Dr Nathalie Bodin
email: nathalie.bodin@ird.fr

Dora Lesperance

Rona Albert

Stephanie Hollanda

Philippe Michaud

Maxime Degroote

Carine Churlaud

Paco Bustamante

Trace elements in oceanic pelagic communities in the western Indian Ocean

Keywords: mercury, selenium, seafood, tuna fisheries' bycatch, environmental risk assessment

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

Introduction

Located in the western-central Indian Ocean, the Republic of Seychelles is one of the 52 member states of the Small Island Development States' group of the United Nations. With a land surface of only 459 km² divided into 115 tropical islands scattered within an Exclusive Economic Zone (EEZ) of 1.3 million square kilometres, the Republic of Seychelles is staking his future growth on the development of a national Blue Economy strategy. The main inter-related challenges for this country include the conservation of ocean ecosystems and biodiversity, the implementation of an integrated cross-sectorial national spatial planning, the increase utilisation of bioresources with respect for those potential new ocean industries as well as for existing ones (eg, shipping, offshore petroleum) of sustainable practises to ensure minimal environmental impact related to their operations, and the maximisation and sustainability of revenues derived from fisheries and fisheries-related sectors. The latter has yet been seriously facing with intense governmental support in the last years to increase national semi-industrial fishing fleets targeting large pelagic species, to facilitate fish export that already represents more than 50% (mainly tuna and swordfish) of Seychelles total exports, and to encourage research and development in the field of seafood value addition, processing and quality. While such actions should directly benefit to coastal ecosystems by reducing fishing pressure on demersal species that are showing signs of overfishing in Seychelles, they have to be balanced with another important source of revenues that comes from the foreign fishing agreements. Indeed, the Republic of Seychelles is receiving economic benefits through access and licence fees, stemming from the large catches of tuna made by European and Asian purse-seine and longline fleets within its EEZ.

Purse-seine and longline fisheries also take non-target, associated and dependent species, referred hereafter as bycatch. In the case of purse-seiners, it has been estimated to be around 5% of the total catches which gives a figure of around 12,500 tons of fish species per year in the western Indian Ocean, with some components discarded at sea [START_REF] Amandé | Precision in bycatch estimates: the case of tuna purse-seine fisheries in the Indian Ocean[END_REF]. Although the magnitude and type of discarded bycatch is fishery-specific and highly variable in space and time, fish species with the highest capture probability are the dolphinfish (Coryphaena hippurus), the wahoo (Acanthocybium solandri), the rough triggerfish (Canthidermis maculata), the silky shark (Carcharhinus falciformis), the rainbow runner (Elagatis bipinnulata) and marlins (Makaira nigricans, M. indica, Tetrapturus audax). Nowadays, it becomes evident that discard practices constitute a purposeless waste of valuable living resources. It is in this context, and with the aim of promoting the responsible and sustainable management of fishing, that the International and European instances are taking a number of actions oriented to the implementation of "no-discard" and "zero-waste" policies to be followed by the fishing fleets in the near future (IOTC, 2010).

With those fisheries policies, a significant increase in bycatch landings is expected in Seychelles. In view of the increasing local population coinciding with a significant drop in the total fish landings from the artisanal fishery, the Republic of Seychelles anticipates that the new influx of bycatch will be a new source of high quality protein for the Nation, hence contributing greatly towards the self-sufficiency and food security of the country where 47% of current daily animal protein stems from fish [START_REF] Monnereau | Unlocking the full potential of the blue economy: Are African Small Island Developing States ready to embrace the opportunities? United Nations Economic Commission for Africa -Technical report[END_REF]. One other potential advantage of bycatch landings for the country is to guarantee a reliable availability of raw material to support the initiative of Seychelles to sustainably develop its fisheries post-harvest sector. In such an economic context, it appeared essential for Seychelles to first conduct a nutritional composition study to better estimate the real potential of fish bycatch.

