Lucien Besnard

Gaël Le Croizier

Felipe Galván-Magaña

David Point

Edouard Kraffe

James Ketchum

Raul Octavio

Martinez Rincon

Gauthier Schaal

Raul Octavio Martinez Rincon

Foraging depth depicts resource partitioning and contamination level in a pelagic shark assemblage: Insights from mercury stable isotopes

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

INTRODUCTION

Sharks are facing worldwide a large variety of threats such as overfishing, pollution, ecosystem degradation and others [START_REF] Dulvy | Extinction risk and conservation of the world's sharks and rays[END_REF][START_REF] Hazen | Predicted habitat shifts of Pacific top predators in a changing climate[END_REF]Queiroz et al., 2019). In recent years, the decline of many shark populations in the global ocean has raised public concern due to the iconic nature of these top predators and their influence on marine ecosystems over various temporal and spatial scales [START_REF] Ferretti | Patterns and ecosystem consequences of shark declines in the ocean[END_REF][START_REF] Heithaus | Predicting ecological consequences of marine top predator declines[END_REF]. Since the consequences of their removal are difficult to assess and predict [START_REF] Baum | Cascading top-down effects of changing oceanic predator abundances[END_REF][START_REF] Ferretti | Patterns and ecosystem consequences of shark declines in the ocean[END_REF], ecological information on habitat use and foraging grounds are needed to better assess future changes in marine ecosystems [START_REF] Shiffman | Stable isotope analysis as a tool for elasmobranch conservation research: a primer for nonspecialists[END_REF].

Large sharks, such as most marine predators, influence their ecosystem mainly through trophic interactions, either by predation on mesopredators, i.e. top-down control [START_REF] Baum | Cascading top-down effects of changing oceanic predator abundances[END_REF][START_REF] Ferretti | Patterns and ecosystem consequences of shark declines in the ocean[END_REF], competition with sympatric (i.e. co-occurring) high trophic level predators [START_REF] Matich | Ecological niche partitioning within a large predator guild in a nutrient-limited estuary[END_REF], or more complex interactions [START_REF] Heithaus | Predicting ecological consequences of marine top predator declines[END_REF][START_REF] Jorgensen | Killer whales redistribute white shark foraging pressure on seals[END_REF].

Competition for a shared trophic resource can result in lower food availability, change in physiological condition, and ultimately, reduced fitness [START_REF] Jorgensen | Killer whales redistribute white shark foraging pressure on seals[END_REF]. Therefore, evolutionary processes tend to favor resource partitioning in co-occurring top predators [START_REF] Heithaus | Apparent resource partitioning and trophic structure of large-bodied marine predators in a relatively pristine seagrass ecosystem[END_REF]. Tracking studies have highlighted differences in habitat use between shark species suspected to compete for food [START_REF] Meyer | A multiple instrument approach to quantifying the movement patterns and habitat use of tiger (Galeocerdo cuvier) and Galapagos sharks (Carcharhinus galapagensis) at French Frigate Shoals, Hawaii[END_REF][START_REF] Musyl | Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean[END_REF], but this method appears limited to discriminate the diet of species sharing a same trophic ground. Isotopic trophic tracers, such as stable carbon and nitrogen isotopes, have been used to assess resource partitioning between sympatric predators [START_REF] Heithaus | Apparent resource partitioning and trophic structure of large-bodied marine predators in a relatively pristine seagrass ecosystem[END_REF]. However, contrary to coastal ecosystems, pelagic trophic webs are most of the time based on phytoplankton production only resulting in homogeneous isotopic signatures and in overlapping isotopic niches between predators [START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF][START_REF] Klarian | New insights on the trophic ecology of blue (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) from the oceanic eastern South Pacific[END_REF][START_REF] Rosas-Luis | Feeding ecology and trophic relationships of pelagic sharks and billfishes coexisting in the central eastern Pacific Ocean[END_REF]. Rather than competition, overlapping regions of isotopic niches might therefore be due to some inherent approach limitations. For instance, stable isotopes have a poor ability to discriminate foraging depth for top-predators likely to feed in deep oceanic habitats [START_REF] Choy | Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions[END_REF][START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF] and only few studies have investigated the vertical dimension of resource partitioning in pelagic predator assemblages (Le Croizier et al., 2020b, 2020a).

The mesopelagic zone (200-1000 m below the ocean surface) contains one of the most important animal biomass on earth [START_REF] Aksnes | Light penetration structures the deep acoustic scattering layers in the global ocean[END_REF][START_REF] Irigoien | Large mesopelagic fishes biomass and trophic efficiency in the open ocean[END_REF], principally gathered inside the "deep scattering layer" community [START_REF] Costello | Ocean Depths: The Mesopelagic and Implications for Global Warming[END_REF]. It is mainly composed of fishes and invertebrates that are commonly targeted by marine megafauna [START_REF] Aksnes | Light penetration structures the deep acoustic scattering layers in the global ocean[END_REF][START_REF] Davison | Mesopelagic fish biomass in the southern California current ecosystem[END_REF][START_REF] Hazen | Meridional patterns in the deep scattering layers and top predator distribution in the central equatorial Pacific[END_REF]. Among these predators, some pelagic shark species display typical deep diving patterns suggesting that they rely on this compartment, such as great white (Le Croizier et al., 2020a), blue [START_REF] Braun | Mesoscale eddies release pelagic sharks from thermal constraints to foraging in the ocean twilight zone[END_REF] or scalloped hammerhead sharks [START_REF] Jorgensen | Scalloped hammerhead shark Sphyrna lewini, utilizes deep-water, hypoxic zone in the Gulf of California[END_REF]. Surprisingly, although the combined effect of climate change and fishing pressure is dramatically changing epipelagic fish biomass and dynamic [START_REF] Pinsky | Unexpected patterns of fisheries collapse in the world's oceans[END_REF][START_REF] Tu | Fishing and temperature effects on the size structure of exploited fish stocks[END_REF], little attention has been paid to the mesopelagic zone which is also predicted to be affected by climate change [START_REF] Proud | Biogeography of the Global Ocean's Mesopelagic Zone[END_REF]. In this context, the importance of deeper mesopelagic prey for different oceanic shark species must be better assessed.

