pacific seaweeds are now the dominant biotic component of some Mediterranean lagoons (e.g., Thau, Mar Piccolo, and Venice lagoons). In addition to direct effects, mussel aquaculture can constitute a source of larvae that flow with currents, the adults of which can overwhelm seaweed forests (e.g., Carpodesmia mediterranea). Shellfish aquaculture is also a source of fecal pellets, resulting in changes in bottom macrophytes, and a vector of diseases of metazoans, the extirpation of which may change the functioning of recipient macrophyte ecosystems. The edible sea urchin Paracentrotus lividus is sometimes erroneously considered as in decline due to overharvesting. However, its abundance in the second half of the 20th century was probably a consequence of human impact (overfishing of its predatory fish, organic pollution. This man-induced proliferation resulted in the extirpation of seaweed forests (e.g., Carpodesmia spp., Treptacantha spp. -formerly Cystoseira spp. -Sargassum spp.; many species are endemic), which play a key role in Mediterranean coastal ecosystems. Therefore, the attempts to restore sea urchin abundance, via seeding of juveniles from hatcheries, has further artificialized the habitats rather than contributing to the restoration of natural ecosystems. Good practices guidelines are proposed aimed at minimizing the impact of aquaculture on macrophytes.

INTRODUCTION

The aquaculture of marine organisms has undergone a spectacular development in the Mediterranean since the beginning of the 20th century, particularly since the 1980s, especially shellfish culture in the western basin and fish farming in the eastern basin [START_REF] Charbonnier | Pêche et Aquaculture en Méditerranée[END_REF][START_REF] Belias | Environmental problems in the development of marine fish-farming in the Mediterranean Sea[END_REF][START_REF] Basurco | The aquaculture situation in the Mediterranean Sea. Prediction for the future[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF][START_REF] Rountos | Indirect effects of fish cage aquaculture on shallow Posidonia oceanica seagrass patches in coastal Greek waters[END_REF][START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF].

Until then, it consisted only of very extensive forms of aquaculture, such as valliculture, in the northern Adriatic and Tyrrhenian seas (Italy). Valliculture, the origin of which dates back at least to the 15th century, and is probably even more ancient, is a practice consisting in the exploitation of the natural seasonal migrations of some fish species (mainly gray mullets) from the sea into coastal lagoons, by keeping fish in enclosures ("valli, " in Italian), preventing the fish from returning to the sea, then capturing them several years later during their descent to the sea [START_REF] Lumare | Italian valliculture and its future development[END_REF]. Aquaculture of brackish species might have been known in the Egypt of the pharaohs, in manmade pools in the Nile Delta [START_REF] Basurco | The aquaculture situation in the Mediterranean Sea. Prediction for the future[END_REF][START_REF] Soliman | Aquaculture in Egypt: status, constraints and potentials[END_REF].

Modern forms of aquaculture, in the Mediterranean Sea, range from the very extensive ones, such as the seeding of juveniles of the sea urchin Paracentrotus lividus, to more intensive methods, such as the fish farms where fish located at a high level in the food webs are reared with artificial food. Here, we review the more or less worrying impact of aquaculture on Mediterranean species of macrophytes and the ecosystems for which they may be an "ordinary" contributor, a key species or an ecosystem engineer. The aim is to present a synthesis of the data dispersed in a large number of publications, by considering both direct and indirect effects of aquaculture, and to propose good practices guidelines aimed at minimizing the impact of aquaculture on macrophytes. In this review, we have taken into all the published references, with the exception of redundant data and gray literature and the Mediterranean Sea. It is important to underline that, despite the peculiarities of the Mediterranean Sea [START_REF] Lejeusne | Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea[END_REF], the conclusions we draw from the Mediterranean are largely generalizable to the world ocean (e.g., [START_REF] Yokoyama | Impact of mariculture on the spatial and temporal patterns of the macrobenthos in Gokasho Bay[END_REF][START_REF] Boyra | Impact of sea-cage fish farms on intertidal macrobenthic assemblages[END_REF][START_REF] Loya | Nutrient enrichment caused by in situ fish farms at Eilat, Red Sea is detrimental to coral reproduction[END_REF][START_REF] Hall-Spencer | Impact of fish farms on maerl beds in strongly tidal areas[END_REF][START_REF] Weitzman | Far-field and near-field effects of marine aquaculture[END_REF]; but see [START_REF] Walls | Impact of kelp cultivation on the Ecological Status of benthic habitats and Zostera marina biomass[END_REF].

The notion of macrophytes actually encompasses a polyphyletic complex [START_REF] Boudouresque | Taxonomy and phylogeny of unicellular eukaryotes[END_REF]Boudouresque et al., 2015a). Macrophytes, or multicellular photosynthetic organisms, belong to both the prokaryotes (namely Cyanobacteria) and the eukaryotes. Within the eukaryotes, macrophytes belong to four taxa, very far apart in the phylogenetic tree of eukaryotes. (i) The Ulvophyceae (phylum Chlorobionta, subkingdom Viridiplantae) are part of what are popularly called "green algae." (ii) The Magnoliophyta (phylum Streptobionta, subkingdom Viridiplantae) are what is popularly called "flowering plants"; some terrestrial flowering plants which had returned to the sea, 100-60 Ma ago, are the ancestors of e.g., modern Posidonia, Cymodocea, and Zostera [START_REF] Kuo | Seagrasses a profile of an ecological group[END_REF][START_REF] Orth | A global crisis for seagrass ecosystems[END_REF]. (iii) The Florideophyceae (subkingdom Rhodobionta) are part of what are popularily called "red algae." Ulvophyceae, Magnoliophyta and Florideophyceae belong to the kingdom Archaeplastida. (iv) Finally, the Phaeophyceae belong to the phylum Ochrophyta (= Chromobionta), within the kingdom Stramenopiles (= Heterokonta); they are popularly called "brown algae."

The Mediterranean is a semi-enclosed sea. It is bordered on its southern coast (from Egypt to Morocco) by arid regions and the contribution of its rivers and groundwater discharge does not compensate for evaporation, so that the water deficit is 0.5-1.0 m a -1 [START_REF] Millot | Circulation of the Mediterranean Sea[END_REF][START_REF] Rodellas | Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea[END_REF]. Last but not least, the construction of the Aswan High Dam in Egypt, completed in 1970, has nearly dried up the Nile River (at its mouth), formerly the most important Mediterranean river, very little of whose waters now reach the Mediterranean Sea [START_REF] Sharaf | Effect of the aswan high dam on the nile flood and on the estuarine and coastal circulation pattern along the mediterranean Egyptian coast[END_REF][START_REF] Stanley | Nile Delta: recent geological evolution and human impact[END_REF][START_REF] Abu-Zeid | Egypt's high aswan dam[END_REF][START_REF] Khadr | Evolution of the Nile deposits input during the quaternary and its effect on Egyptian coastline[END_REF]. This deficit in water of the Mediterranean Sea is offset by the entry of Atlantic water through the Straits of Gibraltar [START_REF] Bethoux | Budgets of the Mediterranean Sea. Their dependence on the local climate and on the characteristics of the Atlantic waters[END_REF][START_REF] Farmer | The flow of Atlantic water through the strait of gibraltar[END_REF][START_REF] Bethoux | Functioning of the Mediterranean Sea: past and present changes related to freshwater input and climate change[END_REF][START_REF] Millot | Circulation of the Mediterranean Sea[END_REF]. Despite the relatively nutrient-rich supply of Atlantic water, the low influx of rivers is responsible for the fact that the Mediterranean is an oligotrophic or highly oligotrophic sea; it is also characterized by high environmental variability and by steep gradients, e.g., of salinity, temperature and stratification, which all tend to increase eastwards, within the relatively restricted Mediterranean area [START_REF] Bethoux | The Mediterranean Sea: a miniature ocean for climatic and environmental studies and a key for the climatic functioning of the North Atlantic[END_REF][START_REF] Turley | The changing Mediterranean Sea -A sensitive ecosystem?[END_REF][START_REF] Lejeusne | Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea[END_REF][START_REF] Rodellas | Submarine groundwater discharge as a major source of nutrients to the Mediterranean Sea[END_REF][START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF]. The Mediterranean Sea has been compared to a giant microcosm of the world's ocean, or to a "miniature ocean" [START_REF] Bethoux | The Mediterranean Sea: a miniature ocean for climatic and environmental studies and a key for the climatic functioning of the North Atlantic[END_REF][START_REF] Lejeusne | Climate change effects on a miniature ocean: the highly diverse, highly impacted Mediterranean Sea[END_REF]. Finally, the opening up of the Suez Canal, in 1869, which joins the Red Sea and the Mediterranean, created a real "highway" for the arrival of exotic species in this region, and was the major modern biogeographical event in the world ocean [START_REF] Por | Lessepsian Migrations. The influx of Red Sea Biota into the Mediterranean by Way of the Suez Canal[END_REF][START_REF] Por | Lessepsian migrations. An appraisal and new data[END_REF]Boudouresque, 1999b).

FISH FARMS

Mediterranean marine fish farming was initially land-based. Since the 1990s, it was transferred to floating cages at sea [START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF][START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF]. In 2013, it was dominated by two main species: the European seabass Dicentrarchus labrax with ∼161,000 t a -1 (tones = metric tons, per year) and the gilthead seabream Sparus aurata with ∼135,000 t a -1 [START_REF] Belias | Environmental problems in the development of marine fish-farming in the Mediterranean Sea[END_REF][START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF]. Farming of these species involves a first phase taking place in a land-based hatchery, then the moving of juvenile fish to floating cages at sea. The Atlantic bluefin tuna Thunnus thynnus is farmed by a different type of process, as it is based on the capture of wild specimens (8-10 kg) which are then grown in cages to market size of at least 30 kg [START_REF] Kružić | Variations in Posidonia oceanica meadow structure along the coast of the Dugi Otok Island (eastern Adriatic Sea)[END_REF][START_REF] Mylonas | Atlantic bluefin tuna (Thunnus thynnus) farming and fattening in the Mediterranean Sea[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF]. Several other finfish species are reared in the Mediterranean, such as the meager Argyrosomus regius and the sharpsnout sea bream Diplodus puntazzo, the latter still at an experimental stage [START_REF] Stipa | Cultured Aquatic Species Information Programme, Argyrosomus regius[END_REF][START_REF] Sánchez-García | Risk assessment for parasites in cultures of Diplodus puntazzo (Sparidae) in the Western Mediterranean: prospects of cross infection with Sparus aurata[END_REF]. Although it is a brackish and freshwater form of aquaculture, it is relevant to also mention the most important Mediterranean aquaculture industry, that of the Nile tilapia Oreochromis niloticus; in 2012, for Egypt alone, the production amounted to ∼769,000 t [START_REF] Soliman | Aquaculture in Egypt: status, constraints and potentials[END_REF].

