L Vanalderweireldt
email: lucie.vanalderweireldt@gmail.com

C Albouy

F Le Loc'h D

R Millot

C Blestel

M Patrissi

M Marengo

J Garcia

C Bousquet

C Barrier

M Lefur

P.-A Bisgambiglia

A Donnay

S Ternengo

A Aiello

P Lejeune

E D H Durieux

Ecosystem modelling of the Eastern Corsican Coast (ECC): Case study of one of the least trawled shelves of the Mediterranean Sea

Keywords:

Ecosystem modelling of the Eastern Corsican Coast (ECC): Case study of one of the least trawled shelves of the Mediterranean Sea

Introduction

The Mediterranean Sea is an oligotrophic semi-enclosed basin known to be a key reservoir of biodiversity [START_REF] Estrada | Life and the productivity of the open sea[END_REF][START_REF] Coll | The biodiversity of the Mediterranean Sea: estimates, patterns, and threats[END_REF]. Its coast has been inhabited for millennia, and the current marine ecosystems reflect the combination of multiple anthropogenic pressures over time [START_REF] Durrieu De Madron | Marine ecosystems' responses to climatic and anthropogenic forcings in the Mediterranean[END_REF][START_REF] Liquete | Ecosystem services sustainability in the Mediterranean Sea: assessment of status and trends using multiple modelling approaches[END_REF]. Moreover, these ecosystems support essential processes and functions that provide valuable ecosystem services and benefits to human societies [START_REF] Liquete | Ecosystem services sustainability in the Mediterranean Sea: assessment of status and trends using multiple modelling approaches[END_REF]. However, the Mediterranean Sea has been negatively altered in numerous ways, including habitat degradation, the spread of invasive species, water pollution, global environmental change, and the overexploitation of resources (Coll et al., 2012;[START_REF] Du Pontavice | Climate change undermines the global functioning of marine food webs[END_REF]. These anthropogenic pressures threaten the ecosystem balance and sustainability, particularly in relation to fishing and its collateral effects, identified as the principal human disturbance of coastal systems [START_REF] Jackson | Historical overfishing and the recent collapse of coastal ecosystems[END_REF]. In this context, the priority is to assess the overall fishing pressure and the relative impact of fishing activities on ecosystems. In 2008, the European Union adopted the Marine Strategy Framework Directive to boost the overall protection of marine ecosystems around Europe [START_REF] Dsf | Directive 2008/56/EC of the European Parliament and of the council of 17 June 2008[END_REF]. The conservation strategies supported by this directive require an initial assessment of the targeted marine ecosystem's current and optimal ecological status and a characterization of the consequences of fishing in the ecosystem.

Since the 1990s, fishery scientists have reconsidered their approach to stock assessment, shifting from single-species or multispecies stock assessments to an understanding of whole-community dynamics when affected by the exploitation activities and the overall consequences for the ecosystem [START_REF] Pauly | Fishing down marine food webs[END_REF][START_REF] Gascuel | The trophic-level based model: a theoretical approach of fishing effects on marine ecosystems[END_REF][START_REF] Cury | Ecosystem oceanography for global change in fisheries[END_REF]. Stock assessments, supported by statistical models, usually focus on complementary aims, i.e., tactical vs. strategic management planning, including the characterization of fishery impacts and the environmental drivers in regard to fish stock condition, as well as estimates of the sustainable yield. However, species within an ecosystem are affected by both direct, e.g., predation, and indirect interactions, e.g., competition or underlying trophic links among species assemblages [START_REF] Corrales | Future scenarios of marine resources and ecosystem conditions in the eastern Mediterranean under the impacts of fishing, alien species and sea warming[END_REF]. Consequently, the ecosystem-based approach to management (EAM) considers how fisheries target species and affect the populations interacting with the targeted species; this approach allows simulating the exploitation impact on the entire marine ecosystem [START_REF] Coll | Contributions of food web modelling to the ecosystem approach to marine resource management in the Mediterranean Sea[END_REF]. Thus, the EAM's holistic approach aims to ensure the sustainable use of marine resources while preserving ecosystem integrity.

With the establishment of the EAM concept and progress in monitoring and computing, much more data have been collected on mar ine ecosystems. These new inputs have driven the development of ecosystem modelling, i.e., the mathematical representation of systems, and several modelling tools have emerged. For example, OSMOSE, an individual-based model (IBM), focuses on fish species grouped into schools of similar size, weight, age, taxonomy, and geographical location (2D model, [START_REF] Shin | Exploring fish community dynamics through size-dependent trophic interactions using a spatialized individual-based model[END_REF]. Atlantis is a 3D ecosystem model with a coarse spatial resolution that accounts for an ecosystem's biophysical, economic, and social components [START_REF] Fulton | Approaches to end to end ecosystem models[END_REF].

Among these ecosystem representations, Ecopath is the most common model applied globally, as it is well documented and freely accessible through a userfriendly interface and permits understanding complex marine systems [START_REF] Christensen | Ecopath with Ecosim: methods, capabilities and limitations[END_REF]. Ecopath is a mass-balance model that characterizes the trophic structure and functioning among the species groups representing the main ecosystem components [START_REF] Christensen | Ecopath with Ecosim: methods, capabilities and limitations[END_REF][START_REF] Christensen | Ecopath with Ecosim Version 6. User Guide[END_REF]. These groups are either monospecific or comprise several ecologically similar species in terms of their diet, predators, size, and/or life cycles. The EcoTroph model, based on Ecopath outputs, enables modelling the ecosystem as a continuum of biomass across trophic levels (TL) ascending the food chain through either predation or ontogenic processes [START_REF] Gascuel | The trophic-level based model: a theoretical approach of fishing effects on marine ecosystems[END_REF]Gascuel and Pauly, 2009). EcoTroph is a trophic level-based model in which the concept of species is no longer explicitly represented, and ecosystem functioning is simplified as a trophic flow of biomass to provide a simple means of ret hinking an ecosystem and the impacts of fisheries on these systems.

The application of ecosystem models such as Ecopath or EcoTroph has been recognized as highly relevant for understanding ecosystem functioning and exploring the effects of fishing on an ecosystem of interest [START_REF] Pauly | Ecopath, Ecosim, and Ecospace as tools for evaluating ecosystem impact of fisheries[END_REF]. Here we apply these integrative tools to the Eastern Corsican Coast (ECC), one of the least exploited marine ecosystems of the Mediterranean Sea [START_REF] Relini | Synthesis of the knowledge on bottom fishery resources in Central Mediterranean (Italy and Corsica)[END_REF][START_REF] Coll | The biodiversity of the Mediterranean Sea: estimates, patterns, and threats[END_REF][START_REF] Manach | Preliminary estimate of total marine fisheries catches in Corsica, France (1950-2008)[END_REF][START_REF] Ferra | Mapping change in bottom trawling activity in the Mediterranean Sea through AIS data[END_REF]. Corsica is the fourthlargest island in the Mediterranean Sea and is considered a hotspot for conservation priorities, given its significant biodiversity. For example, coastal Corsica contains one of the most extensive meadows of Posidonia oceanica seagrass, classified as a Natura 2000 area [START_REF] Miniconi | Inventaire de la faune ichtyologique de la reserve naturelle de Scandola (Corse, ´ Mediterra´ nee nord-occidentale)[END_REF][START_REF] Telesca | Seagrass meadows (Posidonia oceanica) distribution and trajectories of change[END_REF]INPN-MNHN, 2019). In the ECC, marine exploitation is multidisciplinary and includes a wide diversity of small-scale fisheries, including fish nets, spiny lobster nets, longlines, and spiny lobster traps (Le [START_REF] Manach | Preliminary estimate of total marine fisheries catches in Corsica, France (1950-2008)[END_REF][START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF]. The ECC is also distinguished by its wide continental shelf that allows trawling by small fishing vessels-perceived as a large-scale or industrial fishery-whereas the rest of the island has remained untrawlable [START_REF] Ferra | Mapping change in bottom trawling activity in the Mediterranean Sea through AIS data[END_REF]. In addition, the ECC is characterized by one of the lowest fishing intensity rates in the northern Mediterranean Sea, and it can be considered a 'reference state' because of its groundfish diversity (Merigot et al., 2007´ ). Since 1950, the number of fishers has been slowly declining in Corsica, including in the ECC (Le [START_REF] Manach | Preliminary estimate of total marine fisheries catches in Corsica, France (1950-2008)[END_REF]. The decline of fishing as a profession results from both its lack of attractiveness for next-generation candidates and the non-renewal of licenses for trawling (CRPMEM unpublished data). Even so, the decline of the fishing effort and its resulting impacts on this ecosystem have never been assessed.