Mercury is a globally distributed atmospheric pollutant [START_REF] Fitzgerald | Marine Biogeochemical Cycling of Mercury[END_REF] having deleterious toxic effects on marine fauna [START_REF] Eisler | Mercury Hazards to Living Organisms[END_REF]. Entering the ocean in its inorganic form, its bioavailability increases through methylation by microbial activity [START_REF] Sunderland | Mercury sources, distribution, and bioavailability in the North Pacific Ocean: Insights from data and models[END_REF]. The resulting methylmercury (MeHg) is incorporated and bioaccumulated, i.e. increase in concentration with age/length, naturally in marine organisms as well as biomagnified, i.e. increase in concentration with trophic position [START_REF] Biton-Porsmoguer | Mercury in blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) from north-eastern Atlantic: Implication for fishery management[END_REF][START_REF] Lavoie | Biomagnification of Mercury in Aquatic Food Webs: A Worldwide Meta-Analysis[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF]. As longlived predators at the top of food webs, sharks naturally exhibit high mercury concentrations [START_REF] Schartup | Climate change and overfishing increase neurotoxicant in marine predators[END_REF], predominantly in the MeHg form [START_REF] Carvalho | Assessment of Total and Organic Mercury Levels in Blue Sharks ( Prionace glauca ) from the South and Southeastern Brazilian Coast[END_REF][START_REF] Kim | Species-specific accumulation of methyl and total mercury in sharks from offshore and coastal waters of Korea[END_REF][START_REF] Storelli | Polychlorinated biphenyl, heavy metal and methylmercury residues in hammerhead sharks: contaminant status and assessment[END_REF]. Alongside these processes, mercury accumulation in marine predators appears to be also driven by other physiological (e.g. metabolism, ontogeny, detoxification mechanisms) [START_REF] Bolea-Fernandez | High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales ( Globicephala melas )[END_REF][START_REF] Li | Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales[END_REF], ecological (e.g. habitat, systems productivity, food web structure, foraging depth) [START_REF] Ferriss | Does trophic structure dictate mercury concentrations in top predators? A comparative analysis of pelagic food webs in the Pacific Ocean[END_REF][START_REF] Lavoie | Biomagnification of Mercury in Aquatic Food Webs: A Worldwide Meta-Analysis[END_REF][START_REF] Croizier | Stable isotope analyses revealed the influence of foraging habitat on mercury accumulation in tropical coastal marine fish[END_REF][START_REF] Senn | Stable Isotope (N, C, Hg) Study of Methylmercury Sources and Trophic Transfer in the Northern Gulf of Mexico[END_REF] and physical parameters (e.g. oxygen level, sea temperature) [START_REF] Houssard | A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF][START_REF] Schartup | Climate change and overfishing increase neurotoxicant in marine predators[END_REF]. In the ocean, mercury is subject to massindependent isotopic fractionation ("MIF", generally represented trough Δ 199 Hg values) due to its photochemical transformation in the water column [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF]. Following light attenuation with depth, Δ 199 Hg values decrease from the surface to aphotic waters [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF]. Δ 199 Hg values are also not modified during trophic transfer between a prey and its predator [START_REF] Kwon | Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)[END_REF][START_REF] Laffont | Hg Speciation and Stable Isotope Signatures in Human Hair As a Tracer for Dietary and Occupational Exposure to Mercury[END_REF], making this non-traditional isotope a powerful proxy to address trophic resources and feeding depth in marine predators (Le Croizier et al., 2020b;[START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF]. Mercury isotopic composition is also affected by mass-dependent fractionation ("MDF"), studied through δ 202 Hg values [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF][START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF]. δ 202 Hg is modified during physico-chemical processes such as photoreduction [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF] and volatilization [START_REF] Zheng | Mercury isotope fractionation during volatilization of Hg(0) from solution into the gas phase[END_REF], but also during biological processes such as methylation [START_REF] Janssen | Fractionation of Mercury Stable Isotopes during Microbial Methylmercury Production by Iron-and Sulfate-Reducing Bacteria[END_REF] and demethylation [START_REF] Perrot | Natural Hg isotopic composition of different Hg compounds in mammal tissues as a proxy for in vivo breakdown of toxic methylmercury[END_REF]. It is therefore a useful tool to study mercury metabolism in species capable of demethylating MeHg [START_REF] Bolea-Fernandez | High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales ( Globicephala melas )[END_REF][START_REF] Li | Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales[END_REF].

In this study, we used a combination of carbon, nitrogen and mercury stable isotope analyses to address resource partitioning in three sympatric pelagic shark species (blue, shortfin mako and smooth hammerhead sharks) off the west coast of the Baja California peninsula (Mexico) in the northeastern Pacific. Based on stomach contents and carbon and nitrogen stable isotopic analyses, these three species have previously been reported to display highly overlapping trophic niches [START_REF] Klarian | New insights on the trophic ecology of blue (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) from the oceanic eastern South Pacific[END_REF][START_REF] Kone | Diet of two sharks: Sphyrna zygeana (Linnaeus, 1758) and Isurus oxyrinchus (Rafinesque, 1809) of the Ivorian coast[END_REF]. However, because these sharks display different diving behavior patterns [START_REF] Logan | Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management[END_REF][START_REF] Musyl | Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean[END_REF][START_REF] Santos | Migrations and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean[END_REF] leading to different abilities to access potential prey in the mesopelagic layer, we tested the hypothesis that resource partitioning occurs along the vertical dimension of their habitat. From an ecological perspective, this would confirm the key importance of the mesopelagic compartment even for predators spending most of their time in the upper layers (Le Croizier et al., 2020b, 2020a). It would also highlight the limitations of studying only carbon and nitrogen isotopic composition in pelagic assemblages to assess the degree of trophic similarities and the importance of the addition of relevant biomarkers to better address resource partitioning. Finally, as vertical habitat has been suspected to influence mercury contamination in marine predators [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF], we evaluated if total mercury levels found in all three species could be related to foraging depth in sharks.

MATERIALS AND METHODS

Sampling strategy

From 2014 to 2016, samples were collected in the artisanal fishing camp of Punta Lobos (southern Baja California Sur, Mexico, northeast Pacific). Sharks were fished using long lines equipped with hooks. Sex and total length (TL) were recorded for each shark and approximately 1 g of muscle was extracted from the dorsal muscle between the first dorsal fin and snout. Immediately after collection, samples were transported to the laboratory on ice packs, preserved at -20 °C and freeze-dried prior to analysis. In total, 13 blue sharks (Prionace glauca), 10 shortfin mako sharks (Isurus oxyrinchus) and 13 smooth hammerhead sharks (Sphyrna zygaena) were sampled.

Carbon and nitrogen stable isotopes

Following the methodology proposed by Li et al., 2016b, two extractions were performed prior to isotopic ratio determination. To avoid δ 13 C misinterpretation, lipids were extracted by placing each sample in 6 mL of a 2:1 chloroform:methanol Folch solution [START_REF] Folch | A simple method for the isolation and purification of total lipides from animal tissues[END_REF].

Mechanical crushing using a Dounce homogenizer enhanced the extraction. The mixture was then vortexed for 1 minute, left overnight at room temperature and centrifuged for another 10 minutes before tissue extraction. This process was repeated three times. For δ 15 N determination, urea was also removed from the samples. Samples were immerged in 5 mL of distilled water, vortexed for 1 minute and left at room temperature for 24 hours. The aqueous phase was separated from the tissue after another 5 minutes centrifugation and this process was repeated three consecutive times.

Samples were then re-dried and homogenized prior to analysis.

A sample of 0.50 mg of muscle powder was weighted into tin cups using a XPR10 microbalance (METTLER TOLEDO). Stable carbon and nitrogen isotope measurements were carried out using a continuous flow on a Thermo Scientific Flash EA 2000 elemental analyzer coupled to a Delta V Plus mass spectrometer (Pole Spectrométrie Océan, IUEM, Plouzané, France). Based on international standards (Vienna Pee Dee Belemnite for δ 13 C and atmospheric nitrogen for δ 15 N), isotopic ratio (δ) are expressed in per mille (‰) following: δX = ([Rsample/Rstandard] -1) × 1000 where X is 13 C or 15 N and R is the corresponding ratio 13 C/ 12 C or 15 N/ 14 N. We repeatedly measured known international isotopic standards (i.e. IAEA-600 Caffeine, IAEA-CH-6 Sucrose, IAEA-N-1 and IAEA-N-2 Ammonium Sulphate) and an in-lab certified standard substance (i.e. Acetanilide) indicating analytical uncertainties of ± 0.23 ‰ for δ 13 C and ± 0.16 ‰ for δ 15 N. C:N ratios for all samples were ranging from 3.07 to 3.36, validating good extractions as shown by Li et al., 2016b (i.e. mean C:N ratio after lipid and urea removal of 3.2 for blue and smooth hammerhead shark and 3.1 for shortfin mako shark).