Land-based farm wastewater affects the isotopic composition, and mainly δ 15 N values, of benthic macrophytes; the impact of a fairly small farm on the seagrass Posidonia oceanica and the red alga Sphaerococcus coronopifolius was detected at a distance of 500 m from the outfall [START_REF] Vizzini | Stable isotope evidence of the environmental impact of a land-based fish farm in the western Mediterranean[END_REF]. However, the effects can be easily avoided by using land-based wastewater treatment. Off-shore fish farms are also responsible for elevated δ 15 N signature in leaf tissues and epibionts of a deep P. oceanica meadow, up to 3 km from the cages [START_REF] Ruiz | Remote influence of off-shore fish farm waste on Mediterranean seagrass (Posidonia oceanica) meadows[END_REF].

Offshore fish farms, with fish reared in floating cages, which today represent the bulk of marine fish farming, can affect the marine environment, directly and indirectly, in several ways, through (i) the release of nutrients (nitrogen and phosphorus), (ii) the release of particulate organic carbon (e.g., fecal pellets and uneaten feed) and (iii) chemicals (e.g., copper in antifouling treatments and drugs for disease treatments). The effects vary according to the production system, site characteristics, hydrodynamics and movements at the bottom of the water column, the species farmed, the feed used and the sensitivity of the receiving ecosystem [START_REF] Sarà | Use of stable isotopes to investigate dispersal of waste from fish farms as a function og hydrodynamics[END_REF][START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF][START_REF] Marino | Sustainable aquaculture: bioecological aspects[END_REF][START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF].

Many environmental requirements for coastal fish farming (e.g., good water quality and water renewal) correspond almost exactly to the habitat preferences of P. oceanica; as a result, a number of fish farms are placed over or very near P. oceanica meadows [START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]. Studies on fish cage aquaculture located over Posidonia oceanica seagrass meadows have shown a very strong impact on this ecosystem of paramount importance for Mediterranean coastal areas [START_REF] Pergent-Martini | Impact of fish farming facilities on Posidonia oceanica meadows: a review[END_REF][START_REF] Boudouresque | Regression of Mediterranean seagrasses caused by natural processes and anthropogenic disturbances and stress: a critical review[END_REF]Boudouresque et al., , 2012[START_REF] Boudouresque | The necromass of the Posidonia oceanica seagrass meadow: fate, role, ecosystem services and vulnerability[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF][START_REF] Giakoumi | Towards a framework for assessment and management of cumulative human impacts on marine food webs[END_REF][START_REF] Kletou | Seagrass recovery after fish farm relocation in the eastern Mediterranean[END_REF]. The P. oceanica ecosystem thrives between the sea level and 30-40 m depth, in the infralittoral zone [START_REF] Molinier | Recherches sur les herbiers de phanérogames marines du littoral méditerranéen français[END_REF][START_REF] Boudouresque | Découverte de l'herbier de posidonie[END_REF]Boudouresque et al., 2012); for Mediterranean biotic zones and habitats, see [START_REF] Pérès | Nouveau manuel de bionomie benthique de la mer Méditerranée[END_REF] and [START_REF] Pérès | Structure and dynamics of assemblages in the benthal[END_REF]. (i) There is a decrease in light availability under facilities, due to both increased turbidity (including possible phytoplankton enhancement) and the shadow of the cages: 38% of the bottom light is lost beneath the cages in the Gulf of Ajaccio (Corsica). However, the amount of light is still higher than the compensation irradiance level of P. oceanica. Therefore, this factor alone cannot explain the regression of the meadow observed [START_REF] Verneau | Modifications de l'herbier de Posidonies et des populations de poissons dans l'ombre portée sur le fond par des radeaux aquacoles installés dans le golfe d'Ajaccio (Corse-du-Sud, Méditerranée nord-occidentale)[END_REF][START_REF] Delgado | Seagrass regression caused by fish cultures in Fornells Bay (Menorca, Western Mediterranean)[END_REF][START_REF] Mendez | Impact of fish farming on coastal ecosystems[END_REF][START_REF] Pergent | Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean[END_REF]Ruiz and Romero, 2001;Ruiz et al., 2001). This statement may be right for shallow meadows, but not for deeper meadows, which are light limited [START_REF] Romero | Production ecology of Posidonia oceanica (L.) Delile meadows in Nueva Tabarca marine Reserve: growth, biomass and nutrient stocks along a bathymetric gradient[END_REF]. According to [START_REF] Puhr | The effect of in situ shading on a Posidonia oceanica meadow situated within a fish farm induced moderately nutrient enriched environment[END_REF], light availability is the main factor that controls seagrass distribution at fish farm locations. Light available for P. oceanica photosynthesis is also reduced by the high epibiont coating caused by nutrient enrichment [START_REF] Cancemi | Impact of fish farming facility on a Posidonia oceanica meadow[END_REF][START_REF] Cancemi | Effects of organic matter input from a fish farming facility on a Posidonia oceanica meadow[END_REF]; see below). (ii) Because of the input of fecal pellets and uneaten food, the sediment is enriched in organic matter, which alter its physical and chemical characteristics and enhances anoxic conditions and sulfate reduction processes [START_REF] Pergent | Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean[END_REF]Ruiz et al., 2001;[START_REF] Cancemi | Effects of organic matter input from a fish farming facility on a Posidonia oceanica meadow[END_REF][START_REF] Holmer | Benthic primary producers -a neglected environmental problem in Mediterranean maricultures?[END_REF][START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]Boudouresque et al., 2012;[START_REF] Weitzman | Far-field and near-field effects of marine aquaculture[END_REF]. This factor emerges as one of the main drivers of benthic deterioration, via e.g., sediment anoxia [START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]. (iii) The water is enriched in nutrients, especially ammonium [START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF], which may enhance phytoplankton development but especially fosters the growth of the P. oceanica leaves and increases the biomass of their epibionts. It is worth noting that the effect of nutrient enrichment is different according to depth [START_REF] Romero | Production ecology of Posidonia oceanica (L.) Delile meadows in Nueva Tabarca marine Reserve: growth, biomass and nutrient stocks along a bathymetric gradient[END_REF]. The maximum increase in epibiont biomass does not occur beneath the cages, but at a distance of between 20 and 80 m at Figari (Corsica), which may be explained by the large quantities of copper introduced with the fish food and/or antifouling chemicals used for the nets [START_REF] Mendez | Impact of fish farming on coastal ecosystems[END_REF][START_REF] Pergent | Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean[END_REF]Dimech et al., 2000a;[START_REF] Holmer | Benthic primary producers -a neglected environmental problem in Mediterranean maricultures?[END_REF][START_REF] Pergent-Martini | Impact of fish farming facilities on Posidonia oceanica meadows: a review[END_REF][START_REF] Kružić | Variations in Posidonia oceanica meadow structure along the coast of the Dugi Otok Island (eastern Adriatic Sea)[END_REF]. (iv) In contrast with the increase or decrease in leaf growth and epibiont biomass, the vertical growth of rhizomes abruptly declines by about twofold following the onset of fish farm operations; this decline is perceptible up to 400 m from the farm [START_REF] Marbà | Seagrass (Posidonia oceanica) vertical growth as an early indicator of fish farmderived stress[END_REF][START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]. (v) The concentration of trace metals, such as copper and zinc in rhizomes of P. oceanica, increases near the facilities. They derive from uneaten feed, fish excreta and antifouling chemicals used to prevent development of fouling organisms on the cages [START_REF] Pergent | Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean[END_REF][START_REF] Basaran | Impacts of the fish farms on the water column nutrient concentration and accumulation of heavy metals in the sediments of the eastern Aegean Sea (Turkey)[END_REF]Grigorakis and Rigos, 2011). (vi) The epibiontic cover of the leaves, together with the high nitrogen content of leaf tissues and epibionts, are attractive for herbivorous species, such as the teleost Sarpa salpa and the sea urchin Paracentrotus lividus. Sea urchins are actually more abundant in the vicinity of cages. As a result, the grazing of leaves is higher beneath and near facilities than at control sites [START_REF] Mendez | Impact of fish farming on coastal ecosystems[END_REF][START_REF] Pergent | Preliminary data on the impact of fish farming facilities on Posidonia oceanica meadows in the Mediterranean[END_REF]Ruiz et al., 2001;[START_REF] Pérez | Physiological responses of the seagrass Posidonia oceanica as indicators of fish farm impact[END_REF][START_REF] Prado | Seasonal response of Posidonia oceanica epiphyte assemblages to nutrient increase[END_REF][START_REF] Balata | Effects of enhanced loads of nutrients on epiphytes on leaves and rhizomes of Posidonia oceanica[END_REF][START_REF] Rountos | Indirect effects of fish cage aquaculture on shallow Posidonia oceanica seagrass patches in coastal Greek waters[END_REF]. (vii) Fish populations are increased, both in biomass, number of individuals and sometimes point species diversity [START_REF] Verneau | Modifications de l'herbier de Posidonies et des populations de poissons dans l'ombre portée sur le fond par des radeaux aquacoles installés dans le golfe d'Ajaccio (Corse-du-Sud, Méditerranée nord-occidentale)[END_REF][START_REF] Machias | Changes in wild fish assemblages after the establishment of a fish farming zone in an oligotrophic marine ecosystem[END_REF][START_REF] Machias | Response of demersal fish communities to the presence of fish farms[END_REF]. In the Gulf of Ajaccio, biomass and density of fish are increased four to fivefold (from 54 to 268 g 10 m -2 and from 2 to 10 individuals 10 m -2 , respectively); the most successful species are e.g., stripped red mullet Mullus surmuletus, bogue Boops boops and barracuda Sphyraena sphyraena. The trophic structure of fish populations is also changed, with fewer invertebrate consumers and more omnivorous species [START_REF] Verneau | Modifications de l'herbier de Posidonies et des populations de poissons dans l'ombre portée sur le fond par des radeaux aquacoles installés dans le golfe d'Ajaccio (Corse-du-Sud, Méditerranée nord-occidentale)[END_REF]. In Malta, benthic invertebrates (decapods, mollusks and echinoderms) also exhibit an increase in abundance of individuals and point species diversity at intermediate distances (40-160 m) from the cages [START_REF] Dimech | The effects of a marine fish-farm on the species richness and abundance of molluscs, decapods and echinoderms associated with a Posidonia oceanica meadow in Malta (central Mediterranean)[END_REF][START_REF] Dimech | Changes in the structure of a Posidonia oceanica meadow and in the diversity of associated decapod, mollusc and echinoderm assemblages, resulting from input of waste from a marine fish farm (Malta, central Mediterranean)[END_REF]. (viii) Beneath the cages and in the adjacent area, there is an overall regression of the meadow: the shoot density conspicuously declines and extensive areas of dead matte appear (Figure 1; [START_REF] Verneau | Modifications de l'herbier de Posidonies et des populations de poissons dans l'ombre portée sur le fond par des radeaux aquacoles installés dans le golfe d'Ajaccio (Corse-du-Sud, Méditerranée nord-occidentale)[END_REF][START_REF] Delgado | Seagrass regression caused by fish cultures in Fornells Bay (Menorca, Western Mediterranean)[END_REF]Dimech et al., 2000a;[START_REF] Cancemi | Effects of organic matter input from a fish farming facility on a Posidonia oceanica meadow[END_REF][START_REF] Pergent-Martini | Impact of fish farming facilities on Posidonia oceanica meadows: a review[END_REF][START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF][START_REF] Kružić | Variations in Posidonia oceanica meadow structure along the coast of the Dugi Otok Island (eastern Adriatic Sea)[END_REF][START_REF] Pérez | Physiological responses of the seagrass Posidonia oceanica as indicators of fish farm impact[END_REF]Boudouresque et al., 2012;[START_REF] Kletou | Seagrass recovery after fish farm relocation in the eastern Mediterranean[END_REF]. Matte is the structure built by living and dead rhizomes and by the sediment that fills the interstices; because of the very slow or even the absence of degradation of dead rhizomes and roots within the matte, the matte can persist for at least millennia [START_REF] Molinier | Recherches sur les herbiers de phanérogames marines du littoral méditerranéen français[END_REF][START_REF] Boudouresque | Découverte de l'herbier de posidonie[END_REF]Boudouresque et al., 2012[START_REF] Boudouresque | The necromass of the Posidonia oceanica seagrass meadow: fate, role, ecosystem services and vulnerability[END_REF].