To overcome these limitations, we aim to produce relevant tools to guide managers in assessing fishing pressure in the context of ecosystem sustainability within the limits of available knowledge. For this purpose, we develop an ecosystem model to, for the first time, characteriz e the ECC ecosystem structure, understand its ecosystem functioning, and document the effects of fishing on this marine ecosystem. This ECC modelling offers an unprecedented opportunity to jointly assess the impact of small-scale fisheries and trawling within Corsican coastal waters. We first develop an Ecopath model as a snapshot of the system. We then use Ecopath outputs to develop an EcoTroph model and perform simulations of fishery exploitation to characterize the ECC ecosystem response.

Materials and methods

The eastern Corsican coast (ECC): Site description and fishery

Because of its diversity of seascapes and habitats, Corsica hosts many fish and invertebrate species and is considered a biodiversity hotspot [START_REF] Miniconi | Inventaire de la faune ichtyologique de la reserve naturelle de Scandola (Corse, ´ Mediterra´ nee nord-occidentale)[END_REF]. The island is inhabited by many emblematic species protected through regulations, including the dusky grouper (Epinephelus marginatus), common dentex (Dentex dentex), and spiny lobster (Palinurus elephas; [START_REF] Bodilis | Recruitment of the dusky grouper (Epinephelus marginatus) in the North-Western Mediterranean Sea[END_REF][START_REF] Marengo | Catch variation and demographic structure of common dentex (Sparidae) exploited by Mediterranean artisanal fisheries[END_REF][START_REF] Expert | Cartographie d´etaillee des habitats marins[END_REF]. The island's coastal waters are protected by several marine protected areas (MPAs), including the Scandola Natural Reserve, the Bonifacio Straits Natural Reserve (BNSR), and, more recently, the Cap Corse and Agriate Marine Natural Park (OEC, 2011). The ECC forms a singular spatial unit characterized by a continental shelf composed of soft sloping sands and extensive meadows of Posidonia oceanica seagrass, a phanerogam species endemic to the Mediterranean Sea. The P. oceanica habitat of the ECC is among the largest classified Natura 2000 areas in relation to its spread and state of conservation [START_REF] Telesca | Seagrass meadows (Posidonia oceanica) distribution and trajectories of change[END_REF]INPN-MNHN, 2019).

Throughout the ECC, coastal fishery exploitation is limited to artisanal fisheries and consists of a combination of deploying fishing nets and trawling, operated respectively by 26 and 9 fleet vessels. Net fishers, classified as small-scale fisheries, operate with small boats (<12 m long) and use a wide diversity of gears and techniques. This equipment is typically staffed by a single or a pair of fishers. In contrast, trawler vessels are considered large-scale fisheries, having larger boats (>12 m long) handled by several fishers. Three predominant fishing activities are found on the basis of the targeted species, gear, depth, and period during which the gear operates [START_REF] Radley-Gardner | Fundamental Texts on European Private Law[END_REF]: (i) the spiny lobster fleet, defined as any fishing operation using a trammel or gillnet set at 50-150 m depth for a minimum of 48 h; (ii) the fish net fleet, defined as any fishing operation using a trammel or gillnet set at 15-120 m depth for a maximum of 48 h; and (iii) trawling, defined as fish trawling set for one hour at 50-200 m depth. At a smaller scale, bottom longlines, fishing traps, and recreational fisheries are less widely operated in the ECC relative to fishing nets and trawling, and these fisheries are not sufficiently documented for this period (data from the DACOR project used in [START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF] and data from the Marte+ project. Hence, these latter activities were not included in the ECC model.

We developed the Ecopath model of the ECC as an average annual state over two consecutive years (2012)(2013), determined by data availability. The ECC ecosystem covers 1930 km 2 , extending from the cities of Porto-Vecchio to Bastia, and the marine waters have a maximum depth of 200 m (Fig. 1). As a coastal ecosystem, the area deeper than 50 m only covers 489 km 2 , i.e., 25.4% of the total area. The ECC is primarily covered with sand and P. oceanica seagrass, accounting respectively for 1668 km 2 and 258 km 2 , i.e., 86.4% and 13.4% of the ecosystem area. Between 2012 and 2013, the mean near-surface sea temperature (NSST) was 18.7 • C, fluctuating between 13.0 and 26.2 • C (NASA, 2018).

The Ecopath model: Principles and functioning

We modelled the ECC ecosystem using the Ecopath approach via the open-source software Ecopath with Ecosim v.6.6.5. An Ecopath model is a snapshot of the ecosystem structure produced by applying two master equations separately for each model group, one describing the production term and the other presenting the energy balance (Supplementary data 1). To solve both equations, we required two conditions. First, we had to i mplement at least three of four basic parameters of each group i, i.e., the biomass B (t⋅ km -2 ⋅ yr -1 ), the production per unit of biomass P/B (yr -1 ), the consumption rate per unit of biomass Q/B (yr -1 ), and the ecotrophic efficiency EE (unitless). Second, the model requires implementing parameters involving total fishery catch Y (t⋅ km -2 ⋅ yr -1 ), the net migration rate E (t⋅ km -2 ⋅ yr -1 ), the biomass accumulation rate BA (t⋅ km -2 ⋅ yr -1 ), the assimilation of food intake A (t⋅ km -2 ⋅ yr -1 ), and the diet composition DC (unitless, Supplementary data 1 and 2). Ecopath can then perform as many linear equations as there are gr oups implemented within the model to solve for unknown parameters.

The ECC model

The Ecopath model we developed for the ECC encompasses 39 groups, including 5 groups of primary producers and detritus, 14 invertebrate groups, 2 groups of Chondrichthyes, 16 groups of teleosts, 1 seabird group, and 1 group of cetaceans (Table 1). In addition, 14 of Table 1 Parameters of the Ecopath model of the Eastern Corsican Coast (ECC) ecosystem by species or group (Ecopath-estimated values in bold). TL, trophic level (unitless); B, biomass (t⋅ km - 2 ⋅ yr -1 ); P/B, production/biomass (yr -1 ); Q/B, consumption/biomass (yr -1 ); EE, ecotrophic efficiency (unitless); P/Q, production/consumption (unitless); OI, omnivory index (unitless); Z, total mortality (yr -1 ); M2, predation mortality (yr -1 ); KS, keystone index (unitless, [START_REF] Libralato | A method for identifying keystone species in food web models[END_REF]; Y, landings (t⋅ km -2 ⋅ yr -1 ); F/Z, ratio between fishing mortality F and total mortality Z; Acc, the fisheries accessibility (unitless). Groups/species are presented ordered by trophic level. Supra ceph, suprabenthic cephalopods; Benth sharks, benthic sharks; Benth ceph, benthic cephalopods; BIF, benthic invertebrate feeders; SIF, suprabenthic invertebrate feeders. these 39 groups are monospecific, either to isolate species of high interest for fisheries or because of their unique ecological role within the system. Because the model objective is to quantify the effects of fisheries on the ECC ecosystem, groups including exploited species are more represented than nontargetted groups.

An exhaustive list of all species groups (mono-and multispecific groups) inhabiting the ecosystem is provided in Supplementary data 3.