Total mercury concentration

As total mercury concentration (THg) is known to be almost exclusively in the MeHg form in shark muscle, including for the species analyzed here, e.g. 95 to 98% in blue sharks [START_REF] Carvalho | Assessment of Total and Organic Mercury Levels in Blue Sharks ( Prionace glauca ) from the South and Southeastern Brazilian Coast[END_REF][START_REF] Kim | Species-specific accumulation of methyl and total mercury in sharks from offshore and coastal waters of Korea[END_REF][START_REF] Storelli | Polychlorinated biphenyl, heavy metal and methylmercury residues in hammerhead sharks: contaminant status and assessment[END_REF], THg was used as a proxy for MeHg. THg is expressed on a dry weight basis with an analytical detection limit of 0.005 μg•g -1 dw. A 20 mg aliquot section of dry muscle was analyzed using a DMA80 analyzer (Milestone, USA) after combustion, gold trapping and atomic absorption spectrophotometry detection. The analysis accuracy and reproducibility was assessed from repeated measurements of two reference materials, a lobster hepatopancreas (TORT 3, NRCC, 0.292 ± 0.022 μg•g -1 dw) and a homogenate of tuna flesh (IAEA 436, INMM, 4.19 ± 0.36 μg•g -1 dw). Both reference materials were reproduced within the confidence limits (i.e. 0.286 ± 0.024 μg•g -1 dw for TORT 3, n = 10, and 4.20 ± 0.09 μg•g -1 dw for IAEA 436, n = 10).

Mercury isotopes

A segment of 20 mg of dry muscle was immersed into 3 mL of pure bi-distilled nitric acid (HNO3) and left at room temperature overnight. Samples were then digested at 85°C for 6 hours in pyrolyzed glass vessels closed by Teflon caps on a hotplate. After the addition of 1 mL of hydrogen peroxide (H2O2), digestion continued for another 6 hours and 100 µL of BrCl were added to complete the extraction. Finally, the solution was diluted in an inverse aqua regia (3:1 HNO3:HCl with 20 vol.% MilliQ water) to reach a total mercury concentration of 1 ng•mL -1 . Mercury isotopic compositions were measured at the Observatoire Midi-Pyrenées (Toulouse, France) using inductively coupled plasma mass spectrometry (MC-ICP-MS, Thermo Finnigan Neptune) with continuous-flow cold vapor (CV) generation using Sn(II) reduction (CETAC HGX-200), according to a previously published method (e.g. Le Croizier et al., 2020b). Mercury isotopic composition is expressed in δ notation, reported in per mil (‰) deviation from the NIST SRM 3133 standard and determined by sample-standard bracketing according to the following equation: δ XXX Hg (‰) = ([( XXX Hg/ 198 Hg)sample / ( XXX Hg/ 198 Hg)standard] -1) x 1000 where XXX represents mercury isotope different masses. δ 202 Hg represents Hg MDF, and ∆ notation is used to express Hg MIF by the following equation: ∆ XXX Hg (‰) = δ XXX Hg -(δ 202 Hg × a) [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF], where a = 0.252, 0.502, 0.752 and 1.493 for isotopes 199, 200, 201 and 204, respectively. Blanks as well as certified materials (i.e. NRC TORT-3 and ERM-BCR-464) were analyzed with the same procedure. Total mercury concentration in the diluted digest mixtures was monitored by the 202 Hg signals provided by MC-ICP-MS. A recovery rate of 96 ± 7% (n = 37) for shark samples and 95 ± 6% (n = 7) for certified reference materials was obtained. Reproducibility of mercury isotope measurements was assessed by analyzing UM-Almadén (n = 4), ETH-Fluka (n = 4) and the biological tissue procedural standards NRC TORT-3 (n = 3) and ERM-BCR-464 (n = 4). Only one analysis was performed per sample, but measured isotope signatures as well as analytical reproducibility of standards agreed with previously published values (Table S1).

Data analysis

For comparison between shark species, data was first checked for normality using Shapiro-Wilk test and for equality of variances using Bartlett's test. When both conditions were met, one-way ANOVA were performed followed by a post-hoc Tukey's HSD test. Otherwise, we used its nonparametric analogue, the Kruskal-Wallis test, followed by Dunn's post hoc test with Bonferroni's adjustment in the presence of several groups. Between sexes comparisons (Student t test or its nonparametric analogue Wilcoxon test) are described in Table S2.

We used 2D (δ 13 C and δ 15 N) standard ellipse areas encompassing 95% of the data (SEAB) calculated using the SIBER package [START_REF] Jackson | Comparing isotopic niche widths among and within communities: SIBER -Stable Isotope Bayesian Ellipses in R[END_REF] from R programming language (R Core Team, 2019). 3D (δ 13 C, δ 15 N and Δ 199 Hg) standard ellipsoid volumes encompassing 95% of the data (SEVB) were calculated using the SIBER-derived model described by [START_REF] Skinner | Novel triisotope ellipsoid approach reveals dietary variation in sympatric predators[END_REF], for three-dimensional coordinate systems. To allow for comparison between the two model outputs, we expressed isotopic overlap as a proportion of the non-overlapping area of the two 2D ellipses (SEAB) or 3D ellipsoids (SEVB).

Pearson correlation test was used to describe the linear correlation between THg and Δ 199 Hg.

Generalized linear models (GLMs) were used to evaluate the influence of species, total length, C, N and Hg isotope signatures on muscle mercury levels. GLMs were built with the LME4 package [START_REF] Bates | Fitting Linear Mixed-Effects Models Using lme4[END_REF] using THg as the response variable. Based on diagnostic plots of the residuals, a Gaussian distribution and identity link function were used in the GLMs. Predictor variables were species, age, δ 13 C, δ 15 N, Δ 199 Hg and δ 202 Hg. Age was estimated for each individual using growth parameters established for blue [START_REF] Blanco-Parra | Age and growth of the blue shark, Prionace glauca Linnaeus, 1758, in the Northwest coast off Mexico[END_REF], shortfin mako [START_REF] Ribot-Carballal | Age and growth of the shortfin mako shark, Isurus oxyrinchus, from the western coast of Baja California Sur, Mexico[END_REF] and smooth hammerhead sharks [START_REF] Villatoro | Edad y crecimiento del tiburon martillo Sphyrna zygaena (LINNAEUS, 1758) en la costa occidental de baja california sur[END_REF] in the studied region. Models were built using backward stepwise selection which consists in building a model containing all predictor variables and removing gradually each predicator variable until no variable is left in the model (i.e. null model). Akaike's Information Criterion (AIC) was used to define the order of deletion as the model with the lowest AIC is retained for the next step. All models were ranked based on Akaike's Information Criterion adjusted for small sample sizes (AICc) and Akaike weights (w) using the R package WIQID [START_REF] Meredith | wiqid: Quick and Dirty Estimates for Wildlife Populations[END_REF]. Marginal R² were applied to assess each model predictive power using the R package R2GLMM [START_REF] Jaeger | r2glmm: Computes R Squared for Mixed (Multilevel) Models[END_REF].

