Christophe Lett
email: christophe.lett@ird.fr

Nicolas Barrier

Yann Ourmieres

Cécile Petit

Maylis Labonne

Jérôme Bourjea

Audrey M Darnaude

Yann Ourmières

Modeling larval dispersal for the gilthead seabream in the northwestern Mediterranean Sea

Keywords: Sparus aurata, Gulf of Lions, Fish, Larvae, Models-hydrodynamic, Biophysical model, Connectivity, Otolith

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Dispersal is the process by which living organisms expand actively or passively the space or range where they live and is one of the fundamental life-history traits affecting the dynamics of spatially structured populations [START_REF] Cote | Personality-dependent dispersal: characterization, ontogeny and consequences for spatially structured populations[END_REF]. Collecting information on dispersal patterns at the population level is critical not only in terms of basic ecological knowledge, but also for management issues such as marine spatial planning [START_REF] Costello | The value of spatial information in MPA network design[END_REF][START_REF] Grüss | Consequences of adult and juvenile movement for marine protected areas[END_REF][START_REF] Franco | Dispersal patterns of coastal fish: implications for designing networks of marine protected areas[END_REF][START_REF] Magris | Integrating connectivity and climate change into marine conservation planning[END_REF]. The identification of fish spawning and nursery habitats is equally critical to marine population management and conservation [START_REF] Erisman | Fish spawning aggregations: where well-placed management actions can yield big benefits for fisheries and conservation[END_REF]. When spawning and nursery areas are distinct, the dispersal of individuals from the former to the latter is referred to as connectivity. Connectivity is defined as "the exchange of individuals among geographically separated subpopulations" [START_REF] Cowen | Population connectivity in marine systems: an overview[END_REF] and it is "often measured at some point after settlement" [START_REF] Pineda | Larval transport and dispersal in the coastal ocean and consequences for population connectivity[END_REF]. Marine population connectivity is an area of active research [START_REF] Bryan-Brown | Patterns and trends in marine population connectivity research[END_REF] and is best assessed using multiple, complementary approaches [START_REF] Levin | Recent progress in understanding larval dispersal: new directions and digressions[END_REF][START_REF] Cowen | Larval dispersal and marine population connectivity[END_REF][START_REF] Leis | Estimating connectivity in marine fish populations: what works best?[END_REF] in order to encompass the complexity [START_REF] Pineda | Complexity and simplification in understanding recruitment in benthic populations[END_REF] and uncertainty [START_REF] Kaplan | Uncertainty in empirical estimates of marine larval connectivity[END_REF][START_REF] Nolasco | Independent estimates of marine population connectivity are more concordant when accounting for uncertainties in larval origins[END_REF] of the larval life.

The overarching context of this work is to investigate the dispersal and connectivity between spawning and lagoon nursery habitats of gilthead seabream (Sparus aurata) in the Gulf of Lions (GoL), in the northwestern Mediterranean Sea. The GoL is the biggest continental shelf of the northwestern Mediterranean basin. It stretches from Marseille to the Creus cape and has a semicircular shape with an area of 250 km from East to West and 150 km from North to South and an average depth of 90 m [START_REF] Millot | The Gulf of Lions' hydrodynamics[END_REF]. Except for the area directly influenced by the Rhône river plume, the GoL is mainly oligotrophic [START_REF] Lochet | Transfers between a eutrophic ecosystem, the river Rhône, and an oligotrophic ecosystem, the north-western Mediterranean Sea[END_REF]. However, its coastline is punctuated by a chain of highly productive estuarine lagoons, many of which are recurrently used as nursery sites by marine fish species, including the gilthead seabream [START_REF] Mercier | Multi-element otolith fingerprints unravel sea-lagoon lifetime migrations of gilthead sea bream Sparus aurata[END_REF][START_REF] Tournois | Lagoon nurseries make a major contribution to adult populations of a highly prized coastal fish[END_REF]. This is a marine and brackish water finfish distributed along the coasts of the northeastern Atlantic Ocean and the Mediterranean Sea. It has been economically important since the Roman Empire, and has undergone a drastic increase in aquaculture production in Mediterranean countries during the last few decades. It is also one of the most important commercial species for the French artisanal fishery with an estimate of 690 t landed in 2018, representing almost 19 M€ of income for this fishing sector [START_REF] Weiss | Synthèse 2018 de l'observation des efforts et débarquements des pêcheries côtières[END_REF].