The case of the El Hornillo fish farm (Murcia region, Spain), a medium scale farm (up to 700-800 metric tons of fish per year) is particularly instructive (Figure 2). Fish farming began in 1989. Since the onset of fish farming, 11 ha (28%) of the former P. oceanica meadow have been rapidly and completely lost and 10 ha (25%) significantly degraded, with a decline in shoot density; the affected area (a radius of 300 m from the fish farm perimeter) therefore amounts to about 53% of the former meadow, and 7 times the fish farm area [START_REF] Ruiz-Fernandez | Respuesta de la Fanerógama Marina Posidonia oceanica (L.) Delile a Perturbaciones Antrópicas[END_REF]Ruiz et al., 2001).

Another case is also instructive, that of the Fornells Bay (Menorca Island, Spain) fish farm. It was established in 1986, and its activity ceased in 1991. However, the regression of the P. oceanica meadow, which quickly began as soon as the farm came into operation, continued for at least three years; the persistence of the seagrass decline could be due to the excess of organic matter remaining in the sediment [START_REF] Delgado | Effects of fish farming on seagrass (Posidonia oceanica) in a Mediterranean bay: seagrass decline after organic loading cessation[END_REF]. Interestingly, in Cyprus, close to the upper thermal limit of P. oceanica, fish farms initially located over a P. oceanica meadow were relocated offshore; three to five years later, not only had the regression ceased, but the margin of the meadow had progressed, which illustrates an important management success story [START_REF] Kletou | Seagrass recovery after fish farm relocation in the eastern Mediterranean[END_REF].

Similar conclusions can be drawn for coastal detritic bottoms, e.g., maërl beds, which are dominated by calcareous Rhodobionta, in the circalittoral zone, deeper than the infralittoral zone [START_REF] Sanz-Lázaro | Relationship between sedimentation rates and benthic impact on Maërl beds derived from fish farming in the Mediterranean[END_REF][START_REF] Aguado-Giménez | Influence of an experimental fish farm on the spatio-temporal dynamic of a Mediterranean maërl algae community[END_REF]Massa et al., 2017 and references therein); below experimental fish cages, the maërl community was almost completely buried; dead and blackened rhodoliths were covered by a thin layer of sediment and only a few individuals of Gracilaria cylindrica, Lithophyllum racemus, Meredithia microphylla (Rhodobionta) and Fabellia petiolata (Ulvophyceae) remained alive (Aguado-Giménez and [START_REF] Aguado-Giménez | Influence of an experimental fish farm on the spatio-temporal dynamic of a Mediterranean maërl algae community[END_REF]. In a coralligenous ecosystem, from 30 to 40 m 1988, 1994, and 1998 in El Hornillo Bay (Murcia, Spain). Only the complete mortality off (area of dead matte) has been mapped. Original drawing, after [START_REF] Ruiz-Fernandez | Respuesta de la Fanerógama Marina Posidonia oceanica (L.) Delile a Perturbaciones Antrópicas[END_REF] and Ruiz et al. (2001) data.

of depth, at about 30 m of cages, there is a decrease in βdiversity, encrusting Corallinales, erect Rhodobionta, Dictyotales, Fucales (Phaeophyceae) and Halimeda tuna (Ulvophyceae) and an increase in Peyssonnelia spp. (Rhodobionta) and algal turfs [START_REF] Piazzi | Impact assessment of fish cages on coralligenous reefs through the use of the STAR sampling procedure[END_REF]. Unfortunately, most authors focus on the impact of fish farming on P. oceanica meadows, but pay little attention to other macrophyte assemblages. For example, in El Hornillo Bay (Spain), Ruiz et al. (2001) briefly mention other benthic communities, e.g., the presence of the seagrass Cymodocea nodosa on shallow sandy bottoms, but the possible impact of the fish farm was not addressed. In Fornells Bay (Menorca Island, Spain), C. nodosa was less affected than P. oceanica [START_REF] Delgado | Seagrass regression caused by fish cultures in Fornells Bay (Menorca, Western Mediterranean)[END_REF]. On artificial hard substrate, the fouling community, initially dominated by seaweeds, was transformed, in 6 months, into a community dominated by filter-feeders, probably because of the elevated supply of particulate organic matter [START_REF] Cook | The influence of caged mariculture on the early development of sublittoral fouling communities: a pan-European study[END_REF]. In Croatia, near fish cages, the proliferation of sea urchins (Paracentrotus lividus and Arbacia lixula) in rocky reef habitats has been observed, resulting in overgrazing of seaweeds and the occurrence of barren grounds [START_REF] Kušpilić | Impact of fish farming on marine ecosystems. Croatian experiences[END_REF].

Obviously, as pointed out by Ruiz et al. (2001), the impact resulting from fish farming is highly variable. It depends on water exchange, depth, aquaculture practices (fish feeding, fish yield, etc.) and on the vulnerability of the benthic ecosystem, e.g., the presence of P. oceanica meadows or rocky reef seaweed assemblages. Farms may not even have an impact on a given ecosystem; for example, no impact of fish farms on Carpodesmia amentacea (= Cystoseira amentacea; Ochrophyta) distribution along the French coast has been recorded [START_REF] Thibaut | Unexpected abundance and long-term relative stability of the brown alga Cystoseira amentacea, hitherto regarded as a threatened species, in the north-western Mediterranean Sea[END_REF].