RESULTS AND DISCUSSION

Carbon and nitrogen isotopic composition

A summary of the results is presented in Table 1. For all variables (TL, δ 13 C, δ 15 N, Δ 199 Hg, δ 202 Hg and THg), no significant difference between sexes within species was detected (Table S2). In marine ecosystems, δ 13 C is known to vary between habitats (e.g. coastal versus oceanic), according to primary producers supporting the food webs (e.g. benthic producers versus phytoplankton) [START_REF] Fry | δ13C Measurements as Indicators of Carbon Flow in Marine and Freshwater Ecosystems[END_REF]. Here, no significant difference in δ 13 C was detected between the three species (X²35,2 = 6.1, p>0.05). These similarities in δ 13 C profiles (Table 1) between the three species suggest that they forage on equivalent pelagic food webs derived from phytoplankton production, in accordance with reported data on shortfin mako and blue sharks in the study area [START_REF] Hernández-Aguilar | Trophic ecology of the blue shark (Prionace glauca) based on stable isotopes (δ13C and δ15N) and stomach content[END_REF][START_REF] Tamburin | Isotopic niche and resource sharing among young sharks (Carcharodon carcharias and Isurus oxyrinchus) in Baja California, Mexico[END_REF].

We found significant δ 15 N differences among species (F35,2 = 17.2, p<0.001). The smooth hammerhead shark presented significantly higher δ 15 N than both the shortfin mako (Tukey's HSD test, p<0.05) and the blue shark (Tukey's HSD test, p<0.001). In contrast, the blue shark was 15 N depleted compared to the shortfin mako shark (Tukey's HSD test, p<0.05). In the case of mobile toppredator species evolving in the pelagic habitat, these differences in δ 15 N can reflect either differences in relative trophic position [START_REF] Cabana | Modelling food chain structure and contaminant bioaccumulation using stable nitrogen isotopes[END_REF] or foraging in different regions with contrasted δ 15 N baselines [START_REF] Lorrain | Nitrogen isotopic baselines and implications for estimating foraging habitat and trophic position of yellowfin tuna in the Indian and Pacific Oceans[END_REF]. In the northeastern Pacific region, no information is available on smooth hammerhead shark movement, while both blue and shortfin mako are known to perform limited horizontal movement without a clear seasonal pattern that could suggest foraging on broad different δ 15 N ecosystem baselines [START_REF] Musyl | Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean[END_REF]Queiroz et al., 2019;[START_REF] Sepulveda | Movement patterns, depth preferences, and stomach temperatures of free-swimming juvenile mako sharks, Isurus oxyrinchus, in the Southern California Bight[END_REF]. Therefore, their isotopic signatures likely represent an integrated signal of these shared habitat baselines. The significant differences observed in δ 15 N signature between the three species might overall be due to differences in trophic levels, with blue sharks occupying the lowest trophic level and smooth hammerhead sharks the highest. In the study region, all three species are known to principally rely on different cephalopod species as observed in previous stomach content analyses. Shortfin mako sharks feed on Dosidicus gigas [START_REF] Velasco Tarelo | Hábitos alimenticios e isótopos de 13C y 15N del tiburón mako Isurus oxyrinchus (Rafinesque, 1810) en la costa occidental de Baja California Sur[END_REF], blue sharks on Onychoteuthis banksii, Gonatus californiensis and D. gigas and smooth hammerhead sharks on D. gigas, Ancitrocheirus lesueurii and O. banksii [START_REF] Galván-Magaña | Shark predation on cephalopods in the Mexican and Ecuadorian Pacific Ocean[END_REF]. The apparent differences between the trophic levels of the three shark species could be the result of broad differences in cephalopods trophic levels. Indeed, the observed mean difference in δ 15 N values between blue and smooth hammerhead sharks (2.15 ‰) matches the important variation in δ 15 N signatures previously observed for cephalopod species sampled in the region [START_REF] Madigan | Stable Isotope Analysis Challenges Wasp-Waist Food Web Assumptions in an Upwelling Pelagic Ecosystem[END_REF].

Inter-specific differences in foraging depth

Both Δ 199 Hg and δ 202 Hg signals vary vertically throughout the water column due to photochemical transformation of Hg following solar radiation decrease from surface to deep water layers [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF][START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF]. As they are transferred from prey to predator, they therefore can be used as trophic biomarkers to assess foraging depth. However, while Δ 199 Hg is only affected through photochemical reactions and is conserved from prey to predator, δ 202 Hg is also modified by physiological processes like methylation or demethylation of mercury [START_REF] Bolea-Fernandez | High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales ( Globicephala melas )[END_REF][START_REF] Janssen | Fractionation of Mercury Stable Isotopes during Microbial Methylmercury Production by Iron-and Sulfate-Reducing Bacteria[END_REF][START_REF] Li | Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales[END_REF][START_REF] Perrot | Natural Hg isotopic composition of different Hg compounds in mammal tissues as a proxy for in vivo breakdown of toxic methylmercury[END_REF] and undergoes inconstant trophic discrimination factors from prey to predator [START_REF] Kwon | Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)[END_REF][START_REF] Laffont | Hg Speciation and Stable Isotope Signatures in Human Hair As a Tracer for Dietary and Occupational Exposure to Mercury[END_REF], especially in shark species (Le Croizier et al., 2020b). Moreover, for blue, shortfin mako and smooth hammerhead sharks, metabolic MeHg detoxification pathways seems to occur. Indeed, the Δ 199 Hg/δ 202 Hg slope is traditionally used to assess the influence of photodegradation versus microbial transformation on the isotopic signature of mercury before its incorporation into the food web [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF]. Here, no significant linear regression could be obtained between Δ 199 Hg and δ 202 Hg for all shark species at the inter-or intraspecific level (Figure S1). This lack of correlation between Δ 199 Hg and δ 202 Hg is observed in species showing in vivo demethylation modifying the δ 202 Hg values [START_REF] Li | Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales[END_REF] and suggests possible MeHg detoxification processes in the shark species studied here (Le Croizier et al., 2020b). Therefore, as δ 202 Hg in all three shark species appeared to depend on both trophic and physiological features, this isotopic ratio was not taken into account to assess differences in species trophic ecology and subsequently to study the possible food competition between them.