Among the multitude of essential nutrients provided by fish, mineral elements are surprisingly poorly documented despite their important roles in our life functioning as part of numerous enzymes, cofactors and vitamin constituents [START_REF] Goldhaber | Trace element risk assessment: essentiality vs. toxicity[END_REF]. For instance,zinc (Zn) is a cofactor to more than 300 enzymes involved in important functions such as RNA and DNA metabolism and plays a major role in the stabilization of the structure of a large number of proteins [START_REF] Chasapis | Zinc and human health: an update[END_REF]. Manganese (Mn) is involved in protein, lipid and carbohydrate metabolism [START_REF] Nas | Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic[END_REF], and its deficiency results in poor reproductive performance, growth retardation, congenital malformations in offspring, and abnormal function of bone and cartilage [START_REF] Goldhaber | Trace element risk assessment: essentiality vs. toxicity[END_REF]. Selenium (Se) is a critical component of numerous selenoproteins which play major roles in antioxidant systems that actively protect against damage from free radicals and reactive oxygen species, and ultimately could protect against cancer or cardiovascular diseases [START_REF] Flores-Mateo | Selenium and coronary heart disease: a meta-analysis[END_REF] [START_REF] Greenwald | Clinical trials of vitamin and mineral supplements for cancer prevention[END_REF]. Despite their importance in human nutrition and increasing interest in the last decade in the food industry, most of scientific studies focus on the occurrence in fish flesh of the toxic mineral elements that are mercury (Hg), cadmium (Cd) and lead (Pb), and ultimately conclude with the risks for human associated with fish consumption. The nutritional-toxicological conflict related to seafood consumption is being largely discussed at the Commission of the Codex on Contaminants in Food (Codex Alimentarius Commission, 2016), and is of primary concern for both the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO). Efforts should now concentrate on the co-evaluation of essential and toxic nutrients in the different commercial fish species with reference to their fishing origin to better advise the general public.

In this context, the present study aims (i) to determine the levels of eleven essential elements (arsenic As, cobalt Co, chromium Cr, copper Cu, iron Fe, Mn, nickel Ni, Se, silver Ag, vanadium V, and Zn) and the three non-essential ones (Hg, Cd, Pb) in the muscle of target and non-target oceanic fish species caught in the western-central Indian Ocean and being landed in Seychelles, (ii) to study the influence of fish size and feeding habits on the bioaccumulation of those elements, and (iii) to discuss human exposure benefits and risks with regards to International food safety regulations. A particular attention is given to the balance between Hg and Se levels in fish flesh as Se is believed to protect against the toxic effects of Hg, particularly its organic methyl-Hg form [START_REF] Park | Omega-3 Fatty Acids, Mercury, and Selenium in Fish and the Risk of Cardiovascular Diseases[END_REF].

Material and methods

Sample collection

Thirteen oceanic species were caught by industrial fishing vessels in the western-central Indian Ocean during the northeast monsoon season (November 2014-February 2015). Samples and data were carefully collected by fishermen and observers onboard longliners and purse-seiners, respectively. Fishing location and length measurement (lower-jaw-fork length -LJFL -for billfishes; fork length -FL -for other pelagic species) to the nearest cm were recorded for each fish. A sample of around 20 g (wet weight, ww) was then taken from the dorsal white muscle (sampled under the dorsal spine on the left side) and kept frozen until landing (maximum duration 21 days). Samples were then carefully divided into two sub-samples and stored at -80°C until specific lab analyses.

Total lipid content determination and stable isotope analysis

Muscle sub-samples were freeze-dried and ground up to a fine homogeneous powder with a ball mill MM200 (Retsch). The moisture content (i.e., water content) in the muscle of each species was estimated as the difference of the sample's wet mass (before freeze-drying) and dry mass (after freeze-drying), divided by the sample's wet mass and expressed in percent.