SEA URCHIN SEEDING

The edible sea urchin Paracentrotus lividus is, together with the fish Sarpa salpa, the main native macro-herbivore in the Mediterranean Sea [START_REF] Verlaque | Relations entre Paracentrotus lividus (Lamarck) et le phytobenthos de Méditerranée occidentale[END_REF][START_REF] Boudouresque | Paracentrotus lividus[END_REF]. Other herbivorous fish, the native parrotfish Sparisoma cretense and the introduced (from the Red Sea) rabbitfishes Siganus luridus and S. rivulatus, are common in the eastern basin and are currently spreading toward the west, because of the warming of the sea surface water [START_REF] Por | Lessepsian Migrations. The influx of Red Sea Biota into the Mediterranean by Way of the Suez Canal[END_REF][START_REF] Astruch | Provence and Mediterranean warming: the parrotfish Sparisoma cretense is coming[END_REF][START_REF] Karachle | New mediterranean biodiversity records (March 2016)[END_REF]. Paracentrotus lividus is sometimes erroneously considered as in decline, which would have "negative" consequences on the health status of ecosystems; this is due to over-harvesting (see e.g., [START_REF] Guidetti | Effects of the edible sea urchin, Paracentrotus lividus, fishery along the Apulian rocky coast (SE Italy, Mediterranean Sea)[END_REF][START_REF] Couvray | Étude de la Dynamique de Populations d'oursins Comestibles Paracentrotus lividus (Lamarck, 1816) en zone pêchée et anthropisée: évaluation de l'efficacité et de l'impact d'opérations de lâchers de juvéniles pour le repeuplement[END_REF][START_REF] Couvray | Experimental release of juvenile sea urchins (Paracentrotus lividus) in exploited sites along the French Mediterranean coast[END_REF][START_REF] Sartori | Echinoculture: the rearing of Paracentrotus lividus in a recirculating aquaculture system -Experiments of artificial diets for the maintenance of sexual maturation[END_REF], as its gonads (roe) are appreciated as a luxury seafood, especially in France, Spain, Italy, and Greece [START_REF] Ballesteros | La pêche aux oursins en Espagne et plus particulièrement en Catalogne[END_REF][START_REF] Ledireac'h | La pêche des oursins en Méditerranée: historique, techniques, législation, production[END_REF][START_REF] Ledireac'h | Rapport sur l'exploitation des oursins en Méditerranée[END_REF]. However, at least in some Mediterranean areas, e.g., in Corsica, there is no sign of overexploitation, as highlighted by [START_REF] Duchaud | Spatio-temporal patterns based on demographic and genetic diversity of the purple sea urchin Paracentrotus lividus in the area around Corsica (Mediterranean Sea)[END_REF]. Although the actual baseline of the population density of P. lividus in the Mediterranean is unknown, there are reasons to think that its abundance, in the second half of the 20th century, could be, at least partly, a consequence of human impact (overfishing of its predatory fish, organic pollution) [START_REF] Boudouresque | Paracentrotus lividus[END_REF]; it is usually uncommon in No-Take Zones of Marine Protected Areas, i.e., zones where all types of fishing and harvesting (fish, custaceans and echinoderms), both amateur and commercial, are banned [START_REF] Boudouresque | Macrozoobenthos de la réserve naturelle de Scandola (Corse, Méditerranée nord-occidentale). Analyse pluriannuelle de 'l'effet réserve[END_REF], and proliferates in the vicinity of untreated domestic sewage outfalls [START_REF] Harmelin | Impact de la pollution sur la distribution des échinodermes des substrats durs en Provence (Méditerranée Nord-occidentale)[END_REF][START_REF] Boudouresque | Paracentrotus lividus[END_REF] and fish farms [START_REF] Kušpilić | Impact of fish farming on marine ecosystems. Croatian experiences[END_REF]. This man-induced proliferation of P. lividus has resulted in the extirpation of seaweed forests, e.g., Carpodesmia spp., Treptacantha spp. (formerly Cystoseira spp.), and Sargassum spp.; many of them are species endemic to the Mediterranean; they play a key role in Mediterranean coastal ecosystems [START_REF] Thibaut | Long-term decline of the populations of Fucales (Cystoseira spp. and Sargassum spp.) in the Albères coast (France, North-western Mediterranean)[END_REF][START_REF] Thibaut | Decline and local extinction of Fucales in the French Riviera: the harbinger of future extinctions?[END_REF][START_REF] Blanfuné | The fate of Cystoseira crinita, a forest-forming Fucale (Phaeophyceae, Stramenopiles), in France (North Western Mediterranean Sea)[END_REF][START_REF] Thibaut | Unexpected temporal stability of Cystoseira and Sargassum forests in Port-Cros, one of the oldest Mediterranean marine National Parks[END_REF][START_REF] Thibaut | An ecosystem-based approach to assess the status of Mediterranean algae-dominated shallow rocky reefs[END_REF], and their replacement by barren grounds is a concerning issue [START_REF] Fraschetti | Effects of unplanned development on marine biodiversity: a lesson from Albania (central Mediterranean Sea)[END_REF][START_REF] Agnetta | Role of two co-occurring Mediterranean Sea urchins in the formation of barren grounds[END_REF][START_REF] Ling | Global regime shift of catastrophic sea urchin overgrazing[END_REF]. The species could be native, in some parts of its Mediterranean range. b Since the taxonomy of the genus Cladophora is complicated, the confirmation of the identity of this taxon would require further investigation. c The European records of Ulva lactuca over the past 200 years correspond to another species than the real U. lactuca Linnaeus (= U. fasciata Delile) which is referred to here (see [START_REF] Butler | Ancient algal mixup sorted[END_REF][START_REF] Verlaque | CIESM atlas of exotic species[END_REF]. d Cutleria multifida is native to north Atlantic and Japan [START_REF] Kawai | Phylogeographic analysis of the brown alga Cutleria multifida (Tilopteridales), Phaeophyceae) suggests a complicated introduction history[END_REF]. Molecular analysis of Mediterranean specimens from France, Italy and Greece shows a Japanese origin. The species has been recorded from other Mediterranean countries, but, in the absence of molecular analysis, an Atlantic origin cannot be excluded. e Pylaiella littoralis is regarded as introduced only in the northern lagoons of the Mediterranean [START_REF] Verlaque | CIESM atlas of exotic species[END_REF].

As expected, harvesting results in a decline in density, mean size and percentage of individuals of the largest size classes [START_REF] Bertocci | Multiple effects of harvesting on populations of the purple sea urchin Paracentrotus lividus in north Portugal[END_REF][START_REF] Bertocci | Short-term variation of abundance of the purple sea urchin, Paracentrotus lividus (Lamarck, 1816), subject to harvesting in northern Portugal[END_REF]. However, the lack of large individuals (the commercial size class) in a highly harvested area of Sardinia does not seem to be alarming for the self-supporting capacity of the population, as the reproductive potential is mainly due to undersized individuals: the amount of released gametes is similar in harvested and non-harvested areas [START_REF] Loi | Hard time to be parents? Sea urchin fishery shifts potential reproductive contribution of population onto the shoulders of the young adults[END_REF].

The attempts to restore sea urchin abundance, via seeding of juveniles from hatcheries, whether or not effective (see below), and the possible disproportionate financial cost of such operations, cannot be referred to as "ecosystem restoration, " but rather as "extensive aquaculture." It is likely that they contribute to further accentuating the artificialization of the environment, for the possible benefit of humans needs, and not to the restoration of its naturalness. In order to mitigate the supposed decline of P. lividus populations, and to respond to the growing market demand, land-based and offshore aquaculture, and the reseeding of depleted sites, with juveniles reared in hatcheries, has been considered (e.g., [START_REF] Couvray | Étude de la Dynamique de Populations d'oursins Comestibles Paracentrotus lividus (Lamarck, 1816) en zone pêchée et anthropisée: évaluation de l'efficacité et de l'impact d'opérations de lâchers de juvéniles pour le repeuplement[END_REF][START_REF] Couvray | Experimental release of juvenile sea urchins (Paracentrotus lividus) in exploited sites along the French Mediterranean coast[END_REF][START_REF] Sartori | Echinoculture: the rearing of Paracentrotus lividus in a recirculating aquaculture system -Experiments of artificial diets for the maintenance of sexual maturation[END_REF][START_REF] Sartori | Can echinoculture be a feasible and effective activity? Analysis of fast reliable breeding conditions to promote gonadal growth and sexual maturation in Paracentrotus lividus[END_REF][START_REF] Shpigel | The sea urchin, Paracentrotus lividus, in an integrated multi-trophic aquaculture (IMTA) system with fish (Sparus aurata) and seaweed (Ulva lactuca): nitrogen partitioning and proportional configurations[END_REF][START_REF] Volpe | Gonad quality of sea urchin Paracentrotus lividus cultured in an offshore pilot-scale trial on the south-east Italian coast[END_REF][START_REF] Zupo | Roe enhancement of Paracentrotus lividus: Nutritional effects of fresh and formulated diets[END_REF]. For example, 250,000 hatchery produced juveniles were released in eastern Provence (France); a year after release, they represented 3 and 12% of total recaptured urchins at two experimental sites [START_REF] Couvray | Experimental release of juvenile sea urchins (Paracentrotus lividus) in exploited sites along the French Mediterranean coast[END_REF]. In Asturias (Atlantic Spain), all small individuals (10-30 mm) released on substrates with burrows did not survive the first few weeks, and only those that were released on substrates with large individuals successfully settled (12%) (de la [START_REF] De La Uz | Evaluation of tagging and substrate refuges in release of juvenile sea urchins[END_REF]. This preference of juveniles for habitats with abundant adults could explain the relative failure of some reseeding operations, which for intuitively logical reasons, are carried out in depopulated areas rather than in already densely populated areas. Hatchery rearing of P. lividus can cause bottleneck effects (i.e., a substantial reduction in genetic diversity and differentiation from wild populations), as hatchery-reared populations are less diverse than wild populations [START_REF] Segovia-Viadero | Do hatchery-reared sea urchins pose a threat to genetic diversity in wild populations?[END_REF]. This highlights the need to consider the genetic risks of releasing hatchery-reared juveniles into the wild in the framework of stock enhancement and sea ranching programs [START_REF] Segovia-Viadero | Do hatchery-reared sea urchins pose a threat to genetic diversity in wild populations?[END_REF]. However, according to [START_REF] Couvray | Experimental release of juvenile sea urchins (Paracentrotus lividus) in exploited sites along the French Mediterranean coast[END_REF], in eastern Provence, genetic diversity seems not to be affected by introducing juveniles from the hatchery.