Primary producers and detritus

To estimate phytoplankton biomass, we combined satellite ocean colour observations (NASA, 2018) and data from the literature (Supplementary data 3). Next, we determined the biomass of P. oceanica as the average aboveground biomass per meter [START_REF] Gobert | Variations at different spatial scales of Posidonia oceanica (L.) Delile beds; effects on the physico-chemical parameters of the sediment[END_REF] and extrapolated to the meadow surface area (Donia Expert, 2020, Supplementary data 3). We estimated macroalgal biomass using the average weight per meter from [START_REF] Sargian | Contribution a la cartographie de la bande rocheuse infralittorale et de l'herbier a Posidonia oceanica dans la Baie de la Revellata (Calvi)[END_REF] and extrapolated to the surface area of rocky bottom (Donia Expert, 2020, Supplementary data 3). Then, we assessed the detritus biomass from the empirical equation of [START_REF] Christensen | Flow characteristics of aquatic ecosystems[END_REF] using POC values (IFREMER unpublished data). We collected the P/B estimates for primary producer groups in the literature sel ected because of the geographical proximity of these studies to Corsica (Supplementary data 3).

Invertebrates

We assessed invertebrate biomass from field data collected for macro-and megafauna [START_REF] Pelaprat | Mise en oeuvre du controle de surveillance au titre de la directiveˆ cadre eau pour l'indicateur benthos de substrat meuble (Eaux cotˆ ieres)[END_REF], in addition to the information we extracted from the MEDITS trawling survey data (IFREMER, 2019) corrected with catchability parameters [START_REF] Moutopoulos | Toward an ecosystem approach to fisheries in the Mediterranean Sea: multi-gear/multi-species implications from an ecosystem model of the Greek Ionian Sea[END_REF]. We calculated the biomass of the urchin Paracentrotus lividus according to [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] and projected to rock surface area below 10 m (Donia Expert, 2020, Supplementary data 3). We collected the P/B and Q/B parameters either from empirical equations (Eq. 1, [START_REF] Allen | Relation between production and biomass[END_REF]Eq. 2, Brey, 2001;Eq. 3, Pauly, 1989, Christensen and[START_REF] Christensen | Flow characteristics of aquatic ecosystems[END_REF] or from published studies selected because of their geographical proximity to Corsica and temporal overlap (Supplementary data 3). We estimated benth ic groups' diets from literature selected once again for their spatiotemporal overlap with this study and weighted each parameter per the relative biomass of each species (Supplementary data 3). Eq. 1 states

Z = P/B = M +F (1)
where Z represents the total mortality per year, M is the natural mortality per year, and F is the mortality induced by fisheries per year. Eq. 2 has

where Amax is the maximum age observed (year), Mmax is the maximum observed body mass (kJ) converted from wet weight [START_REF] Mills | The structure and dynamics of shelf and slope ecosystems off the north-east coast of North America[END_REF], and T is the mean water temperature ( • C). Eq. 3 is

′ log(Q/B) = 7.964-0.204logW∞ -1.965T +0.083A+0.532h+0.398d (3)
where W∞ is the asymptotic weight (g), T ′ is the mean environment temperature expressed as 1000/( • C + 273.15), A is the aspect ratio of the caudal fin (unitless), h and d, two unitless variables, are indicators of herbivory (h = 1, d = 0), detritivory (h = 0, d = 1), and carnivory (h = 0, d = 0). The asymptotic weight W∞ was estimated using the length-weight relationship based on the asymptotic length L∞ and published allometric coefficients. We chose the groups' parameters from the literature for their spatiotemporal overlap with the study. We then weighted the model's input parameters according to each species' relative biomass.

Fish

We performed cluster analyses on the diet matrix to assign each teleost species to a trophic guild. First, we built the diet matrix from the literature selected for their spatiotemporal overlap, prioritizing the study areas nearest to Corsica and the 2012-2013 period. Second, we conducted cluster analyses on gut-content studies using the average linkage method with Euclidean distance coefficients. The average linkage method provided the highest cophenetic correlation values compared with other clustering methods and was therefore the approach that produced the best clustering model for the distanc e matrix [START_REF] Borcard | Numerical Ecology with R[END_REF]. We selected the optimum number of groups according to silhouette widths and Mantel statistics. Finally, we ensured the consistency between teleost groups by verifying fish length, TL, and water column habitat recognized in the literature, FishBase (Froese and Pauly, 2009), and fish landings. We performed the cluster analyses using R software v.3.6.2 (R Core Team, 2020) running the vegan [START_REF] Oksanen | Vegan: community ecology package, R package version 1[END_REF], the cluster [START_REF] Maechler | Cluster: cluster analysis basics and extensions[END_REF], and the glcus [START_REF] Hurley | Gclus: clustering graphics[END_REF] packages.

We calculated the biomass of Chondrichthyes and teleosts from different sources. For the bathymetric stratum (0-50 m), we used the fish net catch data (fishery database cited in [START_REF] Marengo | Catch variation and demographic structure of common dentex (Sparidae) exploited by Mediterranean artisanal fisheries[END_REF][START_REF] Pere | Evaluation of Homarus gammarus (Crustacea: Decapoda: Nephropidae) catches and potential in a Mediterranean small-scale fishery[END_REF][START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF], and for the bathymetric stratum 50-200 m, we retrieved the information from the MEDITS trawling survey data (IFREMER, 2019), corrected by catchability parameters. These latter parameters were extracted from [START_REF] Moutopoulos | Toward an ecosystem approach to fisheries in the Mediterranean Sea: multi-gear/multi-species implications from an ecosystem model of the Greek Ionian Sea[END_REF] by comparing MEDITS data and the biomass used in an Ecopath model for the Calalan Sea [START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF]. We estimated the P/B and Q/B parameters from empirical equations (Eq. 1, [START_REF] Allen | Relation between production and biomass[END_REF]Eq. 3, Pauly, 1989, Christensen and[START_REF] Christensen | Flow characteristics of aquatic ecosystems[END_REF]Eq. 4, Gascuel et al., 2008) and weighted the parameters according to the relative biomass of each species. Eq. 4 has P/B = 3.55τ -0.66 K 0.80 , where τ is the TL of the group (unitless), and K is the growth coefficient (unitless) extracted from FishBase (Froese and Pauly, 2009).

Shags and dolphins

The ECC model encompassed two monospecific groups: the European shag (Phalacrocorax aristotelis) and the bottlenose dolphin (Tursiops truncatus). We estimated the parameters B, P/B, and Q/B from visual surveys and the literature chosen because of their geographical proximity to Corsica (Supplementary data 3).

Landings, discards, and fish mortality

Fishing data were collected from March to September 2012-2013 by scientists aboard 14 fleet vessels (fishery database cited in [START_REF] Marengo | Catch variation and demographic structure of common dentex (Sparidae) exploited by Mediterranean artisanal fisheries[END_REF], [START_REF] Pere | Evaluation of Homarus gammarus (Crustacea: Decapoda: Nephropidae) catches and potential in a Mediterranean small-scale fishery[END_REF][START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF]). Over the last decade, sampling efforts have been deployed to characterize the Corsican fisheries, including the composition of conserved and discarded catches and estimates of annual production. For all fishing activities (spiny lobster fleets, fish net fleets, and trawling), catches including discards were identified to the lowest possible taxonomic level, counted, measured, and the discard mortality recorded. To estimate each catch weight, we used the weight-length relationship [START_REF] Le Cren | The length-weight relationship and seasonal cycle in gonad weight and condition in the perch (Perca fluviatilis)[END_REF] based on published allometric coefficients selected according to their geographical proximity to Corsica. Then, we assessed catches separately for the spiny lobster fleet, fish net fleet, and trawling nets as the product of the biomass per unit effort (BPUE, g⋅ 50 m of net) and the total fishing effort (Et; [START_REF] Farrugio | A sampling strategy and methodology for assessment and monitoring of Mediterranean small-scale fisheries[END_REF][START_REF] Pere | Evaluation of Homarus gammarus (Crustacea: Decapoda: Nephropidae) catches and potential in a Mediterranean small-scale fishery[END_REF][START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF]Eq. 5).