In the GoL, almost 85% of the adult Sparus aurata captured have spent most of their first year growing in a local lagoon [START_REF] Tournois | Lagoon nurseries make a major contribution to adult populations of a highly prized coastal fish[END_REF]. Seabream post-larvae enter the lagoons in the spring [START_REF] Lasserre | Dynamique des populations ichthyologiques lagunaires; application a Sparus aurata L[END_REF] to feed and grow during the warm season [START_REF] Escalas | Spatial variations in dietary organic matter sources modulate the size and condition of fish juveniles in temperate lagoon nursery sites[END_REF][START_REF] Isnard | Getting a good start in life? A comparative analysis of the quality of lagoons as juvenile habitats for the gilthead seabream Sparus aurata in the Gulf of Lions[END_REF]. Then, at the beginning of the reproductive period in late autumn, yearly individuals migrate back to the sea to avoid cold waters until the next feeding season [START_REF] Audouin | La daurade de l'étang de Thau Chrysophrys Aurata (LINNÉ)[END_REF][START_REF] Lasserre | Dynamique des populations ichthyologiques lagunaires; application a Sparus aurata L[END_REF][START_REF] Zohar | Gonadal cycle of captivity-reared hermaphroditic teleost Sparus-aurata (L) during 1st 2 years of life[END_REF][START_REF] Mosconi | Variabilité spatio-temporell de la croissance des juvéniles de Sparus aurata entre les zones lagunaires et marine du Golfe du Lion[END_REF]. Although the number and the exact location of the spawning sites for the gilthead seabream are still unknown in the area, the mature individuals that exit the Thau lagoon in autumn have been reported to migrate massively towards the East [START_REF] Audouin | La daurade de l'étang de Thau Chrysophrys Aurata (LINNÉ)[END_REF][START_REF] Lasserre | Dynamique des populations ichthyologiques lagunaires; application a Sparus aurata L[END_REF], the last ripe females being captured in front of the Rhône river delta in October [START_REF] Lasserre | Dynamique des populations ichthyologiques lagunaires; application a Sparus aurata L[END_REF]. Then S. aurata adults leave the shallow coastal zone of the GoL, where they massively reappear only in March. Because aggregations of ripe females are repeatedly observed in the winter around the Planier Island, off Marseille (i.e. further East than the Rhône river mouth), we currently consider this area to be the main potential source of S. aurata larvae for the coastal lagoons of the GoL. However, the connectivity between this spawning site and the local lagoons used as nursery areas by the species still has to be assessed. Three-dimensional explicit biophysical modeling of larval dispersal is a good way to investigate the respective roles of physical and biological drivers on larval connectivity [START_REF] Miller | Modelling the effects of physical-biological interactions and spatial variability in spawning and nursery areas on transport and retention of sardine eggs and larvae in the southern Benguela ecosystem[END_REF] and their influence on marine populations [START_REF] Daewel | Coupling ecosystem and individual-based models to simulate the influence of environmental variability on potential growth and survival of larval sprat (Sprattus sprattus L.) in the North Sea[END_REF]. Biophysical models to describe the larval drift in the Mediterranean were scarce ten years ago [START_REF] Calò | A review of methods to assess connectivity and dispersal between fish populations in the Mediterranean Sea[END_REF] but have flourished since then (e.g., [START_REF] Andrello | Low connectivity between Mediterranean marine protected areas: A biophysical modeling approach for the dusky grouper Epinephelus marginatus[END_REF][START_REF] Catalan | Stay off the motorway: Resolving the pre-recruitment life history dynamics of the European anchovy in the SW Mediterranean through a spatially-explicit individual-based model (SEIBM)[END_REF][START_REF] Koeck | Connectivity patterns of coastal fishes following different dispersal scenarios across a transboundary marine protected area (Bonifacio strait, NW Mediterranean)[END_REF][START_REF] Ospina-Alvarez | From egg production to recruits: Connectivity and inter-annual variability in the recruitment patterns of European anchovy in the northwestern Mediterranean[END_REF][START_REF] Bray | Assessing larval connectivity for marine spatial planning in the Adriatic[END_REF][START_REF] Calò | Use of Lagrangian simulations to hindcast the geographical position of propagule release zones in a Mediterranean coastal fish[END_REF][START_REF] Faillettaz | Larval fish swimming behavior alters dispersal patterns from marine protected areas in the North-Western Mediterranean Sea[END_REF]. Numerical studies focusing on the connectivity between coastal lagoons and the adjacent sea areas have also appeared recently [START_REF] Ghezzo | Connectivity in three European coastal lagoons[END_REF][START_REF] Fiandrino | Spatial patterns in coastal lagoons related to the hydrodynamics of seawater intrusion[END_REF][START_REF] Pérez-Ruzafa | Connectivity between coastal lagoons and sea: Asymmetrical effects on assemblages' and populations' structure[END_REF]. However, no biophysical model of larval dispersal has been applied to the gilthead seabream in the Mediterranean Sea so far.

The main objectives of this study were to set up such a model in the GoL and to confront its results on the spatio-temporal variation in the duration and success of dispersal at sea with actual data on the spawning dates and ages of Sparus aurata post-larvae captured at the time of their spring arrival at lagoon entrance. This investigation of the value of three-dimensional biophysical modeling for predicting larval connectivity patterns for the species in the GoL is a prerequisite to its use to help locating the species' regional spawning areas, with potentially high implications for local marine spatial planning and coastal lagoon management.

Material and methods

For this first modeling attempt, we chose to focus on the potential transport of S. aurata larvae between the species supposed main spawning site in the GoL, the Planier Island, and two of its main local nursery areas: the coastal lagoons of Thau and Salses-Leucate [START_REF] Isnard | Getting a good start in life? A comparative analysis of the quality of lagoons as juvenile habitats for the gilthead seabream Sparus aurata in the Gulf of Lions[END_REF][START_REF] Tournois | Lagoon nurseries make a major contribution to adult populations of a highly prized coastal fish[END_REF]; Figure 1). The trajectories followed by the passive larvae dispersing from the spawning site at sea were modeled for four consecutive years using a Lagrangian tool coupled to a hydrodynamic model driven by realistic meteorological forcings. The results obtained were then confronted to the biological information gathered through the structural analysis of the otoliths of local seabream post-larvae collected in both lagoons.