SHELLFISH AQUACULTURE

Mediterranean shellfish aquaculture has grown steadily over the years, although with a lower rate of increase than that of fish farming, and a relative plateau or decline since the 1990s: 17 kt in 1970, 56 kt in 1980, 141 kt in 1990, 190 kt in 2000, FIGURE 5 , FIGURE 5 | Seascape on a shallow rocky substrate near Meze, Thau Lagoon (France). The brown blades: Undaria pinnatifida. The yellowish filaments (above, to the left): Sargassum muticum. The sketch below (Figure 6) shows the rest of the macrophyte stand. Photo: courtesy of Sandrine Ruitton. and 137 kt in 2010; fish production is now higher than that of shellfish (Figure 3; [START_REF] Basurco | The aquaculture situation in the Mediterranean Sea. Prediction for the future[END_REF]FAO, 2019). Three species of bivalves constitute the bulk of the shellfish aquaculture in the Mediterranean Sea [START_REF] Basurco | The aquaculture situation in the Mediterranean Sea. Prediction for the future[END_REF]: the mussel Mytilus galloprovincialis, which is native to the Mediterranean (mainly farmed in Italy, Grece and France), the Pacific oyster Magallana gigas (= Crassostrea gigas) (mainly in France), native to the Pacific coast of Asia [START_REF] Salvi | Molecular taxonomy in 2D: a novel ITS2 rRNA sequence structure approach guides the description of the oysters' subfamily Saccostreinae and the genus Magallana (Bivalvia: Ostreidae)[END_REF], and the Manila clam Ruditapes philippinarum (mainly in Italy), native to the coasts of the Indian and Pacific Oceans, from Pakistan to Japan and the Kuril Islands [START_REF] Goulletquer | Cultured Aquatic Species Information Programme. Ruditapes philippinarum[END_REF]. Three other species are bred, but the quantities produced are very limited: the Atlantic mussel Mytilus edulis, the flat oyster Ostrea edulis and the grooved carpet shell Ruditapes decussatus (Figure 3) Magallana gigas escaped from sea farms and is now introduced (naturalized) in the Mediterranean; in contrast with the European Atlantic coasts, it is not invasive there [START_REF] Zenetos | Origin and vectors of introduction of exotic molluscs in Greek waters[END_REF][START_REF] Wrange | Massive settlement of the Pacific oyster, Crassostrea gigas, in Scandinavia[END_REF]; see Boudouresque andVerlaque, 2002, 2012, for invasion terminology). Ruditapes philippinarum escaped from sea farms and was also deliberately introduced, as in the Venice and Berre lagoons; it is locally invasive and its abundance is considered welcome by local fishermen [START_REF] Mazzola | Allochtonous species and aquaculture[END_REF][START_REF] Occhipinti Ambroggi | Transfer of marine organisms: a challenge to the conservation of coastal biocoenoses[END_REF][START_REF] Pranovi | Mechanical clam dredging in Venice Lagoon: ecosystem effects evaluated with a trophic mass-balance model[END_REF][START_REF] Pranovi | An ecological imbalance induced by a non-native species: the Manila clam in the Venice Lagoon[END_REF].

Shellfish aquaculture can affect macrophytes of Mediterranean ecosystems via four processes: a major source of non-indigenous seaweeds, a flow of propagules that can flood macrophyte forests, a vector of diseases of metazoans of which the extirpation changes the functioning of recipient ecosystems, and a change in the functioning of the recipient ecosystem, including a direct effect of the shower of fecal pellets on bottom macrophytes.

A Major Source of Non-indigenous Seaweeds

Oyster culture has been a mass vector for the introduction of macroalgae in the Mediterranean, as in many regions of the earth: "The greatest agency of all that spreads marine animals [and plants] to new quarters of the world must be the business of oyster culture" [START_REF] Elton | Fishery and Aquaculture Statistics. Global aquaculture production 1950-2017[END_REF][START_REF] Verlaque | Oyster transfers: a major vector for macrophyte introductions[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF]. As many as 65 Non-Indigenous Species (NIS) have been recorded in the Mediterranean (Table 1) of which the arrival is linked, directly or indirectly, to shellfish aquaculture, that of the Pacific oyster Magallana gigas for the most part, but also the Manila clam Ruditapes philippinarum in the case of Aglaothamnion halliae, Gracilaria vermiculophylla, and Uronema marinum [START_REF] Sfriso | On the occurrence of Uronema marinum Womersley (Chaetophorales, Chlorophyta) in the north-western lagoons of the Adriatic Sea, Mediterranean Sea (Italy)[END_REF][START_REF] Verlaque | CIESM atlas of exotic species[END_REF][START_REF] Wolf | Shellfish import and hull fouling as vectors for new red algal introductions in the Venice Lagoon[END_REF]. Between 1968 and 1983, large amounts of oyster spat (up to 200 t a -1 ) were directly imported from Japan (the Bay of Mangoku-Ura, near Sendai) to Thau Lagoon (Occitania, France); in fact, these imports continued well after their official ban in 1983 [START_REF] Verlaque | Checklist of the macroalgae of Thau Lagoon (Hérault, France), a hot spot of marine species introduction in Europe[END_REF]. Theoretically, the spat was to be immerged TABLE 2 | A grid that shows the eligibility of fish farming sites, according to the distance from the nearest Posidonia oceanica meadow, the depth, the water movement (openness of the site) and the size of the farm (in wet mass -metric tons -of fish produced per year). Greater distances would of course be welcome.

Depth

Openness for 4 h in fresh water in order to kill exotic epibionts, before immersion in the lagoon [START_REF] Piquion | Aquaculture: lutte contre les compétiteurs[END_REF], however, this operation was under the sole responsibility of the farmers, who of course did not do it; in addition, freshwater treatment is ineffective in eliminating macroalgae (Verlaque et al., 2007a). Shellfish aquaculture is the main vector of non-indigenous macrophytes in the Mediterranean (42%), ahead of the Suez Canal, fouling, and aquariums. Unsurprisingly, the main region of origin is the North-East Pacific (mainly Japan and Korea) (83% of the taxa) and the main primary recipient area in the Mediterranean is Thau Lagoon (66%). Most of the known attempts of NIS to become introduced have been successful (89%; Table 1). The cases of failure (casual species: recorded once, or for a few years, then apparently extinct) are those of Grateloupia patens, Lomentaria flaccida, Ulva californica, Botrytella parva, Ascophyllum nodosum, Ectocarpus siliculosus var. hiemalis, and Saccharina japonica.

Primary vectors (from the region of origin to the recipient area) can be identical to secondary vectors (from a recipient area acting as a hub to a new recipient area), or different. Undaria pinnatifida arrived at Thau Lagoon via a primary vector, i.e., directly from the Bay of Mangoku-Ura (Japan), with imported oyster spat (Figure 3; [START_REF] Boudouresque | L'algue japonaise Undaria pinnatifida (Phaeophyceae, Laminariales) en méditerranée[END_REF]. Subsequently, Thau Lagoon acted as a hub for the dispersal of the species to new areas, via secondary vectors. In some cases, the secondary vector was similar to the primary one, namely the transfer of adult oysters from one aquaculture basin to another [START_REF] Occhipinti Ambrogi | Biotic invasions in a Mediterranean lagoon[END_REF]; adult transfers are common in the oyster business, oysters beginning to grow in one basin (e.g., Thau) and then being transferred to another (e.g., Marennes-Oleron, Atlantic coast of France) for the final affinage (refining process). Secondary vectors can also be different: fouling on ship hulls or escape from a seaweed farm (Figure 4; [START_REF] Boudouresque | L'algue japonaise Undaria pinnatifida (Phaeophyceae, Laminariales) en méditerranée[END_REF][START_REF] Brault | L'algue alimentaire humaine, perspectives de développement en France[END_REF][START_REF] Floc'h | Undaria pinnatifida (Laminariales, Phaeophyta) 12 years after its introduction into the Atlantic Ocean[END_REF][START_REF] Cecere | Undaria pinnatifida (Fucophyceae, Laminariales) spread in the central Mediterranean: its occurrence in the Mar Piccolo of Tarento (Ionian Sea, southern Italy)[END_REF]. Solieria filiformis probably arrived in the Mediterranean via shipping (fouling on ships' hulls: primary vector), but was subsequently spread by oyster transfers (secondary vector) [START_REF] Verlaque | CIESM atlas of exotic species[END_REF].

The impact of invasive species on native species and/or native ecosystems is well known and therefore of major concern: species extinction, e.g., on islands, local extinctions, functional extinctions and disruption of the ecosystem functioning (e.g., [START_REF] Boudouresque | Population dynamics of Caulerpa taxifolia in the Mediterranean, including the mechanisms of interspecific competition[END_REF][START_REF] Schmitz | Biological invasions: a growing threat[END_REF][START_REF] Balmford | Trends in the state of nature and their implications for human well-being[END_REF][START_REF] Boudouresque | Large-scale disturbances, regime shift and recovery in littoral systems subject to biological invasions[END_REF][START_REF] Clavero | Invasive species are a leading cause of animal extinctions[END_REF]. However, most studies consider the impact of a given invasive species on native species, rather than the collective impact of a pack of invasive species (Boudouresque, 1999a). In fact, a large number of introduced species can coexist in the same ecosystem, as occurs in e.g., coastal lagoons, along the Tuscany coast (Italy) and in the eastern Mediterranean basin [START_REF] Piazzi | Evaluation of benthic macroalgal invasion in a harbour area of the western Mediterranean Sea[END_REF][START_REF] Piazzi | Invasion of alien macroalgae in different Mediterranean habitats[END_REF][START_REF] Boudouresque | Biological Invasion: The Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF]. Introduced species can of course compete with each other [START_REF] Piazzi | Effects of competition between two introduced Caulerpa[END_REF]. Thau Lagoon illustrates the disruption of the ecosystems and seascapes by a host of introduced macroalgae, which may outnumber the native species, and represent the bulk of the biomass of primary producers in some habitats. [START_REF] Boudouresque | Biological Invasion: The Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF] have even compared Thau Lagoon with a "Japanese botanical garden": overall, 121 putatively native species and 58 introduced species, most of them of Japanese origin, currently occur there (Figures 5,6). Putatively native species include cryptogenic species (sensu [START_REF] Carlton | Biological invasions and cryptogenic species[END_REF], i.e., species which are today more or less cosmopolitan, but were probably introduced in most parts of their current range, before the 18th century and the beginning of the Linnean Era; Ulva rigida (Table 2 andFigure 6) is probably a cryptogenic species. On rocky shallow substrates, the seascape is dominated by introduced species, which can account for 97-99% and 48-95% of the biomass in spring and autumn, respectively (Figures 5,6; [START_REF] Boudouresque | Biological Invasion: The Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF]. Many introduced macroalgae are also present, although in lesser numbers, in the Lagoon of Venice (28 species) [START_REF] Occhipinti Ambrogi | Biotic invasions in a Mediterranean lagoon[END_REF][START_REF] Sfriso | Spreading and autoecology of the invasive species Gracilaria vermiculophylla (Gracilariales, Rhodophyta) in the lagoons of north-western Adriatic Sea (Mediterranean Sea, Italy)[END_REF][START_REF] Sfriso | On the occurrence of Uronema marinum Womersley (Chaetophorales, Chlorophyta) in the north-western lagoons of the Adriatic Sea, Mediterranean Sea (Italy)[END_REF][START_REF] Marchini | Current status and trends of biological invasions in the Lagoon of Venice, a hotspot of marine NIS introductions in the Mediterranean Sea[END_REF][START_REF] Sfriso | Invasion of alien macroalgae in the Venice lagoon, a pest or a resource?[END_REF] and in the Mar Piccolo di Taranto lagoon (13 species) [START_REF] Petrocelli | Successions of phytobenthos species in a Mediterranean transitional water system: the importance of long term observations[END_REF].