Et = Nfd ×Nf ×Nn

(5) where Et, in relation to fishing gear, is the total number of either net pieces or trawling hours deployed in one year, Nfd is the annual number of fishing day trips (days), Nf represents the annual number of fishers, and Nn is the number of net pieces or trawling hours occurring per fishing day. We ensured the consistency of the ECC production by comparing our results with Le [START_REF] Manach | Preliminary estimate of total marine fisheries catches in Corsica, France (1950-2008)[END_REF] and [START_REF] Bousquet | Assessment of catch composition, production and fishing effort of small-scale fisheries: the case study of Corsica Island (Mediterranean Sea)[END_REF].

Balancing the model

To balance the model, we automatically adjusted the diet matrix to reduce the proportion of higher TLs consuming food. Groups ' diets were adjusted slightly to account for species' abundances in the ecosystem. In addition, we added diet imports for Sphyranidae, bottlenose dolphin, Seriola dumerili, Chondrichthyes, and zooplankton by assuming intra-annual migrations outside of the system (Supplementary data 2). Then, we verified the model's pedigree, estimated at 0.58, and PREBAL diagnostics [START_REF] Link | Adding rigor to ecological network models by evaluating a set of pre-balance diagnostics: a plea for PREBAL[END_REF]. PREBAL diagnostics include step-by-step verifications of biomass, biomass ratios, vital rates, vital rate ratios, total production, and total removals across all taxa and TLs (Supplementary data 5).

Mixed trophic impact analysis and keystoneness

The mixed trophic impact (MTI) analysis quantifies direct and indirect trophic effects that may be caused by a hypothetical i ncrease of the group's biomass, including fishing fleets [START_REF] Ulanowicz | Mixed trophic impacts in ecosystems[END_REF][START_REF] Christensen | Ecopath with Ecosim Version 6. User Guide[END_REF]. The output of the MTI analysis is an n × n matrix, representing the interaction between the impacting group i and the impacted group j as log(P/B) = 1.672+0.993×log(1/Amax)-0.035×logMmax -300.447×(1/(T +273))

(2)

MTIi,j = DCi,j -FCj,i (6) where DCi,j is the diet composition term expressing how much j contributes to the diet of i, and FCj,i represents the proportion of the predation on j because of i as a predator. In addition, we supplemented the MTI with the keystoneness (KSi) analysis proposed by [START_REF] Libralato | A method for identifying keystone species in food web models[END_REF]. This index has the particularity of attributing high KS values to keystone groups that can have either a low or high biomass proportion in the system.

The EcoTroph model

Starting from the previous Ecopath balanced model as a reference state, we performed the 'transpose' routine to develop the EcoTroph trophic level-based model [START_REF] Gascuel | The trophic-level based model: a theoretical approach of fishing effects on marine ecosystems[END_REF], Gascuel and Pauly, 2009, Gascuel et al., 2009, Supplementary data 9a). We then undertook a sensitivity analysis on the smooth parameter to obtain a continuous parameter distribution along the TLs (Gascuel et al., 2009, Supplementary data 9b). The smooth parameter is based on the idea that the average TL of a group does not fully reflect its function within an ecosystem, as all species within a given group are exactly the average TL. Using EcoTroph, we first simulated the current exploitation state of the ECC by using the default settings of 0.12 for the smooth parameter, 0.60 for the top-down parameter, and 0.80 for the form-D parameter (Gascuel et al., 2009).

The EcoTroph model assumes that only a fraction of the ecosystem biomass is accessible to fisheries, referred to as the fisheries accessibility (unitless, Table 1). We estimated fisheries accessibility (i) for multispecies groups, as the percentage of fished species within a group; (ii) when fisheries targeted all species of a group, accessibility estimated as the fraction of the inhabited habitat area per the overall ecosystem area; and (iii) by setting the accessibility value to zero for unfished groups (Table 1). Then, we performed the 'diagnosis' routine to simulate the spectral distortions induced by fisheries and changes in the exploitation patterns, applying various multiplying factors to the fishing effort (Gascuel et al., 2009). Using the EcoTroph model, we simulated the current exploitation state and compared outputs to three states of exploitation scenarios: i) an extreme scenario in which the total fishing effort increases fivefold; ii) the cessation of the trawl fishery; and iii) the cessation of the trawl fishery combined with a two-fold increase in net fishing. We then plotted isopleths of the simulated fishing effort fluctuations on total catches and catch TLs, i.e., a two-dimensional contour graph of two variable functions along which the function has a constant value so that the curve joins points of equal value.

Comparison of Mediterranean fisheries

We compared the fishing exploitation levels of the ECC to eight Mediterranean models: the Bay of Calvi, Corsica [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF], the Gulf of Lions [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF], the Catalan Sea [START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF], the Adriatic Sea [START_REF] Coll | An ecological model of the northern and Central Adriatic Sea: analysis of ecosystem structure and fishing impacts[END_REF], the Aegean Sea [START_REF] Tsagarakis | Food-web traits of the North Aegean Sea ecosystem (eastern Mediterranean) and comparison with other Mediterranean ecosystems[END_REF], Port-Cros National Park [START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF], the protected marine areas of Portofino [START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF], and the Gulf of Pagasitikos [START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF]. We chose these published models because of their spatial proximity to the ECC and their accessibility on EcoBase, i.e., the open-access database platform (http://ecobase.eco path.org/). To this end, we selected eight indicators of the fishing pressure: 1) the total catch (Y, t⋅ km -2 ⋅ yr -1 ); 2) the mean TL of the catches (TLc); 3) the gross efficiency (GE), defined as the ratio between catch and net primary production [START_REF] Pauly | Primary production required to sustain global fisheries[END_REF][START_REF] Pauly | Fishing down marine food webs[END_REF]; and 4) the probability of an ecosystem being sustainably fished (Psust, Libralato et al., 2008). We also selected 5) the estimated loss in production index (L, Libralato et al., 2008, Eq. 7), defined as

-PPR × TETLC-1 L = 1 × ln(TE) (7) P
where PPR represents the primary production required to sustain fisheries (t⋅ km -2 ⋅ yr -1 ), TE is the transfer efficiency (t⋅ km -2 ⋅ yr -1 ), TLc is the mean trophic level of catches, and P1 represents the autotrophic and detritus production within the trophic web (t⋅ km -2 ⋅ yr -1 ). Finally, we extracted three indicators of the overall fishing pressure from the literature that were not generated from Ecopath models: 6) the estimated number of op erating trawling vessels (FPtrawl) or 7) other fishing activities (FPnot trawl, [START_REF] Colloca | Recent trends and impacts of fisheries exploitation on Mediterranean stocks and ecosystems[END_REF]; and 8) the number of AIS messages (AIS, automatic identification system ship tracking data; [START_REF] Vespe | Mapping EU fishing activities using ship tracking data[END_REF]. AIS messages originated from large trawlers only (bottom otter, beam, and midwater trawls) and contain information on the time, position, direction, and speed of fishing vessels >15 m in length [START_REF] Vespe | Mapping EU fishing activities using ship tracking data[END_REF]. We identified operating trawlers according to speed profiles, which allowed characterizing the fishing grounds and mapping the intensity of use of the offshore fishing areas. We selected the Mediterranean models according to the availability of fishing indicators and used principal component analysis (PCA) to compare the models. PCA was performed using R v.3.6.2 (R Core Team, 2020), running the ade4 [START_REF] Thioulouse | Multivariate Analysis of Ecological Data with ade4[END_REF] and the factoextra [START_REF] Kassambara | Factoextra: extract and visualize the results of multivariate data analyses[END_REF] packages [START_REF] Borcard | Numerical Ecology with R[END_REF].