Study area

Like the whole northwestern Mediterranean Sea, the GoL hydrodynamics is mainly influenced by the two dominant winds (Mistral and Tramontane), by the inputs of the Rhône river [START_REF] Naudin | River discharge and wind influence upon particulate transfer at the land-ocean interaction: case study of the Rhone River Plume[END_REF] and by the Mediterranean Northern Current [START_REF] Barrier | Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling[END_REF]. The Mistral is a North wind coming from the Rhône river valley and blowing from the GoL eastern side. The Tramontane comes from the North too but blows in the GoL western side [START_REF] Millot | The Gulf of Lions' hydrodynamics[END_REF]. The main source of freshwater inputs to the area is the Rhône river discharge, which varies from 500 to 10,000 m 3 s -1 , with a 1,715 m 3 s -1 annual mean flow. For the last 20 years, the highest monthly mean flow (2,355 m 3 s -1 ) occurred in February and the lowest (1,047 m 3 s -1 ) in August [START_REF] Naudin | River discharge and wind influence upon particulate transfer at the land-ocean interaction: case study of the Rhone River Plume[END_REF]. The Northern (or Liguro-Provencal) Current (NC) is part of the large-scale ocean circulation of the Mediterranean Sea. It is the northern branch of the cyclonic gyre circulation occurring in the northwestern Mediterranean [START_REF] Millot | Circulation in the Western Mediterranean Sea[END_REF]. This current is, at first order, in geostrophic balance, and globally flows westwards along the continental shelf, with a seasonal modulation in current amplitude and eddy activity [START_REF] Guihou | A case study of the mesoscale dynamics in the North-Western Mediterranean Sea: a combined datamodel approach[END_REF]. The NC is known to often act as a barrier between the fresh nutrient-rich coastal waters and the salty oligotrophic offshore waters [START_REF] Ourmières | Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products[END_REF]. As evidenced in observations and numerical modeling, the NC occasionally penetrates within the GoL [START_REF] Barrier | Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling[END_REF], causing drastic changes in the biogeochemistry and in turn in the primary productivity along its shelf [START_REF] Ross | Impact of an intrusion by the Northern Current on the biogeochemistry in the eastern Gulf of Lion, NW Mediterranean[END_REF].

The main source of larvae retained for this study, the Planier Island, is located at about 8.6 km from the south-west coast of the city of Marseille (Figure 1). This rocky islet has a very small area (2 ha) and its altitude is not exceeding 5 m. It is worth noting that this island is located on the episodic NC intrusion path. The two arrival sites retained for S. aurata larvae, the Thau and Salses-Leucate lagoons, have similar surface areas, environmental conditions, and number of connections with the sea. However, otholith microchemical analyses revealed that the two lagoons have contrasted importance for the multi-annual renewal of the local population: Thau is the lagoon nursery contributing most to the local exploited stock (18%) while the contribution of Salses-Leucate (7%) is among the lowest in the area [START_REF] Mercier | Multi-element otolith fingerprints unravel sea-lagoon lifetime migrations of gilthead sea bream Sparus aurata[END_REF][START_REF] Tournois | Lagoon nurseries make a major contribution to adult populations of a highly prized coastal fish[END_REF].

Hydrodynamic model

High resolution simulations from the NEMO-GLAZUR64 configuration performed by [START_REF] Ourmières | Assessment of a NEMO-based downscaling experiment for the North-Western Mediterranean region: Impacts on the Northern Current and comparison with ADCP data and altimetry products[END_REF] Daily averages of the PSY2V4R1 velocity and mass fields were used at the GLAZUR64 open boundaries, and the typical damping coefficients for inflow and outflows adjusted consequently to 1 and 10 days, respectively [START_REF] Guihou | A case study of the mesoscale dynamics in the North-Western Mediterranean Sea: a combined datamodel approach[END_REF]. Surface boundary conditions used to force the simulation were retrieved from the Météo-France operational regional model ARPEGE, with a horizontal resolution of around 9.5 by 9.5 km and files provided every 3 h. This atmospheric model features data assimilation and state-of-the-art atmospheric physics [START_REF] Fischer | An overview of the variational assimilation in the ALADIN/France numerical weather-prediction system[END_REF].

Larval dispersal model

Water velocity fields provided by daily stored simulations of GLAZUR64 over the period 2014-2017 were used to simulate the larval dispersal of gilthead seabream in the GoL. We used the Lagrangian tool Ichthyop (http://www.ichthyop.org/), an individual-based model (IBM) designed to study the effects of physical and biological factors on the dynamics of fish eggs and larvae [START_REF] Lett | A Lagrangian tool for modelling ichthyoplankton dynamics[END_REF]. In the IBM, the velocity fields were interpolated in space to provide values at any individual location, and in time to feed the IBM time step (here taken as 15 min). Simulations consisted in releasing randomly 5,000 particles representing virtual larvae in the neighborhood of the Planier Island. For this, the release zone was defined as a cylinder of 5 km radius centered on the Planier Island (43.198855°N, 5.23064°E) and extending from 0 to 20 m depth (other depth range tested: 80 to 100 m). Particle releases for each year were scheduled every 5 days during the main local spawning season for S. aurata, from November to February [START_REF] Audouin | La daurade de l'étang de Thau Chrysophrys Aurata (LINNÉ)[END_REF][START_REF] Lasserre | Dynamique des populations ichthyologiques lagunaires; application a Sparus aurata L[END_REF], and their locations in the GoL were subsequently tracked during 120 days (i.e. four months), the maximum duration for seabream larval life in the area [START_REF] Isnard | Getting a good start in life? A comparative analysis of the quality of lagoons as juvenile habitats for the gilthead seabream Sparus aurata in the Gulf of Lions[END_REF]. We covered three consecutive reproductive seasons fully (Nov. 2014to Feb. 2015, Nov. 2015to Feb. 2016, and Nov. 2016to Feb. 2017) and the last two months of the 2013-2014 reproductive season (Jan. to Feb. 2014). Overall we released 5,000 * 3.5 (years) * 4 (months) * 6 (releases per month) = 420,000 particles.