A Flow of Propagules That Can Flood Macrophyte Forests

The canopy-forming long-lived Carpodesmia mediterranea (Cystoseira mediterranea; Ochrophyta, Stramenopiles), which forms a dense belt on rocky reefs, at the upper part of the infralittoral zone (sensu [START_REF] Pérès | Nouveau manuel de bionomie benthique de la mer Méditerranée[END_REF], from the mean sea level down to 1 m depth, has experienced marked fluctuation of its abundance since the 1970s in French Catalonia [START_REF] Gros | Le genre Cystoseira sur la côte des Albères[END_REF][START_REF] Blanfuné | The ups and downs of a canopy-forming seaweed over a span of more than one century[END_REF]; the most realistic explanation for the episodes of sharp decline of C. mediterranea seems to be the invasion of the habitat by dense stands of the mussel Mytilus galloprovincialis. Mussels could reduce the survival rate of C. mediterranea recruits that settle on its shell because of the vulnerability of this substrate (prone to be pulled out by waves) and the low longevity of the mussels. In addition, there is a light deficit for the recruits of C. mediterranea that grow directly on the rocky substrate. Extreme storms are not uncommon in French Catalonia (Figure 7; [START_REF] Thibaut | Long-term decline of the populations of Fucales (Cystoseira spp. and Sargassum spp.) in the Albères coast (France, North-western Mediterranean)[END_REF][START_REF] Blanfuné | The ups and downs of a canopy-forming seaweed over a span of more than one century[END_REF]FIGURE 7 | A stand of Carpodesmia mediterranea (= Cystoseira mediterranea), in French Catalonia, severely disrupted by mussels of which the recruits probably come from offshore mussel farms.

see Navarro et al., 2011, for Spanish Catalonia). The flow of mussel larvae could be due to the development of offshore mussel farms, off Sète, ∼100 km upstream, transported by the Northern Mediterranean Current toward Catalonia [START_REF] Blanfuné | The ups and downs of a canopy-forming seaweed over a span of more than one century[END_REF]. A similar negative impact of mussel proliferation on seaweeds has already been reported, e.g., in Sweden [START_REF] Lundälv | Long-term trends in algaldominated rocky subtidal communities on the Swedish west coast -A transitional system?[END_REF].

A Vector of Diseases of Metazoans of Which the Extirpation Changes the Functioning of Recipient Ecosystems

The transfer of farmed species from one region to another has not only been a vector for accompanying epibionts ("hitch-hikers"), but also for microbial pathogens and for metazoan parasites [START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF][START_REF] Weitzman | Far-field and near-field effects of marine aquaculture[END_REF]. Usually, it is not in the interests of a pathogen or a parasite to kill its host: if the host dies, they will die too. As a result, in the native range, coevolution has often shaped reciprocal adaptation: an equilibrium between a moderate parasite virulence and a moderate host response [START_REF] Combes | Ecologie et Évolution du Parasitisme[END_REF][START_REF] Combes | Les Associations du Vivant[END_REF]. However, especially in marine environment, some pathogens have interest to kill their host to maximize their dissemination using hydrodynamic connectivity (e.g., [START_REF] Fuhrman | Marine viruses and their biogeochemical and ecological effects[END_REF][START_REF] Van Kan | Licensed to kill: the lifestyle of a necrotrophic plant pathogen[END_REF][START_REF] Avrani | Virus-host swinging party in the oceans: incorporating biological complexity into paradigms of antagonistic coexistence[END_REF]. In the absence of quarantine, farmed non-indigenous species can transfer their pathogens and parasites to native species (host shift). If their virulence is too high, they extirpate native new hosts. Movements of oysters from California to Maine, Washington and France brought the haplosporidian (kingdom Rhizaria) Bonamia ostreae, which caused the severe decline of the flat oyster Ostrea edulis [START_REF] Barber | Impacts of bivalve introductions on marine ecosystems: a review[END_REF][START_REF] Ruesink | Introduction of non-native oysters: ecosystem effects and restoration implications[END_REF]. The actual origin of a number of parasites and viral diseases which affect marine species is unclear, although some of them could be of Asian origin (e.g., [START_REF] Comps | Infection virale associée à la 'maladie des branchies' de l'huître portugaise Crassostrea angulata Lmk[END_REF]Comps et al., 1976;[START_REF] Comps | Les infections virales associées aux épizooties des huîtres du genre Crassostrea[END_REF][START_REF] Ruesink | Introduction of non-native oysters: ecosystem effects and restoration implications[END_REF][START_REF] Miossec | Alien species alert: Crassostrea gigas (Pacific oyster)[END_REF].

In the Mediterranean, the fan mussel Pinna nobilis, the largest Mediterranean mollusk (over 1 m tall), is a key species in the Posidonia oceanica seagrass ecosystem [START_REF] Personnic | An ecosystem-based approach to assess the status of a Mediterranean ecosystem, the Posidonia oceanica seagrass meadow[END_REF]. This long-lived species (over 40 years), endemic to the Mediterranean, is a filter-feeder [START_REF] Butler | Ecology of the pterioid bivalves Pinna bicolor Gmelin and P. nobilis L. Mar[END_REF][START_REF] Trigos | Utilization of muddy detritus as organic matter source by the fan mussel Pinna nobilis[END_REF][START_REF] Rouanet | From youth to death of old age: the 50-year story of a Pinna nobilis fan mussel population at Port-Cros Island (Port-Cros National Park, Provence, Mediterranean Sea)[END_REF]. A wasting disease (90% to near 100% mortality) occurred in Spain in 2016 [START_REF] Darriba | First haplosporidan parasite reported infecting a member of the superfamily Pinnoidae (Pinna nobilis) during a mortality event in Alicante (Spain, Western Mediterranean)[END_REF][START_REF] Vázquez-Luis | Pinna nobilis: a mass mortality event in western Mediterranean Sea[END_REF]. Subsequently, this mortality spread to the whole of the Mediterranean [START_REF] González-Wangüemert | Gene pool and connectivity patterns of Pinna nobilis in the Balearic Islands (Spain, Western Mediterranean Sea): implications for its conservation through retocking[END_REF][START_REF] Pergent | Grande nacre: conséquences pour notre façade suite aux mortalités massives en Espagne[END_REF][START_REF] Bianchi | Consequences of the marine climate and ecosystem shift of the 1980-90s on the Ligurian Sea biodiversity (NW Mediterranean)[END_REF][START_REF] Cabanellas-Reboredo | Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: a collaborative effort of scientists and citizens[END_REF]. Mortality is caused by a unicellular parasite, Haplosporidium pinnae, a species previously undescribed [START_REF] Catanese | Haplosporidium pinnae sp. nov., a haplosporidan parasite associated with mass mortalities of the fan mussel, Pinna nobilis, in the Western Mediterranean Sea[END_REF]. According to [START_REF] Darriba | First haplosporidan parasite reported infecting a member of the superfamily Pinnoidae (Pinna nobilis) during a mortality event in Alicante (Spain, Western Mediterranean)[END_REF] and [START_REF] Catanese | Haplosporidium pinnae sp. nov., a haplosporidan parasite associated with mass mortalities of the fan mussel, Pinna nobilis, in the Western Mediterranean Sea[END_REF], H. pinnae could be either an old symbiont that has changed its relation with the host, due to the environmental changes, or a recently introduced species. The latter hypothesis is by far the most probable: the new parasite presents many of the features that characterize an introduced species (see [START_REF] Ribera | Introduced marine plants, with special reference to macroalgae: mechanisms and impact[END_REF]Boudouresque, 1999a), e.g., the weak genetic diversity and the pattern of expansion. Many haplosporidans are parasites of bivalves in the north-western Pacific; some of them were introduced to Europe via shellfish transfers, with dire consequences [START_REF] Sindermann | Disease risks associated with importation of nonindigenous marine animals[END_REF][START_REF] Cabanellas-Reboredo | Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: a collaborative effort of scientists and citizens[END_REF]. The possible primary vector species of H. pinnae (Magalana gigas? Ruditapes philippinarum?) is still unidentified. Currents and ballast waters seem to have been involved in its spread within the Mediterranean [START_REF] Cabanellas-Reboredo | Tracking a mass mortality outbreak of pen shell Pinna nobilis populations: a collaborative effort of scientists and citizens[END_REF]; in addition, a putative reservoir species (Mytilus galloprovincialis?) could account for long-distance dispersal.