Results

Characterization of the ECC ecosystem

General outputs of Ecopath model

The ECC model was structured into five TLs, ranging from primary producers to top predators, with a TL 4.8 for Sphyraenidae, TL 4.6 for Epinephelus marginatus, and TL 4.4 for Dentex dentex (Table 1, Supplementary data 6, 7). The group of Sarpa salpa, feeding on primary producers, had the lowest TL values for fish at 2.0. Groups of invertebrates obtained TLs from 2.0 for the sea urchin Paracentrotus lividus to 3.9 for suprabenthic cephalopods.

The biomass of primary producers dominated the ecosystem model, followed by invertebrates, teleosts, and Chondrichthyes, at 91.6%, 3.8%, 4.2%, and 0.4%, respectively, of the total biomass, excluding detritus (Table 1). P. oceanica seagrass alone accounted for 90.1% of total biomass. Planktivorous fish were the most abundant group of vertebrates, followed by invertebrate groups of zooplankton and echinoderm, at 3.4%, 1.4%, and 1.2% of the ecosystem biomass, respectively. The ECC ecosystem supported a high biomass of Chondrichthyes, representing 9% of the total fish biomass.

For many groups, ecotrophic efficiency (EE) from inputs or estimated parameters in the Ecopath model were close to one, especially for low TLs (Table 1). Although the biomass of primary producers was dominated by seagrass, seagrass (EE = 0.3) was less assimilated into the system than phytoplankton (EE = 0.6). In contrast, three groups had efficiency values below 0.1, emphasizing their low exposure to predation and fishing (shag, dolphin, and Sphyraenidae). However, the low EE must be interpreted with caution, given that some groups might be subject to predation, especially during the juvenile stage, or are fished outside the system. Moreover, Sarpa salpa is neither fished (its flesh is tasteless) nor hunted, possibly because of its large size and schooling behaviour.

The ratio between production and consumption (P/Q) encompassed expected values [START_REF] Christensen | Ecopath with Ecosim Version 6. User Guide[END_REF]. The highest values were found for groups of Labridae, benthic crustaceans, followed by shrimps, ranging from 0.3 to 0.4 (Table 1). In contrast, (P/Q) values were very low (0.01) for shags and dolphins.

The system omnivory index (OI), measuring the distribution of the trophic interactions among TLs, was highest for dolphins, Seriola dumerili, Sphyraenidae, and benthic sharks, exceeding one (Table 1). In contrast, eight groups had (OI) values below 0.05 (Chromis chromis, benthic invertebrate feeders (BIF), planktivorous taxa, Sciaena umbra, Dentex dentex, worms).

The analysis of the energy fluxes emphasized a main bentho-pelagic pathway, connecting primary producers to the higher TLs in the food web through the benthic invertebrate and benthic fishes compartments to pelagic fishes (Fig. 2). This group of zooplankton proved to be an essential connector to the energy flow ascent, characterized by a high degree of connections to planktivorous fish. Similarly, the model suggested that shrimps were a major link to connect energy flows towards benthic fishes, cephalopods, and Chondrichthyes (Supplementary data 2 and 6). Groups of zooplankton, P. oceanica seagrass, and phytoplankton presented the highest flow values to detritus, at 48.4%, 23.5%, and 19.8%, respectively, of the total flows ( Fig. 2).

In the ECC ecosystem, mortality was primarily induced by predation for most groups of lower TLs, especially for phytoplankton and zooplankton (Table 1). Hence, our results emphasized three groups almost unaffected by predation in terms of their status as top predators in the system: Scorpaena scrofa, Dentex dentex, and Epinephelus marginatus.

Analysis of mixed trophic impact (MTI) and keystone groups

The system comprises top-down types of control from top predators, such as Scorpaena scrofa, piscivorous fish, and sharks (Fig. 3, Supplementary data 8). Some groups of intermediate TLs had a significant trophic effect on the ecosystem, such as benthic cephalopods, planktivorous taxa, and Sparidae. The top-down control of the spiny lobster (P. elephas) on the sea urchin Paracentrotus lividus, a species limited by macroalgal biomass, illustrated a direct impact. A similar form of trophic cascade occurred for BIF, controlled top-down by benthic sharks and at the same time, bottom-up controlled by shrimps.

Groups of echinoderms and zooplankton exerted a high trophic effect on themselves through cannibalism. Finally, few groups in the system were affected by primary producers groups at the base of the food web, such as macroalgae exerting a bottom-up control on sea urchin P. lividus and seagrass P. oceanica on Sarpa salpa. Following the MTI analyses, analyses of keystone groups highlighted the leading role of zooplankton, shrimps, benthic cephalopods, and planktivorous fish (Table 1, Fig. 3).

Fisheries effect on the ECC ecosystem

Along the ECC, biomass extracted by fishing activities represented 463.2 t annually for the entire ecosystem, i.e., 0.5% of the system biomass of combined vertebrates, cephalopods, and crustaceans (Table 1). Total catches of fisheries, estimated at 0.2 t km -2 ⋅ yr -1 , were primarily through trawling (56.1%), lobster net (22.7%), and fish net (21.1%; Table 1; Fig. 4) activities. When all fishery activities were combined, catches included mainly groups of benthic sharks (23.6%), planktivorous fish (21.0%), rays (12.7%), and piscivorous fish (12.2%, Fig. 4a). Fishing fleets targeted distinct ecosystem groups, reflected in the average TLs of total catches (fish net: 3.7, lobster net: 3.7, trawling: 3.5, Table 1). net catches.

Trawling in the area primarily involved planktivorous fish (37.2%), sharks (27.0%), and Sparidae (11.3%, Fig. 4b). The fish netting was marked by high landings of rays (27.0%), piscivorous teleosts (22.4%), Dentex dentex (15.5%), and sharks (14.1%, Fig. 4c). The spiny lobster net fishery affected mainly groups of rays (29.7%), piscivorous teleosts (28.7%), and sharks (24.7%, Fig. 4d). Moreover, two monospecific groups, of Scorpaena scrofa and Palinurus elephas, showed high exploitation rates (F/Z > 0.2) (Table 1). However, our results also highlighted the moderate effect of fisheries, as gross efficiency (GE) was below 0.01%. MTI analysis confirmed the strong control of trawling currently exercised within the ECC system ( Fig. 3). Indeed, trawling has a significant top-down impact on both Sphyranidae and sharks as the top predators of rays and BIF, with consequences continuing down the food chain. MTI analysis also emphasized that fishing nets have lower trophic impacts than trawling. Important top-down controls were noted for fish nets on Dentex dentex and lobster nets on P. elephas (Supplementary data 8).

Comparing the ECC to an unfished state, using the EcoTroph model, our simulation emphasized that the biomass of TLs >3.5 was most affected by fishingleading to a gradual biomass decrease as TL increased (Fig. 5a). The extreme scenario of a fivefold increase in fishing effort showed the most pronounced response relative to the reference state. In the extreme scenario, the biomass flow increased from TL 2.5 to 3.5 and then dropped by half for TLs >3.5. Simulations also suggested that the suppression of trawling resulted in a clear improvement of top-predator abundance. In contrast, the trawling ban and the doubling of net effort resulted in a decrease in top-predator biomass relative to the current state.

Fishing effort fluctuations illustrated using isopleths showed that total catches increased uniformly with fishing pressure and that trawling and net fishing did not compete with one another (Fig. 5b,c). The isopleths produced by EcoTroph indicated that enhanced trawling and net fisheries had a low effect on the mean TL of catches (Fig. 5d).