Aliquot of 350±100 mg were extracted with 8 ml of dichloromethane at 100°C under 1900 psi for 10 minutes using a Dionex ASE 200 Accelerated Solvent Extractor to determine the total lipid content (TLC) expressed in % of dry weight (dw) [START_REF] Bodin | ASE extraction method for simultaneous carbon and nitrogen stable isotope analysis in soft tissues of aquatic organisms[END_REF]. Carbon and nitrogen stable isotopes were analyzed together on dried lipid-free samples. Isotopic ratios were determined on a Delta V Advantage isotope ratio mass spectrometer interfaced to a Flash EA 1112 elemental analyzer with a Conflo IV interface (EA-IRMS, Thermo Scientific) at the LIENSs facility (La Rochelle, France). The results were reported in the δ unit notation and expressed per mil (‰) relative to international standards (Vienna-Pee Dee Belemnite for Carbon and atmospheric N2 for Nitrogen). Estimates using replicate measurements of internal laboratory standards (USGS-24, IAEA-CH6, -600 for Carbon; IAEA-N2, -NO-3, -600 for Nitrogen) gave an analytical variability of less than 0.15‰ for both δ 13 C and δ 15 N.

Trace element analysis

Analysis of Hg was performed on both wet and dried sub-samples of fish muscle at the Seychelles Fishing Authority facility (Victoria, Seychelles). Aliquots of 10 to 50 mg dry weight (dw) and of 50 to 100 mg wet weight (ww) were analysed in triplicate directly on a DMA-80 Dual cell (Milestone, Italy). The calibration of the system was performed within a large range using a multi-point calibration curve to define the linearity range of the detector for Hg (13 points ranging from 5 µg.kg -1 to 10 mg.kg -1 ). Blanks were run between each analysed sample to reach 0.1 ng of Hg, and two laboratory working controls corresponding to large homogenized wet samples of bigeye tuna white muscle BET-M (Hg = 0.141±0.004 mg.kg -1 ww) and liver BET-L (Hg = 0.986±0.036 mg.kg -1 ww) as well as the certified reference material IAEA-436 (tuna fish flesh homogenate; Hg = 4.19±0.36 mg.kg -1 dw) were routinely analyzed every 15-20 samples allowing for comparisons across runs. Satisfactory accuracy (97-104%) was calculated with an analytical variability below 5% (n=29). Quantification limits was calculated from blank measurements with Hg values of 0.008 µg.g -1 dw.

Analysis of 13 trace metals was performed on dried samples of fish muscle at the LIENSs facility (La Rochelle, France) according to [START_REF] Bustamante | Metal and metalloid concentrations in the giant squid Architeuthis dux from Iberian waters[END_REF]. Briefly, aliquots of 50 to 300 mg were digested with a mixture of hydrochloric and nitric acids in a microwave. As, Cr, Cu, Fe, Mn, Ni, Se and Zn were analyzed by inductively coupled plasma atomic emission spectrometry on a Varian Vista-Pro ICP-OES, and Ag, Cd, Co, Pb and V were analyzed by inductively coupled plasma mass spectrometry on an ICP-MS II Series Thermo Fisher Scientific. The analytical performances for each trace element and method were checked using two certified reference materials: dogfish liver NRCC-DOLT-4 and lobster hepatopancreas NRCC-TORT-3. Quality control results are summarized in Table 1 showing recoveries ranging from 80 to 114 % according to the element.

Trace element concentrations measured in fish muscle are presented in µg.g -1 (=ppm=parts per million) on a wet weight basis in the present document. ppm is the unit used by state and federal agencies when communicating with the public generally, and when discussing risk from fish consumption.

Statistics

For each fish, the Hg:Se molar ratio (noted as MHg:MSe) was calculated from the Hg and Se levels by dividing concentration (in ppm ww) by the molecular weight (200.59 for Hg and 78.96 for Se). In addition, the Selenium Health Benefit Value (HBVSe) was calculated according to [START_REF] Ralston | Selenium Health Benefit Values: Updated Criteria for Mercury Risk Assessments[END_REF]:

HBVSe = ([MSe-MHg]/MSe)  (MSe + MHg)
The influence of fish species on fat content, isotopic ratios, trace element levels, MSe:MHg and HBVSe was tested by one way ANOVA followed by Tukey's HSD post hoc test when hypotheses for normality and homoscedasticity were met, or by means of Kruskal-Wallis tests followed by Wilcoxon tests for independent samples. Spearman correlations were used to examine the relationship between the trace metal levels, MSe:MHg or HBVSe, with the biological data (e.g. fish size,  13 C and  15 N). This method was also applied to detect dependencies among trace metal concentrations. All statistical analyses were performed using R 2.15.2 software (access Core Team 2016).