In nearshore ecosystems, Δ 199 Hg can vary seasonally and spatially due to coastal phenomena affecting water turbidity [START_REF] Senn | Stable Isotope (N, C, Hg) Study of Methylmercury Sources and Trophic Transfer in the Northern Gulf of Mexico[END_REF]. In this study, the fact that all three shark species were pelagic species using oceanic habitats (as inferred by δ 13 C values) confirms that reported Δ 199 Hg may vary primarily over a vertical gradient depending on photochemical process affected by depth [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF]. Therefore, the range of individual Δ 199 Hg values (1.40 to 2.13 ‰) observed in this study highlights the importance of mesopelagic prey in the diet of all three oceanic species. In the north Pacific oceanic region, similar Δ 199 Hg signatures in fish muscles have been observed for species foraging in the twilight zone near Hawaii, i.e. 1.00 to 2.56 ‰ [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF], and off the coast of California, i.e. 0.95 to 2.31 ‰ [START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF], including for the great white shark, Carcharodon carcharias, i.e. 1.25 to 1.95 ‰ (Le Croizier et al., 2020a). This conclusion is supported by the consistent presence of mesopelagic species in stomach contents of blue [START_REF] Hernández-Aguilar | Trophic ecology of the blue shark (Prionace glauca) based on stable isotopes (δ13C and δ15N) and stomach content[END_REF][START_REF] Markaida | Food and feeding habits of the blue shark Prionace glauca caught off Ensenada, Baja California, Mexico, with a review on its feeding[END_REF], shortfin mako [START_REF] Lopez | Alimentación del tiburón marrajo Isurus oxyrinchus Rafinesque, 1810 (Lamniformes: Lamnidae) en el Pacífico suroriental[END_REF][START_REF] Preti | Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current[END_REF] and smooth hammerhead sharks [START_REF] Estupiñán-Montaño | Feeding habits and trophic level of the smooth hammerhead shark, Sphyrna zygaena (Carcharhiniformes: Sphyrnidae), off Ecuador[END_REF][START_REF] Galván-Magaña | Shark predation on cephalopods in the Mexican and Ecuadorian Pacific Ocean[END_REF] sampled off the west coast of Baja California and across the Pacific Ocean. As mercury has a similar turnover rate as carbon and nitrogen in fish muscle, i.e. 1 year or more [START_REF] Kwon | Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)[END_REF], deep foraging appears as a constant strategy through time. The three shark species remain most of the time inside the boundaries of the surface mixed layer but exhibit differences in their diving behaviors in the study length range. Indeed, while blue sharks perform frequent deep dives [START_REF] Campana | Migration Pathways, Behavioural Thermoregulation and Overwintering Grounds of Blue Sharks in the Northwest Atlantic[END_REF][START_REF] Queiroz | Short-term movements and diving behaviour of satellite-tracked blue sharks Prionace glauca in the northeastern Atlantic Ocean[END_REF], shortfin mako sharks seem to exploit deep water more sporadically undergoing less frequent dives in the mesopelagic zone [START_REF] Abascal | Movements and environmental preferences of the shortfin mako, Isurus oxyrinchus, in the southeastern Pacific Ocean[END_REF][START_REF] Musyl | Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean[END_REF].

Finally, although no study reported depth habitat use in smooth hammerhead sharks in the Pacific region, data from the Atlantic Ocean demonstrated limited diving frequency associated with shallower dives than both shortfin mako and blue sharks [START_REF] Logan | Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management[END_REF][START_REF] Santos | Migrations and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean[END_REF]. Here, Δ 199 Hg corroborates the deep diving behavior observed for blue and shortfin mako sharks and proves that they are associated with foraging in the mesopelagic layer. It also suggests that smooth hammerhead sharks might feed at depth and that populations from the northeastern Pacific region might differ from the Atlantic ones by using deeper water layers [START_REF] Logan | Seasonal Movements and Habitat Use of Juvenile Smooth Hammerhead Sharks in the Western North Atlantic Ocean and Significance for Management[END_REF][START_REF] Santos | Migrations and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean[END_REF].

We observed significant inter-specific differences in Δ 199 Hg ratio (X²35,2 = 17.8, p<0.001) revealing differences in mean foraging depth. Blue sharks presented Δ 199 Hg value significantly lower than shortfin mako sharks (Dunn's test, p<0.001) and smooth hammerhead sharks (Dunn's test, p<0.01). This could not be explained by different isotope fractionation between prey and predator, as Δ 199 Hg values are conserved during trophic transfers [START_REF] Kwon | Quantifying mercury isotope dynamics in captive Pacific bluefin tuna (Thunnus orientalis)[END_REF][START_REF] Laffont | Hg Speciation and Stable Isotope Signatures in Human Hair As a Tracer for Dietary and Occupational Exposure to Mercury[END_REF]. Thus, the significant gap in Δ 199 Hg between species suggests systematic differences in foraging depths (Le Croizier et al., 2020b). The lower Δ 199 Hg of the blue shark suggests constant foraging in deeper water than the two other species. This conclusion is supported by stomach content analyses, which revealed that this species was the only one with bathypelagic prey in its gut [START_REF] Galván-Magaña | Shark predation on cephalopods in the Mexican and Ecuadorian Pacific Ocean[END_REF]. On the other hand, epipelagic prey were also commonly reported in the stomach of the three shark species [START_REF] Galván-Magaña | Shark predation on cephalopods in the Mexican and Ecuadorian Pacific Ocean[END_REF][START_REF] Hernández-Aguilar | Trophic ecology of the blue shark (Prionace glauca) based on stable isotopes (δ13C and δ15N) and stomach content[END_REF][START_REF] Wood | Recalculated diet and daily ration of the shortfin mako (Isurus oxyrinchus), with a focus on quantifying predation on bluefish (Pomatomus saltatrix) in the northwest Atlantic Ocean[END_REF]. Δ 199 Hg signatures observed in this study may thus be the result of the consumption of both epi-and mesopelagic prey as reported for the Pacific bluefin tuna Thunnus orientalis [START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF].

Differences in the relative importance of prey from these two compartments could therefore result in the observed Δ 199 Hg differences between shark species. Compared to deeper species, epipelagic prey may form dense aggregations with higher nutritive and energetic value [START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF][START_REF] Spitz | Prey selection by the common dolphin: Fulfilling high energy requirements with high quality food[END_REF]. However, they are more scattered across time and space resulting in shark feeding opportunistically on them depending on the season, geographic position and maturity stage [START_REF] Hernández-Aguilar | Trophic ecology of the blue shark (Prionace glauca) based on stable isotopes (δ13C and δ15N) and stomach content[END_REF][START_REF] Maia | Food habits of the shortfin mako, Isurus oxyrinchus , off the southwest coast of Portugal[END_REF][START_REF] Rosas-Luis | Feeding ecology and trophic relationships of pelagic sharks and billfishes coexisting in the central eastern Pacific Ocean[END_REF]. This behavior is frequently observed in shortfin mako sharks, which favor a diet dominated by shallower teleost when seasonally and locally available [START_REF] Harford | Trophic Modeling of Shortfin Mako (Isurus Oxyrinchus) and Bluefish (Pomatomus Saltatrix) Interactions in the Western North Atlantic Ocean[END_REF][START_REF] Maia | Food habits of the shortfin mako, Isurus oxyrinchus , off the southwest coast of Portugal[END_REF]. Compared to the shortfin mako, blue sharks display a more consistent diet targeting less nutritive but more reliable mesopelagic prey [START_REF] Preti | Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current[END_REF][START_REF] Vollenweider | Seasonal cycles in whole-body proximate composition and energy content of forage fish vary with water depth[END_REF]. The mesopelagic food web indeed appears more stable through time and supports high prey biomasses in the northeast Pacific [START_REF] Davison | Mesopelagic fish biomass in the southern California current ecosystem[END_REF][START_REF] Hazen | Meridional patterns in the deep scattering layers and top predator distribution in the central equatorial Pacific[END_REF]. Therefore, foraging on these deeper prey might represent a more reliable feeding strategy and involve less metabolic costs associated to foraging on more scattered epipelagic prey.

Overall, the highest Δ 199 Hg of blue sharks compared to shortfin mako sharks might be the result of different foraging strategies, with the blue shark occupying a deeper ecological niche and the shortfin mako shark favoring opportunistic foraging on epipelagic prey. Smooth hammerhead shark Δ 199 Hg was not significantly different from the shortfin mako shark (Dunn's test, p>0.05), hence, suggesting feeding at shallower depths than blue sharks and/or at equivalent rates on deeper organisms than shortfin mako sharks.