Comparison of Mediterranean fisheries

Overall, the PCA performed on fishing indicators was explained by the first two components, and Mediterranean models were correctly represented in the two-level factorial design (i.e., 68.1%, Fig. 6). The PCA highlighted the importance of four fishing variables: the FPtrawl, TLc, Psust, and L. FPtrawl, the estimated number of operating trawling vessels, and TLc, the mean TL of catches, explained most of the first component (PC1: 45.0%). The right factorial plane included the Catalan Sea, Portofino, and the Adriatic Sea, areas highly frequented by trawlers. In contrast, the right part of the graph, composed of the ECC, Calvi, and Port-Cros models, corresponded to less-trawled systems. TLc is a proxy indicator of the exploitation level that an ecosystem faces. Hence, TLc decreased in highly exploited ecosystems, as fisheries target primarily top predators having high TLs. PC1 emphasized along its axis the high values of TLc in the ECC, Calvi, and Port -Cros systems. Moreover, the second axis was driven by Psust, the probability of the system being in a sustainable fishing situation, and L, the loss in production index (PC2: 23.1%). Psust and L values were lowest for the Gulf of Lions model and highest for the ECC model. 4. Discussion

Ecosystem functioning

The Ecopath model of the ECC reveals a high degree of development, reflecting a mature ecosystem. The model statistics indicate a developed system on the basis of (i) its high value of total system throughput (TST = 3598.56 t⋅ km -2 ⋅ yr -1 ), representing 'the size of the entire system in terms of flow' [START_REF] Ulanowicz | Growth and Development: Ecosystem Phenomenology[END_REF];

(ii) its ratio between primary production and respiration approaching 1 (Pptot/Rtot = 1.73, [START_REF] Odum | Analyse strategique ´ regionale de la´ Corse : enjeux et propositions de creation d'aires marines pro´ t´egees[END_REF]); (iii) its high biomass and low production rates (Ptot/Btot = 1.77), suggesting a high abundance of slow-growing groups [START_REF] Christensen | Flow characteristics of aquatic ecosystems[END_REF]; and (iv) the development of the ecosystem network to the uppermost trophic levels (Table 1, Supplemental material 7). When [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF], Gulf of Lions [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF], Catalan Sea [START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF], Adriatic Sea [START_REF] Coll | An ecological model of the northern and Central Adriatic Sea: analysis of ecosystem structure and fishing impacts[END_REF], Aegean Sea [START_REF] Tsagarakis | Food-web traits of the North Aegean Sea ecosystem (eastern Mediterranean) and comparison with other Mediterranean ecosystems[END_REF], Port-Cros National Park [START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF], protected marine areas of Portofino [START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF], and the Gulf of Pagasitikos [START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF]. Y, total catch of all fisheries combined (t⋅ km -2 ⋅ yr -1 ), TLc, mean TL of catches; GE, gross efficiency; Psust, probability of an ecosystem being sustainably fished; L, loss of production index; FPtrawl, estimated number of trawling vessels; FPnot trawl estimated number of vessels engaged in other fishing activities; AIS, density of AIS messages classified as 'fishing'. comparing these results with other Mediterranean ecosystems-selected for their proximity and similarities-we note that the TST values are similar to those of the Gulf of Gabes model (TST = 3798.99 t⋅ km -2 ⋅ yr -1 , [START_REF] Halouani | Modelling food web structure using an end-to-end approach in the coastal ecosystem of the Gulf of Gabes (Tunisia)[END_REF]. Both models present similar, gently sloping continental shelves and a maximum depth not exceeding 200 m. In regard to previous Corsican region models, the Pp/R and P/B of the ECC ranged between values recorded for Calvi (Pp/R = 0.62, P/B = 1.56; [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF] and those for the Bonifacio Straits Natural Reserve (BSNR, Pp/R = 1.12, P/B = 8.91; [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF]. Although these systems share very similar biodiversity values, the Calvi and BSNR systems differ from the ECC by the former systems' rocky coasts and dominant species. Species inhabiting soft substrates, such as Chondrichthyes, are favoured in the ECC and are more prominent than observed for the Calvi and BSNR regions [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF][START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF][START_REF] Lauria | Predictive habitat suitability models to aid conservation of elasmobranch diversity in the Central Mediterranean Sea[END_REF]. The dissimilarity between these systems may involve significant changes in trophic cascades affecting the overall system balance. The top TLs of the ECC are consistent w ith the range of previous models developed for the Mediterranean Sea, as the highest TLs were > 4.00 for Port-Cros [START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF], the Gulf of Lions [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF], the Gulf of Gabes [START_REF] Halouani | Modelling food web structure using an end-to-end approach in the coastal ecosystem of the Gulf of Gabes (Tunisia)[END_REF], and Portofino [START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF]. The Corsican models of Calvi and BSNR had Sphyraenidae and pelagic and piscivorous fish as the top predators, having TLs from 4.30 to 4.96 [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF][START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF].

In the mature ecosystem of the ECC, we identified several TL controls, including a top-down control exercised by top predators, a wasp-waist control at intermediate TLs, and a bottom-up control of primary producers. The mixed trophic analysis (MTI) emphasizes that top predators, including red scorpionfish (Scorpaena scrofa), piscivorous fish, and sharks, shape their prey dynamics; these prey are striped red mullet (Mullus surmuletus), brown meagre (S. umbra), BIF, and rays. The MTI also suggests that benthic cephalopods exert a wasp-waist control in the ECC, their abundance affecting positively dusky grouper (Epinephelus marginatus) and negatively lobster (Homarus gammarus). Moreover, over the last decades, the imposed fishing moratorium on grouper around Corsica prohibits recreational fishing, both spearfishing and hook-and-line fishing (Arretˆ´e n • 2013,357-0001). In agreement with [START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF], we suggest that benthic cephalopods could provide an effective monitoring tool for conservation considering that their biomass could become-or already is-a limiting food item for the dusky grouper. However, using benthic cephalopods as an indicator for management must be undertaken with caution given the short lifespan, fast growth, and opportunist feeding behaviour of these species can lead to marked population fluctuations [START_REF] Arkhipkin | Stock assessment and management of cephalopods: advances and challenges for short-lived fishery resources[END_REF]. The observed benthic cephalopod increase could also involve the decrease of Homarus gammarus, as the exploited stock of European lobster is suspected to be low and vulnerable around Corsica [START_REF] Pere | Evaluation of Homarus gammarus (Crustacea: Decapoda: Nephropidae) catches and potential in a Mediterranean small-scale fishery[END_REF].

MTI also highlights a bottom-up control of P. oceanica seagrass that controls the abundance of the sparid teleost Sarpa salpa, its most important grazer [START_REF] Bunuel | Warming intensifies the˜ interaction between the temperate seagrass Posidonia oceanica and its dominant fish herbivore Sarpa salpa[END_REF]. Similar to [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF], we had previously assumed that the foraging activity of S. salpa induced a top-down control of the seagrass. Our results suggest that the seagrass meadows are so well developed in the ECC that herbivory does not threaten their expansion. Moreover, we suspect that the ECC provides an optimum habitat for S. salpa with its shallow habitats of mixed meadows/sand cover. Because S. salpa is not targeted by fisheries or predation, we assume that this population has already reached the asymptotic stage, probably limited by the ecology of its young stages, growth rates, its strategy of monandric protogyny, or even the number of eggs. Considering these points, it is unlikely that the top-down control of S. salpa on Posidonia meadows would increase unless the meadows were to decline.