Ichthyop outputs files gathered all possible larval locations for every hour within the 120-days of simulated dispersal. From these, we plotted the larval dispersal trajectories and histograms of ages for larvae reaching the Thau or Salses-Leucate lagoons. In the simulations, virtual larvae were considered to have reached the Thau or the Salses-Leucate lagoon when they arrived at a minimum distance of 5 km from their main connection with the sea, the "Port des Quilles" (43.3920126°N, 3.665203°E) or the "Port Leucate" (42.872259°N, 3.049062°E) harbor, respectively (Figure 1). An additional run of simulations was also made with a minimum distance of 10 km to both lagoon entrances for larval arrival. In each case, the number of larvae reaching each lagoon along the whole simulated period was computed.

Oceanographic drifters

To confront simulated results with actual oceanographic data, we used the Global Drifter Program (GDP) database (https://www.aoml.noaa.gov/phod/gdp/) and the Coriolis database (http://www.coriolis.eu.org) to find oceanographic drifters trajectories passing within the GoL. We found one such drifter in each database, drifter id 2442647 from the GDP, an Argos-2 Surface Velocity Profiler (SVP) with a drogue centered at 15 m released October 6 th 2005, and drifter id 61794 from the Coriolis database, another SVP released October 28 th 2014.

Otolith data

To confront simulated results with actual biological data, Sparus aurata post-larvae were sampled at the time of their first arrival at the entrance of local coastal lagoons (in February-March), for two successive years (in 2016 and 2017). The right sagittal otoliths of these individuals were extracted, washed and prepared for daily ring counts following [START_REF] Tomás | Otolith microstructure examination and growth patterns of Vinciguerria nimbaria (Photichthyidae) in the tropical Atlantic Ocean[END_REF]. They were embedded in Polyester resin and transversal sections of approximately 1 mm, including the central nucleus, were made with a precision saw (Buehler, Isomet 1000). Otolith cuts were polished to approximately 10 µm depth with the nucleus exposed on the surface. These sections were photographed with a digital camera system (Olympus ProgRes C5) and ProRes Capture pro 2.5 software linked to a microscope (Olympus BX41, x400). All pictures were imported into Perfect Image software.

Examination of the otolith sections revealed multiple concentric rings from the nucleus to the edge, clearly visible on all otoliths. In accordance with previous literature validating their daily deposition [START_REF] Morales-Nin | Patterns of primary growth increments in otoliths of Sparus aurata larvae in relation to water temperature and food consumption[END_REF][START_REF] Isnard | Getting a good start in life? A comparative analysis of the quality of lagoons as juvenile habitats for the gilthead seabream Sparus aurata in the Gulf of Lions[END_REF], these increments were counted to assess fish age (in days). For this, each otolith was read twice by two independent readers, from the first visible increment corresponding to mouth opening, 3 days after hatching [START_REF] Bodinier | Ontogeny of osmoregulation and salinity tolerance in the gilthead sea bream Sparus aurata[END_REF] to the edge of otolith, namely the capture date. Otoliths were read again if the two estimated ages in days differed by more than 5%, to achieve a consensual age estimate between the two readers. The two separate readings of otolith daily increments were compared by paired t-tests after checking normality and homoscedasticity and when there were non-significant differences, the mean of the two readings was used. This allowed estimation of the age (in days) of the post-larvae at their arrival at the lagoon entrances and back-calculation of the date of spawning for each individual, with an average potential error in otolith reading of ±3.2 days.

Results

Larval dispersal model

Among the 420,000 virtual gilthead seabream larvae released around the Planier Island (5,000 for 84 dates), 13,531 reached a distance < 5 km of the Thau lagoon entrance "Port des Quilles" at some Virtual larvae from the Planier Island followed a great variety of trajectories to reach the Thau lagoon along the simulated period (Figure 3). There were direct transport trajectories following the coastline and generally reaching "Port des Quilles" quickly (within 10-20 days), either from the East or from the West (Figure 3a). We also found longer transport trajectories entering the GoL from mid-shelf (Figure 3b), and a few trajectories following the continental shelf offshore before entering the GoL coastal zone from its western side (Figure 3c). There were also many indirect trajectories, e.g. some following the coast to pass offshore the "Port des Quilles" toward the western side of the GoL, and then crossed back the GoL to arrive to "Port des Quilles" from mid-shelf, or larvae entering and leaving the GoL several times (Figure 3d). Overall, possible larval dispersal routes from the Planier Island to "Port des Quilles" spread over the entire GoL, but were mostly concentrated along the coast, with most larvae reaching the lagoon entrance from the West, an unexpected result (Figure 4).

The age at which the virtual larvae reached "Port des Quilles" in the simulations was also very variable, ranging from 8 to 120 days (the total duration of the simulations). However, a clear mode was observed between 10 and 60 days (Figure 5, mean = 46.7 days, SD = 23.9 days, median = 41.2 days, IQR = 31.2 days).