Results

Biological data

The characteristics of the 13 oceanic pelagic fish are presented in Table 2. Species differed with their muscle fat content (Kruskal-Wallis, H = 72.56, p-value < 0.001) and were categorized into three groups: (ii) lean fish with a mean TLC below 5% dw includes the four tunas (skipjack, yellowfin, bigeye and albacore), the dolphin fish, the blue marlin and the wahoo, (ii) low fat fish with a mean TLC between 5 and 10% dw (silky shark, rainbow runner, rough triggerfish and giant grouper), and (iii) fatty fish with a mean TLC higher than 10% dw including the swordfish and Indian mackerel (14 and 26% dw respectively).

Species also differed with their isotopic signatures ( 15 N: ANOVA, F = 18.89, p-value < 0.001;  13 C: ANOVA, F = 24.21, p-value < 0.001). Nitrogen and carbon isotopic ratios were highly positively correlated in the studied pelagic food web (Pearson's test: rho=0.64, p-value < 0.001; Supplementary material Fig. S1), and both trophic tracers were positively correlated with fish size (Spearman's test on all species mixed;  13 C: rho = 0.43, p-value < 0.001;  15 N: rho = 0.61, p-value < 0.001; Supplementary material Fig. S1).  13 C ranged from -17.7±0.2‰ (min=-18.4‰) for rough triggerfish to -15.4‰ for giant grouper, and the total length of the carbon isotopic chain was 3.0‰.  15 N ranged from 11.5±0.6‰ (min=10. 5‰) for skipjack tuna to 14.8±0.4 (max=14.9‰) for swordfish, and the total length of the nitrogen isotopic chain was 4.4‰.

Distribution of trace elements in oceanic pelagic fish

Except for Ag, Co, and V whose levels were below the method quantification limits, other trace elements were successfully detected in the muscle of oceanic pelagic fish (Table 2; online supplementary material Table S1). Fe and Zn, both essential nutrients, had the highest concentrations of all elements found in all biota, ranging from 1.0 ppm ww Fe in yellowfin tuna to 12.7 ppm ww Fe in swordfish, and from 2.2 ppm ww Zn in albacore tuna to 17.9 ppm ww Zn in swordfish. With only 37 and 40% of data above the method quantification limits respectively, Ni and Pb exhibited the lowest concentrations, ranging from 4.2e-4 ppm ww Ni and 1.7e-4 µg.kg -1 ww Pb in albacore tuna to 12.2e-2 ppm ww Ni in swordfish and 8.7e-2 ppm ww Pb in swordfish.

Species differed with their muscle Hg content (Kruskal-Wallis, H = 111.20, p-value < 0.001; Table 2, Fig. 2).

The highest mean Hg concentrations, ranging from 0.74 to 1.33 ppm ww, were measured in wahoo, followed by swordfish, giant grouper, and blue marlin. Albacore, bigeye and yellowfin tunas exhibited intermediate Hg levels, around 2 to 4 times lower than the former group. Finally, skipjack tuna, dolphinfish and silky shark had mean Hg levels ranging from 0.18 to 0.20 ppm ww, while the lowest concentrations (Hg <0.07 ppm ww) were found in rainbow runner, Indian mackerel and rough triggerfish. The strongest intraspecific variability of Hg concentrations was observed for the largest pelagic species (e.g., variance = 0.75, 0.48 and 0.20 for blue marlin, wahoo and swordfish, respectively). Hg was positively correlated with fish size (Spearman's test; rho = 0.72, p-value < 0.001; Table 3) and  15 N (Spearman's test; rho = 0.52, p-value < 0.001; Table 3, Supplementary material Fig. S2).

Significant differences between species were found in all other elements' concentrations (Table 3, Fig. 3).