A Change in the Functioning of the Recipient Ecosystem Shellfish aquaculture can profoundly change the functioning of the ecosystems of the recipient lagoon or embayment by (i) removing Particulate Organic Matter (POM; including phytoplankton), via the activity of filter-feeder mollusks, (ii) removing nutrients via the export of farmed shellfish to markets, outside the aquaculture area, and (iii) via the shower of fecal pellets on the bottom compartment (with or without macrophytes), with mineralization which induces anoxy of the sediment, anoxy which can spread to the water column under low wind conditions [START_REF] Deslous-Paoli | Oyster farming impact on the environment of a Mediterranean lagoon (THAU) (Preliminary results of the OXYTHAU programme)[END_REF][START_REF] Plus | Modelling of oxygen and nitrogen cycling as a function of macrophyte community in the Thau lagoon[END_REF][START_REF] Gaertner-Mazouni | Exploring new issues for coastal lagoons monitoring and management[END_REF][START_REF] Boudouresque | Qu'est-ce qu'une lagune méditerranéenne?[END_REF][START_REF] Weitzman | Far-field and near-field effects of marine aquaculture[END_REF].

In Thau Lagoon (Occitania, France), benthic macrophytes are well developed below shellfish facilities: Zostera marina (seagrass), Chaetomorpha linum (Chlorobionta), Gracilaria spp., Halopitys incurvus, and Rytiphloea tinctoria (Rhodobionta) [START_REF] Plus | Modelling of oxygen and nitrogen cycling as a function of macrophyte community in the Thau lagoon[END_REF]. However, the baseline, before the development of shellfish aquaculture, is poorly known. According to De [START_REF] De Casabianca | Preliminary results on eelgrass regression and red seaweed dominance under increasing eutrophication (Thau Lagoon, France)[END_REF], eutrophication, including the effects of shellfish farming, drives the regression of Zostera meadows and the expansion of Gracilaria and other seaweed stands. In another lagoon, Salses-Leucate (Occitania, France), the isotopic ratio of macrobenthos (including macrophytes) was not modified underneath the oyster facilities [START_REF] Carlier | Spatial heterogeneity in the food web of a heavily modified Mediterranean coastal lagoon: stable isotope evidence[END_REF].

DICUSSION AND CONCLUSION

Global marine captures by fisheries are declining, although this trend is greatly underestimated as small-scale fishery, recreational fishery, discards and of course Illegal, Unreported and Unregulated (IUU) fishery are poorly or not at all taken into account [START_REF] Zeller | Reconstruction of coral reef fisheries catches in American Samoa, 1950-2002[END_REF][START_REF] Kaiser | Food subsidies from fisheries to continental shelf benthic scavengers[END_REF][START_REF] Pauly | Reported' versus 'likely' fisheries catches of four Mediterranean countries[END_REF][START_REF] Piroddi | Reconstruction of Italy's marine fisheries removals and fishing capacity[END_REF][START_REF] Pauly | Catch reconstructions reveal that global marine fisheries catches are higher than reported and declining[END_REF]; but see [START_REF] Hilborn | Effective fisheries management instrumental in improving fish stock status[END_REF] for an alternative view). This trend is of course dependent upon the nature and extent of fisheries management systems [START_REF] Hilborn | Effective fisheries management instrumental in improving fish stock status[END_REF]. In the Mediterranean and Black Sea, the production from fisheries declined from about 1.4-1.2 t a -1 , from 1993 to 2013, because most fisheries stocks are overfished [START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF]. In this context, aquaculture is an important option to feed a growing world population, which is estimated to reach 9 billion people by 2050 [START_REF] Massa | Aquaculture in the Mediterranean and the Black Sea: a blue growth perspective[END_REF]. It is often associated with the concepts of blue growth and blue economy, aiming at providing food and jobs and at the same time limiting the degradation of oceans and revitalizing ecosystems (Le Gouvello and Simard, 2017;[START_REF] Barbesgaard | Blue growth: savior or ocean grabbing?[END_REF]. This optimistic view of aquaculture should be seen in a more nuanced way by taking into account not only the benefits, but also the costs for natural habitats.

Fish farming is responsible for organic matter and nutrient loads that can be compared to those of small untreated sewage outfalls. At a time when, at least for the EU countries on the northern shore of the Mediterranean, the vast majority of terrestrial wastewater now passes through a sewage treatment plant [START_REF] Kalavrouziotis | Current status in wastewater treatment, reuse and research in some Mediterranean countries[END_REF], the rapid, strong increase in fish farming, and the resulting untreated loads of organic matter and nutrients, represents a paradox. A number of authors have developed rather optimistic views concerning nutrients, organic matter and trace elements which are released by fish farms and enter the recipient ecosystems. They consider that they are spatially limited, within the range of tolerable levels for the marine ecosystem and that an oligotrophic sea such as the Mediterranean is able to assimilate these loads [START_REF] Basaran | Impacts of the fish farms on the water column nutrient concentration and accumulation of heavy metals in the sediments of the eastern Aegean Sea (Turkey)[END_REF][START_REF] Puhr | The effect of in situ shading on a Posidonia oceanica meadow situated within a fish farm induced moderately nutrient enriched environment[END_REF][START_REF] De Biasi | An asymmetrical sampling design as a tool for sustainability assessment of human activities in coastal systems: a fish farming study case[END_REF]. According to [START_REF] Puhr | The effect of in situ shading on a Posidonia oceanica meadow situated within a fish farm induced moderately nutrient enriched environment[END_REF], when carefully planned, fish farms do not necessarily degrade the health status of P. oceanica meadow, but in fact facilitate a transition to an alternative stable state that is "characterized by a delicate balance between organic input from fish farms and the capacity of the system to process it"; the difference between a degraded meadow and this "alternative stable state, " as Puhr and Pikelj (2012) named it, is however unclear. Whatever the surface area of dead P. oceanica meadows resulting from fish farming, its is worth highlighting that seagrass dead mattes constitutes a kind of time bomb: the huge amounts of carbon which have been sequestrated within it over millennia can be mineralized into carbon dioxide and returned to the environment, contributing to global warming [START_REF] Pergent | Mediterranean Seagrass Meadows: Resilience and Contribution to Climate Change Mitigation[END_REF][START_REF] Pergent | Climate change and Mediterranean seagrass meadows: a synopsis for environmental managers[END_REF]. Some authors have rightly pointed out that fish farms cause the increase, by a factor of up to 4, of the overall fish abundance and species diversity, at a spatial scale larger than the immediate vicinity of the fish cages, which constitutes a positive effect on local fisheries (e.g., [START_REF] Machias | Changes in wild fish assemblages after the establishment of a fish farming zone in an oligotrophic marine ecosystem[END_REF][START_REF] Machias | Response of demersal fish communities to the presence of fish farms[END_REF]. However, it would be naïve, on the basis of a partial and erroneous view of the biodiversity concept, to extend this positive effect to the ecosystem and the environment (see [START_REF] Boudouresque | Insights into the diversity of the biodiversity concept[END_REF], for the biodiversity concept). A partial recovery of the annelid fauna of a soft bottom ecosystem, near Murcia (Spain), one year after the cessation of the activities of a fish farm, gives grounds for optimism (Aguado-Giménez et al., 2012), although the annelid assemblage is just a compartment of an ecosystem which also encompasses teleosts, echinoderms, mollusks, etc., compartments which are not addressed by the authors. It has been suggested that multitrophic aquaculture, i.e., the simultaneous farming of low trophic level organisms, such as bivalves and sea urchins, together with fish, would reduce fish waste impact, while enhancing total productivity [START_REF] Troell | Integrated mariculture: asking the right questions[END_REF][START_REF] Israel | Testing the digestibility of seabram waste in three candidates for integrated multi-trophic aquaculture: grey mullet, sea urchin and sea cucumber[END_REF]. However, this optimistic hypothesis has been contradicted by [START_REF] Navarrete-Mier | Does bivalve mollusc polyculture reduce marine fin fish farming environmental impact?[END_REF].

With the exception of Egypt, where fish farming is based on the semi-extensive production of mullet, which are omnivorous and opportunistic, most of the farmed fish species are carnivorous [START_REF] Basurco | The aquaculture situation in the Mediterranean Sea. Prediction for the future[END_REF]. In aquaculture facilities, they are fed both pelleted and mainly extruded feeds rich in protein (up to 600 g kg -1 ) and lipids (up to 260 g kg -1 ) [START_REF] Kousoulaki | Review of European sea bass (Dicentrarchus labrax), Linnaeus, 1758) nutrition and feed management: a practical guide for optimizing feed formulation and farming protocols[END_REF]. Most farmed fish diets include fishmeal and fish oil; they come from ∼14 Mt whole fish and 5 Mt by-products of seafood processing, which produces 4-5 Mt fishmeal and ∼1 Mt fish oil. Most of the fishmeal is now used to feed farmed fish [START_REF] Iffo | The Marine Ingredients Organisation[END_REF]. According to Le Gouvello and Simard (2017) and IFFO (2019), the concern that expanding aquaculture would use more and more fishmeal and fish oil, and so would contribute either to fish depletion in the seas, or to the removal of a fish resource for populations of southern countries, is misplaced: the percentage of fishmeal in used fish diet is on the decline. In contrast, other authors accuse farming of carnivorous fish of being "the aquatic equivalent of robbing Peter to pay Paul": it transforms small pelagic and other fishes perfectly fit for human consumption into animal feeds the nutritive value of which is lost to humans [START_REF] Goldburg | Future seascapes, fishing and fish farming[END_REF][START_REF] Pauly | Global trends in world fisheries: impacts on marine ecosystems and food security[END_REF][START_REF] Jacquet | Funding priorities: big barriers to smallscale fisheries[END_REF][START_REF] Grigorakis | Aquaculture effects on environmental and public welfare -the case of Mediterranean mariculture[END_REF]. It is worth noting that the pilot trial of offshore aquaculture of the sea urchin Paracentrotus lividus, on the Italian coast, was based upon anchovy and krill flour [START_REF] Volpe | Gonad quality of sea urchin Paracentrotus lividus cultured in an offshore pilot-scale trial on the south-east Italian coast[END_REF], although this sea urchin is actually a herbivore (Boudouresque andVerlaque, 2013, 2020).