Among the same 420,000 virtual larvae, 23,060 reached a distance < 5 km to the Salses-Leucate lagoon entrance "Port Leucate", i.e., a mean transport success of 5.5%. Unexpectedly, transport success to "Port Leucate" was therefore higher, on average, than to "Port des Quilles". Like for "Port des Quilles", transport success to "Port Leucate" was very variable according to the release date (Figure 6a, min = 0%, max = 45.7%, SD = 8.1%, median = 3.4%, IQR = 4.8%). Lowest and highest transport successes occurred generally for the same release dates for the two lagoons (Figure 2, Figure 6a). As for Thau, virtual larvae were mostly transported to the Salses-Leucate lagoon by currents following the coastline, but they generally reached "Port Leucate" from the East (Figure 6b). The age distribution of the virtual larvae reaching "Port Leucate" was bimodal, with modes between 20-30 and 50-60 days (Figure 6c, min = 10.5 days, max = 120 days, mean = 50.9 days, SD = 24.6 days, median = 49.6 days, IQR = 37.2 days). Interestingly, the 30-40 days peak in the age distribution of larvae reaching "Port des Quilles" (Figure 5) was a trough in the age distribution of larvae reaching "Port Leucate" (Figure 6c). Among the 420,000 virtual dispersal trajectories simulated for this work, only 4,080 (< 1%) reached distances < 5 km to both "Port Leucate" and "Port des Quilles" within the 120 days of larval drift modeling. Most of these trajectories (4,010) reached "Port des Quilles" before "Port Leucate", spending on average 18.5 days at sea traveling between the two sites. Unsurprisingly, increasing the distance area around the lagoon main entrances, from 5 to 10 km, for considering larval arrival to both lagoons, increased the transport success of virtual larvae to 9.3% and 13.0% on average for "Port des Quilles" and "Port Leucate", respectively. However, it also changed the age distribution of larvae reaching "Port des Quilles", which tended to become bimodal with peaks at age 10-20 and 30-40 days (Appendix Figure Aa), while for larvae reaching "Port Leucate" it remained unchanged with two peaks at age 20-30 and 50-60 days (Appendix Figure Ab). Conversely, releasing particles deeper, i.e. at 80-100 m instead of 0-20 m, decreased the transport success of virtual larvae to 0.5% and 1.1% on average for "Port des Quilles" and "Port Leucate", respectively, and delayed the ages at which they reached the lagoon entries by ~10 days (Appendix Figure Bb).

Oceanographic drifters

The trajectory followed within the GoL by the oceanographic drifter 2442647 from the GDP is shown in Figure 1

Otolith data

Otolith reading (N = 145, Appendix Table) revealed large variation in the age of the S. aurata postlarvae collected at their time of first arrival at lagoon entrances (in February-March for both 2016 and 2017, Figure 7). However, the minimum and maximum ages observed were of 60 days (for a post-larva captured in mid-February 2016) and 121 days (for a post-larva captured in late March 2017), respectively. Moreover, most of the post-larvae collected (88%) had ages between 60 and 90 days (Figure 7), including 80% of the individuals sampled at their arrival in Thau and 100% of those caught at their arrival in Salses-Leucate. Interestingly, the oldest larvae were all caught in March (Figure 7), irrespective of the year or the lagoon, suggesting similar spawning dates for all fish, irrespective of their final age at lagoon entrance. As a matter of fact, estimated spawning dates for all lagoons and both sampling years ranged between November 23 rd (for an individual caught in 2015) and December 29 th (for an individual caught in 2017), with a majority (> 51%) of fish spawned in early December (1-15 th ) irrespective of the year of capture (Figure 8). However, small differences in fish spawning dates could be observed between the two sampling years. Thus, the percentage of fish spawned in November was close to zero (2 individuals only) in the post-larvae collected in 2016, but reached 14% in those captured in 2017, especially at the Salses-Leucate entrance (29%). Conversely, the percentage of post-larvae spawned after December 20 th was greater in the fish collected in 2016 (14%) than in those collected in 2017 (6.5%). This suggests that the peak of spawning of S. aurata and/or the most favorable time for the dispersal of its larvae at sea occurred later in 2015-16 than in 2016-17, but with no major influence on fish age at lagoon entrance.