Cd, Ni, Pb, and Zn exhibited similar trends with low levels and variations among species, the highest being measured in swordfish. The rough triggerfish had the highest Mn content while the lowest ones were observed in the two billfish species. Cr and Fe were about 4 times higher in the mackerel compared to other oceanic fish, although some specimens of swordfish and rainbow runner also exhibited Cr and Fe levels in the same range as the mackerel. Cu had the highest levels in bony fish and the lowest in billfish, while the opposite pattern was found for Se. Finally, As was low in most oceanic fish, except in rough triggerfish and silky shark which had As levels more than 7-fold higher. Among elements, Mn showed strong negative correlations with fish size, carbon and nitrogen isotopic values (Table 3, Supplementary material Fig. S2). Cu and Fe tend to decrease with  15 N and fish size respectively, whereas Cd seems to increase with fish size (Table 3, Supplementary material Fig. S2). Finally, Hg concentrations were positively correlated with Cd, but negatively with Cu and Mn levels (Table 3).

Mercury-selenium interactions

Mercury:selenium molar ratio (MHg:MSe) and Selenium Health Benefit Value (HBVSe) were calculated for the eight oceanic pelagic fish from the measured muscle Hg and Se concentrations (Fig. 4).

MHg:MSe ranged from 0.010 in rough triggerfish to 1.01 in wahoo and showed significant inter-specific differences (Kruskal-Wallis, H = 54.56, p-value < 0.001): wahoo and swordfish had the highest mean MHg:MSe value (0.660.36 and 0.430.24, respectively), followed by blue marlin (0.230.17), dolphinfish (0.120.04), rainbow runner (0.100.04), and silky shark (0.090.10), and finally Indian mackerel and rough triggerfish with mean ratio below 0.03 (Fig. 4A).

HBVSe ranged from -0.89 in wahoo to 83.74 in swordfish and significantly varied according to the species (Kruskal-Wallis, H = 14.33, p-value < 0.05). The lowest mean HBVSe were observed in rainbow runner (19.812.02) and wahoo (21.1722.45), followed by Indian mackerel, dolphinfish, swordfish and rough triggerfish with mean HBVSe from 30.403.56 to 32.256.02, to reach finally the highest mean values in blue marlin (51.2919.23) and silky shark (52.3021.64) (Fig. 4B).

Although those risk exposure indices gave different distribution profiles among the studied oceanic fish, they both highlighted large intra-species variations in the largest pelagic species, i.e. wahoo, blue marlin, and swordfish, with the exception of silky shark which only displayed large variance for HBVSe. Finally, positive relationships were obtained between MHg:MSe and fish size,  15 N and  13 C values (Spearman's test: rho=0.64, 0.60 and 0.64, p-value < 0.001 for all), but none between HBVSe and the three biological parameters (i.e. fish size,  15 N,  13 C).

Discussion

The Food and Agriculture Organisation and the World Health Organisation both recommend that governments monitor the levels of chemical contaminants in food. This recommendation comes in response to increasing concern over the health effects of various contaminants, including metals, found in the food supply. The present study determined the levels and patterns of essential and non-essential trace elements in the flesh of oceanic pelagic communities caught in the western-central Indian Ocean and landed in Seychelles. Compared to existing international food safety regulations and literature data, our results contribute to the identification of benefits and risks for Seychelles' population associated with fish consumption. Moreover, we provide information of primary interest on the mineral composition of nontarget pelagic fish for future potential economic valorisation.

Comparison with literature and reference values for toxic elements

Three general categories of Hg bioaccumulation were identified: high range (mean Hg >0.5 ppm ww), middle range (between 0.1 and 0.5) and low range (< 0.1). High range includes the largest pelagic species, i.e. wahoo, swordfish, giant grouper and blue marlin; middle range includes the four tuna species, dolphinfish, and silky shark; low range includes the smallest pelagic species, i.e. rainbow runner, rough triggerfish and Indian mackerel. From those values, none of the individual fish from the 2 latter groups exceeds the maximum limit (ML) of 0.5 ppm ww defined for fish in general by international instances, and only 10% of the individuals composing the high range group showed Hg levels higher than the value of 1 ppm ww set up specifically for predatory fish (Codex Alimentarius Commission, 2011;EC, 2011EC, , 2006)).