The recovery times of Posidonia oceanica meadows is in the order of at least decades and may be more than a century, so that losses can be considered to be irreversible at human time scales [START_REF] Molinier | Recherches sur les herbiers de phanérogames marines du littoral méditerranéen français[END_REF][START_REF] Boudouresque | Découverte de l'herbier de posidonie[END_REF][START_REF] Holmer | Benthic primary producers -a neglected environmental problem in Mediterranean maricultures?[END_REF][START_REF] Boudouresque | Regression of Mediterranean seagrasses caused by natural processes and anthropogenic disturbances and stress: a critical review[END_REF]Boudouresque et al., , 2012)). According to [START_REF] Holon | A predictive model based on multiple coastal anthropogenic pressures explains the degradation status of a marine ecosystem: implications for management and conservation[END_REF], the tipping point (shift from a healthy P. oceanica meadow to a significantly degraded meadow) is situated, on average, 320 m from a fish farm. A safety distance of 400 m has been suggested between fish farms and the nearest P. oceanica meadow [START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]. Boudouresque et al. (2012) have proposed the following recommendations: (i) No fish farm facilities should be directly established over a P. oceanica meadow. In Spain, no fish farm facilities have been directly established over P. oceanica meadows, and the existing ones have been moved offshore; however, this is not the case in most other Mediterranean countries. (ii) If there is a meadow nearby, minimum distances from the cages should be respected. They depend upon the depth, the openness of the site (embayment or well-flushed open sea) and the size of the farm (Table 2); deep meadows are more sensitive to turbidity and overshadowing, so that farms established above deep bottoms should respect an increased safety distance from the nearest meadow. According to recent literature, these safety distances seem to be under-estimated, as nutrients release by fish farms may reach several kilometers of distance (e.g., [START_REF] Ruiz | Remote influence of off-shore fish farm waste on Mediterranean seagrass (Posidonia oceanica) meadows[END_REF]. (iii) An installation of fish farm facilities on a 45-50 m deep seabed should be given priority, whenever possible. (iv) Permission to set up a fish farm should be reviewed every 4 years for possible extension, on the basis of a demonstration that the P. oceanica meadows located nearby have not regressed. In addition, permanent monitoring plots should be established and revisited annually to assess the health of the meadow [START_REF] Holmer | Effects of fish farm waste on Posidonia oceanica meadows: synthesis and provision of monitoring and management tools[END_REF]. Physiological parameters of P. oceanica leaves and rhizomes (e.g., total nitrogen content, free amino acid concentration and composition, stable nitrogen isotope ratio) are useful indicators of marine environmental degradation [START_REF] Pérez | Physiological responses of the seagrass Posidonia oceanica as indicators of fish farm impact[END_REF]. For monitoring methods for P. oceanica meadows, see [START_REF] Boudouresque | Monitoring methods for Posidonia oceanica seagrass meadows in Provence and the French Riviera[END_REF]Boudouresque et al. ( , 2012)), [START_REF] Personnic | An ecosystem-based approach to assess the status of a Mediterranean ecosystem, the Posidonia oceanica seagrass meadow[END_REF], and [START_REF] Boudouresque | Ecosystem-based versus species-based approach for assessment of the human impact on the Mediterranean seagrass Posidonia oceanica[END_REF]. Similar recommendations could apply to coralligenous and coastal detritic ecosystems, although no minimum distance has been proposed for them. Whatever the ecosystem concerned, monitoring programs are of paramount importance (e.g., [START_REF] Borja | Assessing the suitability of a range of benthic indices in the evaluation of environmental impact of fin and shellfish aquaculture located in sites across Europe[END_REF][START_REF] De Biasi | An asymmetrical sampling design as a tool for sustainability assessment of human activities in coastal systems: a fish farming study case[END_REF].

Overall, in the Mediterranean, fish farming can have very negative impact; the impact has mostly been studied on Posidonia oceanica seagrass beds, but obviously it also concerns coralligenous and coastal detritic ecosystems. It can be direct (shading, fecal pellets) and indirect (overgrazing). Shellfish aquaculture is the major source (ahead of the Suez Canal and fouling on ship hulls) of the introduction of exotic macrophyte species which deeply disrupt lagoon landscapes and ecosystems. As far as the sea urchin seeding is concerned, fortunately, it is not very effective, which avoids aggravation of the overgrazing of marine forests.

The above conclusions do not call into question marine aquaculture, but underline the importance of taking into account its possible negative effects, in order to avoid them. This requires taking them into account at the very beginning of the project design process. In addition, some knowledge gaps for future research avenue on the thematic deserve to be more developed, e.g., (i) surveillance networks, not only of species, groups of species and functional compartments, but also on the whole ecosystem functioning; (ii) active restoration, after the cessation of aquaculture activities, versus natural restoration (in many cases, nature do things better than humans); (iii) assessing in a less empirical way, using maps of ecosystem distribution and local scale models of currents, the safety distance between aquaculture facilities and high heritage value ecosystems.

FIGURE 1 |

 1 FIGURE 1 | A Posidonia oceanica meadow severely degraded under and in the vicinity of a fish farm. Bay of Calvi (Corsica). Photo © Gérard Pergent.

FIGURE 2 |

 2 FIGURE 2 | Distribution of Posidonia oceanica meadow in1988, 1994, and 1998 in El Hornillo Bay (Murcia, Spain). Only the complete mortality off (area of dead matte) has been mapped. Original drawing, after[START_REF] Ruiz-Fernandez | Respuesta de la Fanerógama Marina Posidonia oceanica (L.) Delile a Perturbaciones Antrópicas[END_REF] andRuiz et al. (2001) data.

FIGURE 3 |

 3 FIGURE 3 | Shellfish production in the Mediterranean. For the countries which have both Atlantic and Mediterranean shores (France and Spain), Atlantic production has been removed from the statistics. Data from FAO (2019).

FIGURE 4 |

 4 FIGURE 4 | Primary (red arrow) and secondary (blue arrows) vectors of spread of Undaria pinnatifida ("brown alga," Stramenopiles) in the Mediterranean and western Europe. Only some of the sites where U. pinnatifida occurs, along the western coasts of Europe, have been represented.

FIGURE 6 |

 6 FIGURE 6 | A sketch of the dominant macrophytes on shallow (down to 1 m depth) rocky substrates of the Thau Lagoon in spring. Names on a yellow background: introduced species; blue background: probably cryptogenic introduction; white background: native species. From Boudouresque et al. (2011), modified and updated.

TABLE 1 |

 1 Seaweeds probably non-indigenous to the Mediterranean, of which the vector (or one of multiple vectors) is oyster aquaculture.

	Species	Origin	First mediterranean	Countries of current	Habitat and status
			record	occurrence
	Rhodobionta (Archaeplastida)
	Aglaothamnion halliae	E Atlantic	Venice (It), 2016	It	Lagoons, Mid
	Agardhiella subulata	W Atlantic or NE Pacific	Thau (Fr), 1984	Fr, It	Lagoons, Intro, C

The possible area of origin can be only a part of the native range of a species. Data from

[START_REF] Verlaque | Inventaire des plantes marines introduites en Méditerranée: origine et répercussions sur l'environnement et les activités humaines[END_REF]

;

[START_REF] Ribera | Introduced marine plants, with special reference to macroalgae: mechanisms and impact[END_REF]

,

[START_REF] Boudouresque | Biological Invasion: The Thau Lagoon, a Japanese biological island in the Mediterranean Sea[END_REF]

;

[START_REF] Petrocelli | Alien marine macrophyes in transitional water systems: new entries and reappearances in a Mediterranean coastal basin[END_REF]

,

[START_REF] Marchini | Current status and trends of biological invasions in the Lagoon of Venice, a hotspot of marine NIS introductions in the Mediterranean Sea[END_REF]

;

[START_REF] Verlaque | CIESM atlas of exotic species[END_REF]

,

[START_REF] Kawai | Phylogeographic analysis of the brown alga Cutleria multifida (Tilopteridales), Phaeophyceae) suggests a complicated introduction history[END_REF]

;

[START_REF] Ocaña | Massive proliferation of a dictyotalean species (Phaeophyceae, Ochrophyta) through the Strait of Gibraltar (Research note)[END_REF]

,

[START_REF] Wolf | Shellfish import and hull fouling as vectors for new red algal introductions in the Venice Lagoon[END_REF][START_REF] Sempere-Valverde | Expansion of the exotic brown alga Rugulopteryx okamurae (E.Y. Dawson)[END_REF]

, updated. Status: Cas, casual; Intro, introduced (established); Inv, Invasive; Mid, midlittoral; RR, very rare; R, rare; C, common; CC, very common. Countries of current occurrence: Al, Algeria; Cy, Cyprus; Eg, Egypt; Es Spain; Fr, France; Gib, Gibraltar; Gr, Greece; Hr, Croatia; Is, Israel; It, Italy; Lb, Lebanon; Ly, Libya;, Mo, Morocco, including Spanish cities),

Mon, Monaco; Slo, Slovenia; Sy, Syria; Tn, Tunisia; Tr, Turkey. a

  In gray, the combination of factors resulting in exclusion. Open sites or not open sites: located outside a bay or within an embayment. AfterBoudouresque et al. (2012).

			<100 m 100-200 m 200-300 m 300-400 m >400 m
	<5 m	Open			<100 t	<500 t
		Not open				<100 t
	5-10 m Open		<100 t	<500 t	<1 000 t
		Not open			<100 t	<500
	10-20	Open	<100 t	<500 t	<1 000 t <2000 t
		Not open		<100 t	<500 t	<1 000 t
	20-40 m Open		<100 t	<500 t	<1 000 t
		Not open			<100 t	<500 t
	>40 m	Open	<500 t	<1000 t	<2 000 t <5000 t
		Not open	<100 t	<500 t	<1 000 t <2000 t

Frontiers in Marine Science | www.frontiersin.org

April 2020 | Volume 7 | Article 218

ACKNOWLEDGMENTS

We are indebted to Michael Paul, a native English speaker, for proofreading the text, to four reviewers for the valuable suggestions, and to the editors of this Research topic, especially to Margarida Casadevall and Jonathan Richir.

AUTHOR CONTRIBUTIONS

All authors equally contributed to the writing of the manuscript.

Conflict of Interest:

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.