Discussion

Information on the location of fish spawning grounds and the scale of dispersal towards their nursery areas is crucially important for elucidating connectivity patterns between populations, and for providing the optimal background knowledge in order to design efficient protection and management strategies [START_REF] Fogarty | Population connectivity and spatial management of marine fishes[END_REF][START_REF] Erisman | Fish spawning aggregations: where well-placed management actions can yield big benefits for fisheries and conservation[END_REF]. The method used here combining model simulations with information on otolith age-reading of gilthead seabream larvae in the Gulf of Lions (GoL) is a first attempt to fill information gaps on the species potential spawning areas, larval drift duration, larval dispersal pathways, and age at recruitment. The large variability in simulated trajectories found for the virtual larvae transported from the Planier Island to "Port des Quilles" corresponds to different well-documented conditions. Coastal simulated trajectories (Figure 3a) are consistent with the trajectories observed for the two oceanographic drifters (Figure 1) and can correspond to two types of events, either a classical Northern Current (NC) intrusion on the shelf (for causes and consequences, see [START_REF] Barrier | Strong intrusions of the Northern Mediterranean Current on the eastern Gulf of Lion: insights from in-situ observations and high resolution numerical modelling[END_REF], or a current induced by strong easterlies, pushing the Rhône river plume to the coast, creating a light water current along the GoL coast. Mid-shelf trajectories (Figure 3b) also correspond to classical NC intrusions. This place of the shelf is favorable to water intrusion into the GoL, as shown by hydrological data, drifters and models [START_REF] Petrenko | Circulation in a stratified and wind-forced Gulf of Lions, NW Mediterranean Sea: in situ and modeling data[END_REF][START_REF] Leredde | Hydrodynamics in the Gulf of Aigues-Mortes, NW Mediterranean Sea: In situ and modelling data[END_REF][START_REF] Poulain | Aspects of the surface circulation in the Liguro-Provençal basin and Gulf of Lion as observed by satellite-tracked drifters (2007-2009[END_REF]. Some intrusions are forced by easterlies, through either Ekman transport or a shoreward displacement of the NC. Both the Mistral and Tramontane winds may further favor intrusions through the associated positive wind stress curl. This curl would provide a source of vorticity to the NC that may drive intrusions [START_REF] Gatti | Intrusions du courant Nord méditerranéen sur la partie Est du plateau continental du Golfe du Lion[END_REF]. In the eastern part of GoL, intrusions are also likely to occur after the relaxation of upwelling-favourable winds, as shown by [START_REF] Millot | The effect of Mistral wind on the Ligurian current near Provence[END_REF]. When the wind relaxes, the frontal zone between the cold upwelled waters and warm water originating from the NC tends to move northwestward and to penetrate over the shelf. Using analytical model and numerical simulations, [START_REF] Echevin | Interaction of a Coastal Current with a Gulf: Application to the Shelf Circulation of the Gulf of Lions in the Mediterranean Sea[END_REF] suggested that Ekman transport associated with south-easterlies induces downwelling, which in turn generates a westward coastal current that transports NC waters onto the shelf. There are also meanders and eddies allowing waters to flow into the shelf. [START_REF] Estournel | Observation and modeling of the winter coastal oceanic circulation in the Gulf of Lion under wind conditions influenced by the continental orography (FETCH experiment)[END_REF] showed an anticyclonic eddy located at the center of the GoL continental shelf. On its eastern part, eddies are generated by local winds [START_REF] Allou | Submesoscale vortex structures at the entrance of the Gulf of Lions in the Northwestern Mediterranean Sea[END_REF]. Backloops identified on many of our simulated trajectories reaching "Port des Quilles" from the West are likely induced by the presence of an eddy preventing the water to flow southwards, called the Latex eddy [START_REF] Hu | Numerical study of eddy generation in the western part of the Gulf of Lion[END_REF][START_REF] Petrenko | A review of the LATEX project: mesoscale to submesoscale processes in a coastal environment[END_REF]. Trajectories entering from the western side of the GoL (Figure 3c) can be due to a current flowing northwards from the Creus cape when the Tramontane blows homogeneously over the GoL [START_REF] Petrenko | Barotropic eastward currents in the western Gulf of Lion, north-western Mediterranean Sea, during stratified conditions[END_REF], or are possibly trapped by the Latex eddy onshore branch [START_REF] Hu | Numerical study of eddy generation in the western part of the Gulf of Lion[END_REF][START_REF] Petrenko | A review of the LATEX project: mesoscale to submesoscale processes in a coastal environment[END_REF].

The high diversity in the hydrodynamic conditions that larvae face in the GoL explains the large variability obtained in our simulations for their transport success to the vicinity of "Port des Quilles" (Figure 2) or "Port Leucate" (Figure 6a), as well as for their age at arrival (Figure 5, Figure 6c). The most common ages predicted for larval arrival (20 to 60 days) are broadly consistent with previous modeling studies. Indeed, using a Lagrangian approach for the whole Mediterranean Sea based on a NEMO model configuration at 1/16° horizontal resolution, [START_REF] Rossi | Hydrodynamic provinces and oceanic connectivity from a transport network help designing marine reserves[END_REF] identified the GoL as a more coherent hydrodynamic province (i.e., well connected internally and with limited external exchanges) for a larval life duration of 30 days than for a duration of 60 days. Similarly, Nicolle et al.'s (2009) results using a configuration of the MARS model at 1.2 km resolution predicted residence time of 1 to 2 months for the GoL shelf waters. Possibility for diel vertical migration from 0 to 50 m in the larvae, as tested for anchovy larvae in the GoL, was not shown to significantly affect this result [START_REF] Nicolle | Modelling for anchovy recruitment studies in the Gulf of Lions (Western Mediterranean Sea)[END_REF]. Possibly, however, the resolution and forcing of the hydrodynamic models used so far may not have been sufficient to reproduce adequately small retentive coastal structures that may play a crucial role in larval retention [START_REF] Briton | High-resolution modelling of ocean circulation can reveal retention spots important for biodiversity conservation[END_REF]. In addition, the simulated currents used are typically daily averaged, which means that a part of the information regarding the currents modulation might have been lost by the averaging process, though being important for the larvae trajectory computing [START_REF] Blanke | Sensitivity of advective transfer times across the North Atlantic Ocean to the temporal and spatial resolution of model velocity data: Implication for European eel larval transport[END_REF]. With high-resolution (200 m) and high-frequency (3 h) currents simulated with a configuration of the MARS-3D model covering the Bay of Marseille, [START_REF] Thibaut | Connectivity of populations of the seaweed Cystoseira amentacea within the Bay of Marseille (Mediterranean Sea): genetic structure and hydrodynamic connections[END_REF] showed that particles released around the Planier Island could reach sites located 20 km away (~ 15% of the distance to the Thau lagoon) in 12 h under strong wind conditions.