Moreover, considering each large pelagic species separately, the main concerns appeared for wahoo with 60% of the individuals showing a Hg muscle concentration exceeding 1ppm ww and with the highest measured Hg level reaching 2.2 ppm ww. Swordfish and blue marlin both had maximum Hg levels of 1.7 ppm ww, with 30% of the analysed specimens exceeding 1 ppm ML. Swordfish from Seychelles waters showed similar muscle Hg concentrations than swordfish from Sri Lanka and Reunion Island waters (Jinadasa et al., 2014a(Jinadasa et al., , 2014b;;Kojadinovic et al., 2006), whereas a 2-fold lower mean value was reported in specimens caught in the Mozambique Channel (see online supplementary material Table S2).

On a global scale, X. gladius from the Indian Ocean had similar Hg bioaccumulation than in the Pacific Ocean and Eastern Atlantic Ocean (e.g., Japan: 1.03 ppm ww, [START_REF] Yamashita | Selenoneine, total selenium, and total mercury content in the muscle of fishes[END_REF]; Hawaii: 1.07±0.60 ppm ww, [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF]; Azores: 0.06-4.91 ppm ww, [START_REF] Branco | Mercury and selenium in blue shark (Prionace glauca, L. 1758) and swordfish (Xiphias gladius, L. 1758) from two areas of the Atlantic Ocean[END_REF][START_REF] Monteiro | Assessment of Arsenic, Cadmium and Mercury Level in Commonly Consumed Coastal Fishes from Bay of Bengal, India[END_REF]; Equatorial region: 0.90-2.3 ppm ww [START_REF] Branco | Mercury and selenium in blue shark (Prionace glauca, L. 1758) and swordfish (Xiphias gladius, L. 1758) from two areas of the Atlantic Ocean[END_REF]), whereas swordfish in the Western Atlantic Ocean was 1.5-2-fold less contaminated (e.g., Brazil and Uruguay waters: 0.60.3 ppm ww, [START_REF] Mendez | Total Mercury Content -Fish Weight Relationship in Swordfish (Xiphias gladius) Caught in the Southwest Atlantic Ocean[END_REF][START_REF] Rodrigues | Mercury Concentrations in South Atlantic Swordfish, Xiphias gladius, Caught off the Coast of Brazil[END_REF]; US southern waters: 0.46±0.24 ppm ww [START_REF] Cai | Bioaccumulation of mercury in pelagic fishes from the northern Gulf of Mexico[END_REF][START_REF] Cai | Bioaccumulation of Mercury in Pelagic Fishes in NW Gulf of Mexico and its Relationship with Length, Location, Collection Year, and Trophic level[END_REF]). Hg values of wahoo reported in this study were surprisingly high (from 0.60 to 2.17 ppm ww), exceeding by far those reported for similar-size specimens in Reunion Island and Hawaii (0.13±0.08 and 0.25±0.20 ppm ww, respectively; [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF]Kojadinovic et al., 2006). In the 80s, Hg content 2-fold lower were measured in this species caught in Seychelles waters (see online supplementary material Table S2). On the opposite, blue marlin from Seychelles surrounding waters appeared 3 to 15 times less contaminated than similar-sized fish analysed in Hawaii [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF][START_REF] Shultz | The distribution of total and organic mercury in seven tissues of the Pacific blue marlin, Makaira nigricans[END_REF][START_REF] Shultz | Mercury and selenium in blue marlin, Makaira nigricans, from the Hawaiian Islands[END_REF] and Southern US Atlantic waters [START_REF] Cai | Bioaccumulation of mercury in pelagic fishes from the northern Gulf of Mexico[END_REF][START_REF] Cai | Bioaccumulation of Mercury in Pelagic Fishes in NW Gulf of Mexico and its Relationship with Length, Location, Collection Year, and Trophic level[END_REF][START_REF] Luckhurst | Evidence Of blue marlin (Makaira nigricans) spawning in Bermuda waters and elevated mercury levels in large specimens[END_REF]. Regarding the middle Hg range group, the values measured in the four tuna species and the dolphinfish in this study were in the same range as in the Indian Ocean (see online supplementary material Table S2), Atlantic Ocean and north Pacific Ocean, whereas (Araújo and Cedeño-Macias, 2016) recently reported Hg concentrations higher than the ML of 1 ppm ww in the flesh of dolphinfish and yellowfin tuna in Ecuador (1.6±1.4 and 1.4±1.3 ppm ww respectively). Finally, within the low Hg range group, only data for Indian mackerel have been reported in the literature revealing also weak Hg bioaccumulation for this species in Malaysian waters [START_REF] Agusa | Concentrations of trace elements in marine fish and its risk assessment in Malaysia[END_REF][START_REF] Khandaker | Assessment of Radiation and Heavy Metals Risk due to the Dietary Intake of Marine Fishes (Rastrelliger kanagurta) from the Straits of Malacca[END_REF], whereas consumption risks were reported for Indian mackerel caught in Mumbai waters and Bay of Bengal (see online supplementary material Table S2) (Deshpande et al., 2008;[START_REF] Mukherjee | Assessment of Arsenic, Cadmium and Mercury Level in Commonly Consumed Coastal Fishes from Bay of Bengal, India[END_REF].