Keystone groups/species revealed by the model highlight the importance of zooplankton, shrimps, benthic cephalopods, and planktivorous teleosts for maintaining ecosystem balance. Zooplankton, the most abundant group of animals in the ECC model, include a wide range of phyl a, sizes, and functions, e.g., ichthyoplankton, thereby explaining its keystone function. As expected, we noted widespread cannibalism within the zooplankton and a notable MTI effect on this group. Shrimps, as the third-most abundant group of animals, include a wide diversity of suprabenthic species of crustaceans and provides a key food to forage fish species, favouring the rapid transfer of energy to higher TLs. However, these results should be interpreted with caution, given that both zooplankton and shrimp biomass were estimated by the model, thereby involving a high degree of uncertainty in terms of the related energy flows. In the ECC model, benthic cephalopods also play a crucial role as opportunists, feeding on 12 different benthic groups. Benthic cephalopods are also pr eyed upon mainly by higher TLswith benthic cephalopods also preying on a wide range of TLs. As revealed by MTI, the benthic cephalopods exercise a wasp-waist control, and their biomass fluctuation thus has a marked impact on coastal food webs. Cephalopods also have high keystone index values in the Portofino [START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF], Port-Cros [START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF], and the Gulf of Lions [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF] models. Finally, the planktivorous group was dominated by Spicara smaris and S. maena, in agreement with M´erigot et al. (2007). Recently, Androm´ede Oceanologie identified the ECC as ´ one of the largest habitats for Spicara spp., containing many hectares of spawning grounds. The ECC model emphasizes planktivorous fish as a key group probably because of its dominance in biomass, i.e., 43.34% of the total animal biomass of the model.

Fisheries pressure on the ECC

Our results underline the moderate impact of fisheries on the ECC, one of the least exploited ecosystems of Mediterranean sys tems. These findings also emphasize the unequal fishing pressure among groups, with fisheries having a particular impact on TLs >3.50 and untargeted groups, including benthic sharks and rays.

The biomass value extracted by fisheries (0.24 t⋅ km -2 ⋅ yr -1 ) is close to the estimates for the BNSR (0.19 t⋅ km -2 ⋅ yr -1 ; [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF] and Port-Cros (0.32 t⋅ km -2 ⋅ yr -1 ; [START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF] and are distinctly lower than for other Mediterranean models [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF][START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF][START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF][START_REF] Tsagarakis | Food-web traits of the North Aegean Sea ecosystem (eastern Mediterranean) and comparison with other Mediterranean ecosystems[END_REF][START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF][START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF][START_REF] Michailidis | Modelling the role of alien species and fisheries in an Eastern Mediterranean insular shelf ecosystem[END_REF]. Fishing pressure in the ECC model, however, proved to be unequal among groups as: (i) sharks, planktivorous fish, rays, and piscivorous fish represent the highest catches, and (ii) exploitation rates (F/Z > 0.20) are notable for Scorpaena scrofa and Palinurus elephas. In addition, most catches include untargeted groups, particularly sharks, rays, and planktivorous fish, primarily discarded dead because of their low market value and poor resistance to stress when entangled in nets. Chondrichthyes, caught dead, remain essentially unexploited in the present state throughout the Mediterranean Sea [START_REF] Guijarro | Inter-and intra-annual trends and status indicators of nektobenthic elasmobranchs off the Balearic Islands (northwestern Mediterranean)[END_REF]. Caught Chondrichthyes in the ECC encompass protected species classified as vulnerable (Myliobatis aquila, Dasyatis pastinaca, [START_REF] Serena | Myliobatis aquila. The IUCN Red List of Threatened Species[END_REF][START_REF] Serena | The IUCN Red List of Threatened Species[END_REF] and even endangered (Rostroraja alba, [START_REF] Ellis | Rostroraja alba. The IUCN Red List of Threatened Species[END_REF]. Moreover, the Chondrichthyes biomass estimated for the ECC (2.75 t⋅ km -2 ⋅ yr -1 ) is much higher than for other Mediterranean systems-from 0.02 t⋅ km -2 ⋅ yr -1 in the Ionian Sea [START_REF] Moutopoulos | Toward an ecosystem approach to fisheries in the Mediterranean Sea: multi-gear/multi-species implications from an ecosystem model of the Greek Ionian Sea[END_REF] to 0.69 t⋅ km -2 ⋅ yr -1 in the Gulf of Gabes [START_REF] Halouani | Modelling food web structure using an end-to-end approach in the coastal ecosystem of the Gulf of Gabes (Tunisia)[END_REF] from studies where Chondrichthyes are included, as this is not always the case [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF][START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF][START_REF] Corrales | Future scenarios of marine resources and ecosystem conditions in the eastern Mediterranean under the impacts of fishing, alien species and sea warming[END_REF][START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF]. The high abundance of Chondrichthyes in the ECC is probably because of vast expanses of sandy bottom [START_REF] Lauria | Predictive habitat suitability models to aid conservation of elasmobranch diversity in the Central Mediterranean Sea[END_REF] and low fishing pressure, especially from trawlers [START_REF] Walls | Tracking the rising extinction risk of sharks and rays in the Northeast Atlantic Ocean and Mediterranean Sea[END_REF], leading to their high catch rates.

By comparing the exploited EEC to an unfished state, our simulation emphasizes that all exploitation activities combined affe ct, in particular, groups and species having a TL above 3.50, reducing the abundance of these groups/species by nearly 30%. These results can be explained by the fact that fisheries value and retain resources of high TL, thereby shaping the structure of marine ecosystems. In the EEC, the mean TL of catches is 3.60, whereas it ranges from 3.07 (Adriatic Sea, [START_REF] Coll | An ecological model of the northern and Central Adriatic Sea: analysis of ecosystem structure and fishing impacts[END_REF]) to 3.94 (Aegean Sea, Tsagarakis et al., 2010) in the Mediterranean Sea [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF][START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF][START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF][START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF][START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF]. Previous models developed for Corsica show higher values of TL catches, i.e., 3.75 for the Calvi model [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF]) and 3.94 for the BSNR [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF]. This difference may be explained by the unselective gear used by trawling fisheries included in the ECC model; this significantly reduces the mean TL of catches. Considering only the catches by fish net and spiny lobster net, values for TL catches ranged between 3.71 and 3.77. In addition, the other two existing Corsican models are based on different fishing data sources. T he BNSR model distinguished itself by integrating recreational fishing activities (spearfishing, boat fishing, and shore fishing), which may justify this contrast. Yet the ECC model encompasses three species of intermediate TLs having a high economic value: Palinurus elephas, Homarus gammarus, and Paracentrotus lividus. Catches of spiny lobster P. elephas, are often discarded alive, as regulations prohibit catches of spiny lobsters carrying eggs, individuals captured outside the fishing season (October-February), and lobsters below a size limit [START_REF] Marengo | A typical Mediterranean fishery and an iconic species: focus on the common spiny lobster (Palinurus elephas, Fabricius, 1787) in Corsica[END_REF]. These three species are poorly represented within the ECC, as their ecological niche of rocky habitats is scarce [START_REF] Pere | Evaluation of Homarus gammarus (Crustacea: Decapoda: Nephropidae) catches and potential in a Mediterranean small-scale fishery[END_REF][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], explaining their low catch rates. Finally, the structure of the models themselves may lead to attribute distinct TLs to the same groups, explaining the difference.

Trawling, accounting for half of the total catches, was the most non-selective gear with the broadest ranging impact on the ecosystem. This activity exerts the strongest top-down control on the ecosystem, followed by a significant trophic cascade, as revealed by MTI. Yet in recent years, the number of trawl licenses has been reduced continuously, with expectations that trawling exploitation will likely cease over the next decades (CRPMEM, unpublished data). Simulations also suggest that the trawling fisheries removal induces a noticeable improvement in the abundance of top predators, which can be counterbalanced or even enhanced by an increase in net fishing effort. It is worth noting that trawling was responsible for 64% to 92% of Chondrichthyes catches; hence, dealing with trawling issues is critical for managing their conservation. Furthermore, our study confirms that trawling does not compete w ith the lobster net and fish net fisheries, given that the increase in one fishery will not impact the landings from another. Our results also suggest that the current exploitation rate only marginally influences mean-catch TLs. We expect the ecosystem TL to diminish in an overexploited system as fisheries eliminate the top TL predators.