The most common ages predicted for larval arrival at the coastal lagoons in the simulations (20 to 60 days) are also consistent with the gilthead seabream pelagic larval duration (45 days in [START_REF] Chaoui | Strong genetic differentiation of the gilthead sea bream Sparus aurata (L., 1758) between the two western banks of the Mediterranean[END_REF]50 days in Franchini et al., 2012) and the ages over which the species fins develop (20 to 50 days; [START_REF] Chatain | Estimation et amélioration des performances zootechniques de l'Elevage larvaire de Dicentrarchus labrax et de Sparus auratus[END_REF][START_REF] Patruno | Muscle growth in response to changing demands of functions in the teleost Sparus aurata (L.) during development from hatching to juvenile[END_REF][START_REF] Moretti | Manual on hatchery production of seabass and gilthead seabream. Food and Agriculture Organization of the United Nations[END_REF]. However, they contrast with the ages actually observed in the post-larvae collected in 2016 and 2017 (60 to 90 days, Figure 7).

Endurance tests of raised gilthead seabream larvae of age 50 days showed that they could swim at a constant 12 cm s -1 speed during 42.2 ± 11.1 h, covering a mean distance of nearly 20 km [START_REF] Faria | Effects of starvation on swimming performance and body condition of pre-settlement Sparus aurata larvae[END_REF]. The critical swimming speed of larvae of ages 58 to 96 days collected at sea was recently estimated as 10.4 ± 1.7 cm s -1 [START_REF] Rossi | Relationship between swimming capacities and morphological traits of fish larvae at settlement stage: a study of several coastal Mediterranean species[END_REF]. The individuals collected here had similar ages and therefore probably also a significant capacity of movement, which they may have used to depart from drifting currents and swim to the coast and/or to remain close to the lagoon entrances, until they have developed sufficient ability to osmoregulate and sense lagoon waters. [START_REF] Bodinier | Ontogeny of osmoregulation and salinity tolerance in the gilthead sea bream Sparus aurata[END_REF] showed that seabream larvae increase their osmoregulatory ability sharply from age 30 to 70 days. [START_REF] Morais | Response of gilthead seabream (Sparus aurata L., 1758) larvae to nursery odor cues as described by a new set of behavioral indexes[END_REF] reported that young (< 25 days) seabream larvae had insufficient swimming capacity to response to water cues and that older larvae (25 to 57 days) were still predominantly unresponsive. Overall, ages from 25 to 70 days appear to be critical for seabream larvae to acquire sufficient swimming, osmoregulatory, and olfactory abilities to enter the coastal lagoons. The full morphological development of seabream larvae takes approximately 90 days in aquaculture [START_REF] Chatain | Estimation et amélioration des performances zootechniques de l'Elevage larvaire de Dicentrarchus labrax et de Sparus auratus[END_REF]. The fact that the larvae collected at the lagoon entrances in March were generally older than those collected earlier (in February, Figure 7) suggests that they actually stayed offshore for some time once arrived in front of the lagoon entrance, possibly in order to complete their development. It is also possible that larvae younger than 60 days did arrive at the lagoon entrances but were missed in the sampling due to their small size, or quickly died because they were not developped enough either to successfully feed on coastal prey or escape local predators.

To conclude, we believe that horizontal swimming plays a critical role in the transport and retention of seabream larvae within the GoL, explaining the discrepancy between our observed data and simulation results. Recently, [START_REF] Faillettaz | Larval fish swimming behavior alters dispersal patterns from marine protected areas in the North-Western Mediterranean Sea[END_REF] showed that including shoreward swimming in their model of Sparidae larvae dispersal in the northwestern Mediterranean Sea increased the simulated coastal settlement significantly. Another explanation would be that the larvae we collected come from spawning grounds located further East than the Planier Island. Contrasting results concerning the genetic structure of the gilthead seabream in the western Mediterranean were reported, with a strong genetic differentiation obtained between samples collected from the GoL and from Algerian waters [START_REF] Chaoui | Strong genetic differentiation of the gilthead sea bream Sparus aurata (L., 1758) between the two western banks of the Mediterranean[END_REF], but low levels of structure along the Italian coast [START_REF] Franchini | Low levels of population genetic structure in the gilthead sea bream, Sparus aurata, along the coast of Italy[END_REF]. Comparing samples from the GoL and the Ligurian Sea could indicate if a Ligurian origin of the GoL larvae is plausible. More broadly, there is a crucial need to better identify where the key reproduction sites of the seabream found in the coastal lagoons of the GoL are located. If biotelemetry is now widely used to study the spatial ecology of large migratory marine species [START_REF] Block | Tracking apex marine predator movements in a dynamic ocean[END_REF][START_REF] Hussey | Aquatic animal telemetry: A panoramic window into the underwater world[END_REF], we believe that acoustic telemetry is the best approach for a small coastal migratory fish such as the gilthead seabream in order to avoid affecting its natural behavior [START_REF] Forin-Wiart | Why implantation of biologgers may improve our understanding of how animals cope within their natural environment[END_REF]. Acoustic telemetry is widely used to study the spatial ecology of fish [START_REF] Donaldson | Making connections in aquatic ecosystems with acoustic telemetry monitoring[END_REF][START_REF] Abecasis | A review of acoustic telemetry in Europe and the need for a regional aquatic telemetry network[END_REF] and it has already helped following the movements of seabream adults within a coastal lagoon in Portugal [START_REF] Abecasis | Site fidelity and movements of gilthead sea bream (Sparus aurata) in a coastal lagoon (Ria Formosa, Portugal)[END_REF]. By using a large network of hydrophones deployed at lagoon entrances along the GoL coastline as well as around expected offshore reproduction sites such as the Planier Island, and by putting acoustic tags on seabream adults while in their lagoon feeding grounds, we expect to obtain valuable information on these fish movement from foraging areas to spawning sites in the near future.