Resource partitioning between co-occurring predators

The overlaps between the three species SEAB (i.e. δ 13 C and δ 15 N, Figure 1) and SEVB (i.e. δ 13 C, δ 15 N and Δ 199 Hg, Figure 2) are presented in Table 2. The limited differences between δ 13 C and δ 15 N isotopic compositions resulted in significant isotopic overlaps between species except between blue and smooth hammerhead sharks (16.8%). The overlap decreased by adding Δ 199 Hg for all pairs of species. This decrease was the strongest for shortfin mako and blue sharks (from 42.0% to 23.0%). A smaller overlap decrease was observed between shortfin mako and smooth hammerhead sharks, which presented no significant difference in Δ 199 Hg, and between blue and smooth hammerhead sharks, already well separated by δ 15 N and to a less extent by δ 13 C (i.e. 5.9% and 3.9% decrease respectively). In the latter case, even if significant differences occurred in Δ 199 Hg, SEAB and SEVB overlapping areas appeared equivalent suggesting that carbon and nitrogen isotopic ratios can be sufficient to depict resource partitioning in the case of co-existing shark species feeding on different habitats or prey [START_REF] Curnick | Evidence for dynamic resource partitioning between two sympatric reef shark species within the British Indian Ocean Territory[END_REF], although always overlooking the vertical dimension.

Resource partitioning within pelagic shark assemblages has been extensively studied using δ 13 C and δ 15 N, yielding frequent records of important overlapping areas [START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF]Li et al., 2016a) suggesting similar foraging niches. In the eastern Pacific, similar overlaps were recorded between shortfin mako and blue shark [START_REF] Klarian | New insights on the trophic ecology of blue (Prionace glauca) and shortfin mako sharks (Isurus oxyrinchus) from the oceanic eastern South Pacific[END_REF][START_REF] Rosas-Luis | Feeding ecology and trophic relationships of pelagic sharks and billfishes coexisting in the central eastern Pacific Ocean[END_REF]. From a methodological perspective, these similarities could however be expected in the case of these pelagic species. Indeed, by foraging in the same region and on similar phytoplankton derived food web, sympatric pelagic sharks would exhibit similar δ 13 C signatures [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF], and possibly similar δ 15 N depending on prey trophic position, even when feeding on different prey. The observed similarities in carbon and nitrogen isotopic niches between blue and shortfin mako sharks (overlapping at 42.0%) in this study may therefore be due to these processes, rather than to a clear similarity in dietary habits.

Moreover, important overlaps in isotopic signatures are not in accordance with previous stomach content analysis of both species in the northeastern Pacific region [START_REF] Preti | Comparative feeding ecology of shortfin mako, blue and thresher sharks in the California Current[END_REF][START_REF] Rosas-Luis | Feeding ecology and trophic relationships of pelagic sharks and billfishes coexisting in the central eastern Pacific Ocean[END_REF]. Hence, the differences in mean foraging depth between the two species increased resource partitioning estimation between the blue shark and the shortfin mako shark (19.0% decrease in overlapping area).

Our results therefore demonstrate that carbon and nitrogen isotopic signatures overestimate overlapping areas by not reflecting the importance of foraging depth, which was estimated here by Δ 199 Hg. In the case of pelagic sharks migrating vertically, carbon and nitrogen stable isotopes analysis has shown some limitations in its capacity to address foraging depth issues. Indeed, even if vertical patterns in nitrogen isotopic baselines have been reported in zooplankton [START_REF] Hannides | Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective[END_REF], such patterns are rarely observed for top predators [START_REF] Choy | Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions[END_REF]. However, the vertical foraging component has been suspected of critical importance in the ecology of marine predators.

For example, computational models based on prey distribution systematically resulted in the emergence of vertical movements in tropical oceanic predatory fishes [START_REF] Dagorn | Co-evolution of movement behaviours by tropical pelagic predatory fishes in response to prey environment: a simulation model[END_REF].

Furthermore, in pelagic systems, differences in vertical movement patterns in sympatric toppredator species, including sharks, have already been demonstrated [START_REF] Choy | Trophic structure and food resources of epipelagic and mesopelagic fishes in the North Pacific Subtropical Gyre ecosystem inferred from nitrogen isotopic compositions[END_REF][START_REF] Musyl | Postrelease survival, vertical and horizontal movements, and thermal habitats of five species of pelagic sharks in the central Pacific Ocean[END_REF]. In our study, the systematic decreases in overlapping area for all pair of comparisons, after the incorporation of Δ 199 Hg, demonstrates that differences in foraging depth better explain trophic niche partitioning between shark species.

Influence of foraging depth on mercury exposure

Muscle THg (Table 1) significantly differed among the three species (X²35,2 = 15.2, p<0.001).

Blue sharks presented higher THg compared to shortfin mako (Dunn's test, p<0.05) and smooth hammerhead sharks (Dunn's test, p<0.001), as already observed in a previous study in the area [START_REF] Maz-Courrau | Bioaccumulation and Biomagnification of Total Mercury in Four Exploited Shark Species in the Baja California Peninsula, Mexico[END_REF]. The blue shark was the species presenting both higher THg and lower Δ 199 Hg.

Regardless of the species, THg was higher for individuals foraging on the deepest mesopelagic prey (i.e. exhibiting the lowest Δ 199 Hg) as shown by the significant and negative correlation between the two variables (Figure 3). In the open Pacific Ocean, MeHg, the most bioavailable form of mercury, is mainly produced in the mesopelagic layer and especially in the Oxygen Minimum Zone [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Fitzgerald | Marine Biogeochemical Cycling of Mercury[END_REF]. As MeHg is trophically incorporated in food webs, sharks feeding in the mesopelagic zone will be exposed to higher MeHg levels compared to shallow feeding sharks, as highlighted by our results. This is in agreement with previous observation on fish [START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF], sea birds [START_REF] Thompson | Seabirds as biomonitors of mercury inputs to epipelagic and mesopelagic marine food chains[END_REF] and pelagic predators [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Houssard | A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF][START_REF] Madigan | Mercury Stable Isotopes Reveal Influence of Foraging Depth on Mercury Concentrations and Growth in Pacific Bluefin Tuna[END_REF] that demonstrated higher mercury contamination in deeper foraging individuals or species.

In generalized linear models (GLMs), Δ 199 Hg and species were the main factor explaining shark THg compared to other variables (Table S3), confirming that foraging depth was a key driver of shark mercury concentration in our dataset. Foraging depth has rarely been assessed in THg accumulation studies of marine predators except by qualitative approaches such as attributing a median depth of occurrence [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF] or habitat preference (Le [START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF] to the studied species. It is interesting to note that this approach would not have been relevant for blue, shortfin mako and smooth hammerhead sharks that remain in the upper oceanic layers and occasionally undergo deep bounce dives to feed on mesopelagic prey [START_REF] Abascal | Movements and environmental preferences of the shortfin mako, Isurus oxyrinchus, in the southeastern Pacific Ocean[END_REF][START_REF] Campana | Bycatch and discard mortality in commercially caught blue sharks Prionace glauca assessed using archival satellite pop-up tags[END_REF][START_REF] Queiroz | Short-term movements and diving behaviour of satellite-tracked blue sharks Prionace glauca in the northeastern Atlantic Ocean[END_REF][START_REF] Sepulveda | Movement patterns, depth preferences, and stomach temperatures of free-swimming juvenile mako sharks, Isurus oxyrinchus, in the Southern California Bight[END_REF]. In our study, mercury isotopic composition offers new opportunities to implement quantitative approaches of foraging depth. As Δ 199 Hg was the main driver of THg (along with species), we suggest that this factor should be investigated in future research regarding mercury accumulation patterns in elasmobranchs.