When comparing fishing exploitation in the Mediterranean Sea, PCA-driven by the density of trawlers (FPtrawl), the mean TL of catches (TLc), the probability that the system would be in a sustainable fishing state (Psust), and the loss in production index (L)-confirmed the ECC as one of the least exploited Mediterranean ecosystems. Our PCA designates the Catalan Sea [START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF], the Adriatic Sea [START_REF] Coll | An ecological model of the northern and Central Adriatic Sea: analysis of ecosystem structure and fishing impacts[END_REF], and the Gulf of Lions [START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF] as the most exploited systems. The ECC is a unique system in the Mediterranean Sea and is distinguished from the Corsican model of Calvi [START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF], despite the two models having the same artisanal fishing pressures. However, our PCA results have some caveats given that (i) Mediterranean models differ in their periods of simulation, development, and compartment structure; (ii) the availability of the models' database forced us to omit some models having characteristics similar to the ECC, such as the Gulf of Gabes [START_REF] Halouani | Modelling food web structure using an end-to-end approach in the coastal ecosystem of the Gulf of Gabes (Tunisia)[END_REF], Cyprus Island [START_REF] Michailidis | Modelling the role of alien species and fisheries in an Eastern Mediterranean insular shelf ecosystem[END_REF], and the BNSR [START_REF] Albouy | Simulation of the combined effects of artisanal and recreational fisheries on a Mediterranean MPA ecosystem using a trophic model[END_REF]; and (iii) some fishing activities of the ECC model are missing, including bottom longlines, fishing traps, and recreational fisheries (CRPMEM unpublished data).

Conclusions

-The Ecopath model of the Eastern Corsican Coast (ECC) shows this mature ecosystem to have a high degree of development.

-Four groups represent critical links in the ecosystem for moving biomass from primary producers to predators: zooplankton, shrimps, benthic cephalopods, and planktivorous teleosts. Benthic cephalopods exercise a wasp-waist control, and fluctuations in their biomass thus have significant consequences on the coastal food web, including for their predator Epinephelus marginatus. -MTI analysis emphasizes that trawling fisheries induce significant controls on top-predator biomass and thus substantially alter the ecosystem balance.

-Benthic sharks, planktivorous fish, and rays represent the most important groups in terms of catch, despite often not being targeted or even kept by fisheries.

Trawling fisheries accounted for 64% to 92% of Chondrichthyes catches; therefore, trawling issues are critical for managing C hondrichthyes conservation, especially considering that some Chondrichthyes species are classified as vulnerable or endangered. Furthermore, our study suggests that the ECC waters, among the least harvested of the Mediterranean Sea, provide a suitable habitat and a shelter for Chondrichthyes development as reflected by their high biomass. These results raise into question the contribution of the ECC as a sink of Chondrichthyes and whether populations from nearby areas may supply this system. -Fishery indicators confirm that fishing pressure on the ECC ecosystem is low to moderate, although being at the same time one of the least exploited trawlable shelves in the Mediterranean Sea. Simulations performed using the EcoTroph model suggest that the ecosystem would eventually be able to support an increase in the fishing effort as a whole system, although impacting more drastically high trophic levels. However, these res ults must be taken with caution, given that we did not include bottom longlines, fishing traps, and recreational fisheries in our model. -This first ecosystem model of the ECC is an initial component of a more far-reaching project that aims to model the entire Corsican coast. This research will provide a relevant tool for managers, offering guidance for future decisions and conservation measures.

help regarding trawling catchability, and the Projet GDEGeM for the Tursiop data. We are also especially grateful to all Corsican fishermen for their tremendous goodwill and kindness in welcoming scientific observers onboard their boats. A special thanks to Dr. Murray Hay from Maxafeau Editing Services for linguistic revisions.

Fig. 1 .

 1 Fig. 1. Map of the Eastern Corsican Coast (ECC) ecosystem in the north-western Mediterranean Sea. The two heavy lines delimit the northern and southern boundaries of the eastern shelf ecosystem, and the 200 m isobath demarks the eastern border. Table 1 Parameters of the Ecopath model of the Eastern Corsican Coast (ECC) ecosystem by species or group (Ecopath-estimated values in bold). TL, trophic level (unitless); B, biomass (t⋅ km -

Fig. 2 .

 2 Fig. 2. Flow diagram of the ECC ecosystem aggregated into a Lindeman spine and organized by integer trophic level (TL) and detritus ( D). TST, total system throughput; TE, transfer efficiency.

Fig. 3 .

 3 Fig. 3. Mixed trophic impact (MTI) analysis of the Eastern Corsican Coast (ECC). We included only the absolute values of MTI >0.5. Arrow thickness is in function of impact value for both positive (dotted grey arrows) and negative (solid red arrows) impacts. SIF, suprabenthic invertebrate feeders; BIF, benthic invertebrate feeders; Benth. ceph., benthic cephalopods; Zoopl., zooplankton. Keystone groups/species (represented in bold green) were identified according to Libralato et al. (2006). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 4 .

 4 Fig. 4. Composition of total catches (%) of the Eastern Corsican Coast (ECC) ecosystem of (a) all combined fleets, (b) trawling, (c) fish net, and (d) spiny lobster

Fig. 5 .

 5 Fig. 5. (a) Trophic spectra of relative biomass (unitless) by fishing scenario of the Eastern Corsican Coast (ECC) ecosystem. We calculated the relative biomass as the ratio between biomass values of the simulated fishing scenario and unexploited state. The horizontal line of relative biomass equal to one corresponds to the unexploited state. Isopleths of scenarios simulating the fluctuation of fishing effort of the ECC ecosystem on (b) the trawling catches (t⋅ km -2 ⋅ yr -1 ), (c) the net catches (t⋅ km -2 ⋅ yr -1 ), and (d) the catch trophic level (TL). Net fisheries included the spiny lobster and fish netting; mE is a multiplier of the fishing effort.

Fig. 6 .

 6 Fig. 6. Principal component analysis (PCA) of fishing pressure indicators estimated in the models of the Mediterranean Sea: the Bay of Calvi, Corsica[START_REF] Pinnegar | Predicting indirect effects of fishing in Mediterranean rocky littoral communities using a dynamic simulation model[END_REF], Gulf of Lions[START_REF] Banaru | Trophic structure in the˘ Gulf of lions marine ecosystem (North-Western Mediterranean Sea) and fishing impacts[END_REF], Catalan Sea[START_REF] Coll | Trophic flows, ecosystem structure and` fishing impacts in the south Catalan Sea, northwestern Mediterranean[END_REF], Adriatic Sea[START_REF] Coll | An ecological model of the northern and Central Adriatic Sea: analysis of ecosystem structure and fishing impacts[END_REF], Aegean Sea[START_REF] Tsagarakis | Food-web traits of the North Aegean Sea ecosystem (eastern Mediterranean) and comparison with other Mediterranean ecosystems[END_REF], Port-Cros National Park[START_REF] Valls | Modeling trophic interactions to assess the effects of a marine protected area: case study in the NW Mediterranean Sea[END_REF], protected marine areas of Portofino[START_REF] Prato | Assessing interacting impacts of artisanal and recreational fisheries in a small marine protected area (Portofino, NW Mediterranean Sea)[END_REF], and the Gulf of Pagasitikos[START_REF] Dimarchopoulou | Ecosystem models and effort simulations of an untrawled gulf in the Central Aegean Sea[END_REF]. Y,

Acknowledgments

This work forms part of the MoonFish project (Modelisation tOOls for the management of FISHeries resources in Corsica) sponsored by the 'Collectivit´e Territoriale de Corse (CdC)' within the funding framework of the European project PO-FEDER 2014-2021. The MoonFish project is a consortium involving the University of Corsica/CNRS (project leader), the Environmental Office of Corsica (OEC), STARESO, and CRPMEM. We thank all data providers: IFREMER for the MEDITS data, Dr. Nathalie Malet (IFREMER) for the POC data, Gilles Faggio (OEC) for the marine bird data, Dr. Dimitrios K. Moutopoulos (University of Patras) for

Data availability

The authors do not have permission to share data.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.jmarsys.2022.103798.