Figures

  over the GoL were used to model local hydrodynamic circulation at sea over the years of interest (2014-2017). GLAZUR64 is based on the primitive equation ocean circulation model of the "Nucleus for European Modelling of the Ocean" (NEMO) modeling framework (Madec and the NEMO team, 2016). The resolution on the horizontal is 1/64° (about 1.3 km) on a regular grid with 130 vertical z-levels. The vertical resolution ranges from 1 m near the surface to 30 m near the bottom. Ocean boundary conditions used to force the model came from the NEMO-based operational configuration developed by MERCATOR-OCEAN (http://www.mercator-ocean.fr) PSY2V4R1.

  time along their 120-day drift, i.e. a mean transport success of 3.2%. Transport success varied greatly with release date, though, ranging from 0% for five dates, all in November(2014/11/05, 2015/11/15, 2015/11/20, 2015/11/25, 2016/11/10), to 31.6% for a release on 2015/02/25 (Figure2, standard deviation SD = 4.8%, median = 1.7%, and interquartile range (IQR) = 3.2%). There were two periods of one month, from 2015/11/05 to 2015/11/30 and from 2016/12/05 to 2017/01/05, when particles releases led to transport successes constantly below 0.5%. Conversely, release periods with high subsequent transport success were short (< 5 days) irrespective of the year, and represented by only a single release date (e.g. 2016/11/25 and 2017/01/25, Figure2).

  with locations every 6 hours. It passed ~ 20 km south of the Planier Island then drifted westwards off the coast passing ~ 30 km from "Port des Quilles" after ~ 5 days and ~ 20 km from "Port Leucate" after ~ 8 days, before leaving the GoL from its southwestern part after ~ 14 days. The trajectory of Coriolis drifter 61794 is shown in Figure 1 with locations every 1/2 hour. It came very close to the Planier Island then drifted westwards passing ~ 10 km from "Port des Quilles" after ~ 14 days and eventually drifted southwards along the coast, passing very close to "Port Leucate" after ~ 24 days.

Figure 1 :

 1 Figure 1: Map of the Gulf of Lions in the northwestern Mediterranean Sea showing the bathymetry (right colorbar), the current speed averaged over the period 2014-2017 and depth range 0-20 m from GLAZUR64 daily outputs (bottom colorbar), trajectories of two oceanographic drifters (top colorbar) and locations cited in the text.

Figure 2 :

 2 Figure 2: Percentage of the virtual gilthead seabream larvae released around the Planier Island and successfully transported to the Thau lagoon entrance "Port des Quilles", for each release date. Dashed grey lines separate breeding seasons (Nov.-Feb.).

Figure 4 :

 4 Figure 4: Spatial distribution of the virtual gilthead seabream larvae released around the Planier Island and successfully transported to the Thau lagoon entrance "Port des Quilles". Locations were summed along the simulated trajectories and among all release dates on a 0.01° * 0.01° map.

Figure 5 :Figure 6 :

 56 Figure 5: Age distribution of the virtual gilthead seabream larvae released around the Planier Island and successfully transported to the Thau lagoon entrance "Port des Quilles" for all release dates combined.

Figure 7 :

 7 Figure 7: Age distribution of 145 gilthead seabream larvae collected at the entrance of four lagoons in February-March 2016-2017. Grey bars are for the 108 larvae collected in February. Black bars are for the 37 larvae collected in February at the Thau and Salses-Leucate lagoon entrances "Port des Quilles" (16 larvae) and "Port Leucate" (21 larvae) combined.

Figure 8 :

 8 Figure 8: Distribution of estimated spawning dates for 145 gilthead seabream larvae collected at the entrance of four lagoons in February-March 2016-2017.

Acknowledgements

This work was supported by the BOUCLEDOR project (http://www.boucledor-project.com) funded by the European Research Fund and the French Languedoc-Roussillon Region. We are grateful to Sadjia Belkacemi, Franck Ferraton, and Chloé Vagnon for their assistance in fish sampling and data acquisition, and to Skandar Hentati for preliminary modeling results. We thank the developers of the marmap R package (https://doi.org/10.1371/journal.pone.0073051) and the GEBCO Compilation Group (https://doi.org/ 10.5285/836f016a-33be-6ddc-e053-6c86abc0788e). SVP drifters data were collected and made freely available by the Global Drifter Program (https://www.aoml.noaa.gov/phod/gdp/) and by the Coriolis project (http://www.coriolis.eu.org) and programmes that contribute to it. The GLAZUR64 simulations were performed using GENCI-IDRIS resources (Grant 2016011707). We thank Ignacio Catalán and two anonymous reviewers for their useful comments.

Appendix figures

Appendix table