In shark species, the pattern observed for THg in muscular tissue has been generally linked to changes in trophic level, generally estimated through δ 15 N values [START_REF] Biton-Porsmoguer | Mercury in blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) from north-eastern Atlantic: Implication for fishery management[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF]. Here, δ 15 N did not explain THg variation. Although the limited number of samples may not cover the entire spectrum of length and trophic levels for all shark species, influence of such small δ 15 N spectrum on THg has been previously demonstrated [START_REF] Croizier | Stable isotope analyses revealed the influence of foraging habitat on mercury accumulation in tropical coastal marine fish[END_REF]. This therefore implies that foraging depth could affect more significantly THg in pelagic shark species than their respective trophic levels, but further investigations at a specific level or with broader size and δ 15 N range are needed to confirm this effect.

THg in marine organisms depends not only on trophic features [START_REF] Ferriss | Does trophic structure dictate mercury concentrations in top predators? A comparative analysis of pelagic food webs in the Pacific Ocean[END_REF][START_REF] Bourg | Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean[END_REF][START_REF] Thompson | Seabirds as biomonitors of mercury inputs to epipelagic and mesopelagic marine food chains[END_REF] but also on species physiological characteristics such as longevity, metabolic, growth and feeding rates and/or possible detoxication processes [START_REF] Bolea-Fernandez | High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales ( Globicephala melas )[END_REF][START_REF] Houssard | A Model of Mercury Distribution in Tuna from the Western and Central Pacific Ocean: Influence of Physiology, Ecology and Environmental Factors[END_REF][START_REF] Li | Selenium and stable mercury isotopes provide new insights into mercury toxicokinetics in pilot whales[END_REF][START_REF] Senn | Stable Isotope (N, C, Hg) Study of Methylmercury Sources and Trophic Transfer in the Northern Gulf of Mexico[END_REF]. This can be seen in our results as the best-fitted model incorporated species as a key variable explaining THg (along with Δ 199 Hg). However, all these parameters could not be tested in this study, except for longevity and demethylation mechanisms which are known to increase δ 202 Hg values [START_REF] Bolea-Fernandez | High-precision isotopic analysis sheds new light on mercury metabolism in long-finned pilot whales ( Globicephala melas )[END_REF][START_REF] Senn | Stable Isotope (N, C, Hg) Study of Methylmercury Sources and Trophic Transfer in the Northern Gulf of Mexico[END_REF]. Here, δ 202 Hg values varied significantly between species (Table 1;F35,2 = 4.4,p<0.05), as previously observed for other co-occurring shark species (Le Croizier et al., 2020b). Still, δ 202 Hg variations did not affect THg in GLMs (as it can also be seen in the relation between the two variables in Figure S2) showing no link between mercury contamination and the demethylation process previously highlighted by the absence of correlation between Δ 199 Hg and δ 202 Hg (Figure S1). Age was also not affecting THg while differences between species were also significant (F35,2 = 8.6, p<0.001), with blue sharks encompassing the oldest individuals and shortfin mako sharks the youngest (Table S5). Overall, the fact that metabolic demethylation and individual age did not critically affect THg strengthens the hypothesis that ecological characteristics were mainly driving muscle mercury contamination in all shark species. As all physiological characteristics could not be analyzed in this study, future investigations on physiological intra-specific differences between blue, shortfin mako and smooth hammerhead sharks might help to identify the entire mechanisms behind their THg.

Insights into mercury cycle

In the northeastern Pacific region, atmospheric mercury deposition dominates mercury inputs to the water column as river influence appears negligible [START_REF] Masbou | Hg-Stable Isotope Variations in Marine Top Predators of the Western Arctic Ocean[END_REF][START_REF] Zhang | Natural biogeochemical cycle of mercury in a global threedimensional ocean tracer model[END_REF]. This deposition at the atmosphere/ocean interface has two different origins: inorganic mercury (iHg) via precipitation and atmospheric gaseous mercury (Hg(0)) through dissolution [START_REF] Gratz | Isotopic Composition and Fractionation of Mercury in Great Lakes Precipitation and Ambient Air[END_REF][START_REF] Zhang | Natural biogeochemical cycle of mercury in a global threedimensional ocean tracer model[END_REF]. While the mechanisms causing mass-independent isotopic fractionation (MIF) of even-mass Hg isotopes are still poorly understood, processes such as photooxidation of Hg(0) to iHg in the atmosphere (e.g. tropopause) may be involved leading to isotopic fractionation [START_REF] Chen | Unusual fractionation of both odd and even mercury isotopes in precipitation from Peterborough, ON, Canada[END_REF]. Therefore, iHg deposition by precipitation presents slightly different Δ 200 Hg signatures ranging from 0 to 0.3 ‰ than atmospheric Hg(0) dissolution characterized by Δ 200 Hg between -0.11 and -0.01 ‰ [START_REF] Enrico | Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition[END_REF][START_REF] Gratz | Isotopic Composition and Fractionation of Mercury in Great Lakes Precipitation and Ambient Air[END_REF]. Once in the water column, Δ 200 Hg values are conserved, making it a good tracer of atmospheric deposition pathways even when analyzed in top-predator species [START_REF] Enrico | Atmospheric Mercury Transfer to Peat Bogs Dominated by Gaseous Elemental Mercury Dry Deposition[END_REF]Le Croizier et al., 2020b). Here, Δ 200 Hg values ranged from -0.07 to 0.14 ‰ with no significant differences between species (F35,2 = 1.4, p>0.05; i.e. 0.05 ± 0.05 ‰ for blue and smooth hammerhead sharks and 0.02 ± 0.05 ‰ for shortfin mako sharks). These values are similar to those observed in bottomfish from Hawaii, i.e.

-0.04 to 0.10 ‰ [START_REF] Sackett | Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish[END_REF], and to other coastal shark species sampled off La Réunion Island, i.e. 0.08 ± 0.04 ‰ in bull sharks, Carcharhinus leucas, and 0.06 ± 0.04 ‰ in tiger sharks, Galeocerdo cuvier (Le Croizier et al., 2020b). For the three shark species, Δ 200 Hg values suggest a common origin in the mercury precursors of MeHg in shark tissues, probably from a combined source of both iHg and Hg(0) because of both positive and negative Δ 200 Hg compositions (Le Croizier et al., 2020b).

Due to solar radiation, photodemethylation can transform dissolved MeHg into iHg and photoreduction can convert iHg into Hg(0) in the water column. Both of these reactions are characterized by a Δ 199 Hg/Δ 201 Hg ratio of respectively 1.36 and 1.00 [START_REF] Bergquist | Mass-Dependent and -Independent Fractionation of Hg Isotopes by Photoreduction in Aquatic Systems[END_REF]. All shark species considered, Δ 199 Hg/Δ 201 Hg slope was 1.16 (Figure S3), not clearly indicating the prevalence of one reaction over another. Surprisingly this is not in accordance with previous studies reporting the dominance of MeHg demethylation in oceanic island marine ecosystems (Le Croizier et al., 2020b;[START_REF] Sackett | Carbon, Nitrogen, and Mercury Isotope Evidence for the Biogeochemical History of Mercury in Hawaiian Marine Bottomfish[END_REF]. Indeed, the ratio observed here seems to represent a mixed signature of both phenomena. Interestingly, in Hawaii, deep-foraging species exhibit a flatter Δ 199 Hg/Δ 201 Hg slope (1.05 ratio) compared to shallower species (1.21 ratio) [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Masbou | Hg-Stable Isotope Variations in Marine Top Predators of the Western Arctic Ocean[END_REF]. The intermediate slope in this study is therefore consistent with shark species foraging on mesopelagic prey exhibiting a flatter slope than epipelagic ones, reinforcing the conclusion that all three shark species foraged mainly at depth.

Conclusions

Based on δ 13 C, δ 15 N and Δ 199 Hg, this study provided new information on the trophic ecology of blue, shortfin mako and smooth hammerhead sharks in the eastern Pacific region. Mesopelagic prey appeared as a major component in the diet of the three species. Our study demonstrated the importance of considering foraging depth when studying resource partitioning between co-occurring pelagic predators. In the northeastern Pacific, blue sharks appeared to forage deeper than mako and smooth hammerhead sharks and these foraging strategies seemed to reduce trophic competition between them. Since carbon and nitrogen isotopes did clearly underestimate resource partitioning, this study confirmed the usefulness of multi-isotopic approaches to help fine scaling resource partitioning in top-predators depending on the ecology of the studied species. Our results highlighted the underexploited potential of mercury stable isotopes for marine ecology studies. This new tool has the potential to elucidate possible ontogenetic variation in depth utilization for these species, as deep diving at early life stages is a rare pattern for blue and shortfin mako sharks [START_REF] Nosal | Movement ecology of young-of-the-year blue sharks Prionace glauca and shortfin makos Isurus oxyrinchus within a putative binational nursery area[END_REF] and could be limited inside shallow coastal nursery areas for smooth hammerhead sharks [START_REF] Francis | Distribution, habitat and movement of juvenile smooth hammerhead sharks (Sphyrna zygaena) in northern New Zealand[END_REF][START_REF] Santos | Migrations and habitat use of the smooth hammerhead shark (Sphyrna zygaena) in the Atlantic Ocean[END_REF]. Moreover, there are increasing evidences of vertical habitat partitioning between different predator species in other pelagic ecosystems [START_REF] Madigan | Water column structure defines vertical habitat of twelve pelagic predators in the South Atlantic[END_REF] and mercury stable isotopes could help to precise the mechanisms behind such vertical structuring.

As foraging depth appeared a key factor influencing mercury exposure for the three species, it should be more extensively studied to understand mercury accumulation in top predators. In the context of climate change, Oxygen Minimum Zones are observed at shallower depth in tropical and subtropical regions, acting as a physical barrier and preventing sharks to forage deeper in the water column [START_REF] Vedor | Climate-driven deoxygenation elevates fishing vulnerability for the ocean's widest ranging shark[END_REF], particularly in the tropical eastern Pacific [START_REF] Trucco-Pignata | Ventilation of the Upper Oxygen Minimum Zone in the Coastal Region Off Mexico: Implications of El Niño 2015-2016[END_REF]. This habitat compression could therefore limit the possibility for co-occurring shark species to forage at significant different depths and could lead to new competition processes between pelagic predators that should be carefully monitored. Comparison is made between the SIBER ellipse metric for δ 13 C and δ 15 N, SEAB [START_REF] Jackson | Comparing isotopic niche widths among and within communities: SIBER -Stable Isotope Bayesian Ellipses in R[END_REF], and the ellipsoid approach combining δ 13 C, δ 15 N and Δ 199 Hg, SEVB [START_REF] Skinner | Novel triisotope ellipsoid approach reveals dietary variation in sympatric predators[END_REF].

δ

Figure 1 :

 1 Figure 1: δ 13 C and δ 15 N of blue ( ), shortfin mako ( ), and smooth hammerhead ( ) sharks. The represented ellipses encompass 95% of the data.

Figure 2 :

 2 Figure 2: 95% ellipsoids using δ 13 C, δ 15 N and Δ 199 Hg for blue, shortfin mako and smooth hammerhead sharks respectively represented in blue, grey and yellow.

Figure 3 :

 3 Figure 3: Variation of THg with Δ 199 Hg values in the muscle of blue ( ), shortfin mako ( ) and smooth hammerhead ( ) sharks. Data fit a linear curve. Pearson correlation (r² value) was significant as indicated by its p-value.

Table 1 :

 1 Number of individuals, total length, C, N and Hg isotope composition, and total mercury concentration (THg) in muscles of blue, shortfin mako and smooth hammerhead sharks. Data are means (± standard deviation). THg is expressed on a dry weight basis. Different letters indicate significant differences between species for each variable.

	Species	N	Total length (m)	δ 13 C (‰)	δ 15 N (‰)	Δ 199 Hg (‰)	δ 202 Hg (‰) THg (ng•g -1 )
	Blue	13	1.98 (± 0.36)	-16.81 (± 0.91) A	18.15 (± 1.07) A	1.56 (± 0.10) A	0.63 (± 0.16) AB	7804 (± 2699) A
	Shortfin mako	10	1.49 (± 0.43)	-16.36 (± 0.55) A	19.13 (± 1.10) B	1.94 (± 0.23) B	0.53 (± 0.15) A	4772 (± 3892) B
	Smooth hammerhead	13	1.67 (± 0.16)	-16.25 (± 0.85) A	20.30 (± 0.60) C	1.82 (± 0.15) B	0.71 (± 0.12) B	3600 (± 1524) B

Table 2 :

 2 Isotopic overlaps between blue, shortfin mako and smooth hammerhead sharks.

13 C/δ 15 N δ 13 C/δ 15 N/Δ 199 Hg Blue | Shortfin mako

		42.0%	23.0%
	Shortfin mako | Smooth hammerhead	33.3%	27.4%
	Blue | Smooth hammerhead	16.8%	12.9%

Acknowledgements

Lucien Besnard doctoral grant was funded by the École Doctorale des Sciences de la Mer et du Littoral (EDSML) and by the Université de Bretagne Occidentale (UBO). The project is supported by ISblue project, interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) and cofunded by a grant from the French government under the program "Investissements d'Avenir". It is also backed by MIC ERASMUS+ and Ecos Annuies projects. Gaël Le Croizier was supported by a postdoctoral grant from the French National Research Institute for Sustainable Development (IRD). Mercury concentrations and stable isotopic analysis was financially supported by the French National Research Agency (project ANR-17-CE34-0010 MERTOX, PI David Point). Felipe Galván-Magaña thanks the Instituto Politecnico Nacional for fellowships (COFAA, EDI). The authors would like to thank the Centro Interdisciplinario de Ciencias Marinas and the Centro de Investigaciones Biológicas del Noroeste, S. C. for their contribution in shark tissue sampling and processing, especially Bertha Olivia Arredondo Vega, Olivia Arjona López and Rosa Linda Salgado Garcia.

CRediT authorship contribution statement

FIGURES AND TABLES (Colors should be used for any figures in the print)

Graphical abstract KEYWORDS

Trophic ecology; Top-predator; Mercury stable isotopes; Resource partitioning; Foraging depth; Mercury accumulation

HIGHLIGHTS

• δ 13 C, δ 15 N, Δ 199 Hg and δ 202 Hg were determined in three pelagic shark species.

• Hg isotopes suggested shark species foraged on mesopelagic prey.

• δ 13 C and δ 15 N overestimated overlaps between trophic niches.

• Differences in foraging depth better explained resource partitioning.

• Foraging depth influenced mercury contamination level.