Cd and Pb showed low concentrations in the muscle of the fish from the oceanic pelagic communities, the highest being observed in swordfish (mean values = 0.06 and 0.01 ppm ww respectively). While a unique ML of 0.3 ppm ww Pb exists for all fish, three MLs have been defined in the case of Cd: 0.3, 0.1 and 0.05 ppm ww for billfishes, mackerels, and other fish species, respectively (Codex Alimentarius Commission, 2011;EC, 2011EC, , 2006)). Considering those limits, only 2 swordfish from the total number of 76 oceanic fish and 28 swordfish were reported to exceed Cd ML, and all fish appeared safe for consumption regarding Pb exposure. Our data were in accordance with Pb concentrations measured in fish from the Indian Ocean (Annex 2), while higher values ranging from 0.04 to 0.8 ppm ww were reported in dolphinfish, yellowfin and Indian mackerel from Ecuador and Malaysia [START_REF] Agusa | Concentrations of trace elements in marine fish and its risk assessment in Malaysia[END_REF][START_REF] Araújo | Heavy metals in yellowfin tuna (Thunnus albacares) and common dolphinfish (Coryphaena hippurus) landed on the Ecuadorian coast[END_REF].

Regarding Cd, fish from Seychelles appeared less contaminated than in other regions. For instance, around 3 to 7-fold higher Cd concentrations were observed in swordfish and dolphinfish from the south-west Indian Ocean see online supplementary material Table S2), and levels exceeding more than 100 times our data were measured in Malaysian Indian mackerel and Ecuadorian dolphinfish [START_REF] Agusa | Concentrations of trace elements in marine fish and its risk assessment in Malaysia[END_REF][START_REF] Araújo | Heavy metals in yellowfin tuna (Thunnus albacares) and common dolphinfish (Coryphaena hippurus) landed on the Ecuadorian coast[END_REF].

Comparison with literature and reference values for essential elements

Essential elements are micronutrients that need to be daily consumed in adequate amounts in order to sustain normal physiological functions [START_REF] Goldhaber | Trace element risk assessment: essentiality vs. toxicity[END_REF]. In general, seafood constitutes a good source for metalloid nutrients, but concentrations can diverge greatly between species and thus have to be monitored.

The comparison of observed data with available literature revealed that dolphinfish and swordfish in Seychelles waters had 2-3 times less Cu, Fe and Zn but similar levels of Mn than these two species caught either in the Mozambique Channel or Reunion Island (Kojadinovic et al., 2007). Moreover, those four elements had levels 3 to 10 times lower in Indian mackerel from Seychelles compared to Malaysia waters [START_REF] Agusa | Concentrations of trace elements in marine fish and its risk assessment in Malaysia[END_REF][START_REF] Khandaker | Assessment of Radiation and Heavy Metals Risk due to the Dietary Intake of Marine Fishes (Rastrelliger kanagurta) from the Straits of Malacca[END_REF]. Legal thresholds are inexistent for essential elements in Europe. Guidance values in muscle are, however, recommended in some countries, such as 20 ppm ww for Cu and 50 ppm ww for Zn (MAFF, 1995). While a previous study reproted individuals of skipjack and yellowfin tunas from south-west Indian Ocean with Zn concentrations above the recommended value of 50 ppm (maximum Zn levels around 85 and 125 ppm ww, respectively), none of the fish caught in Seychelles waters reached this limit or the one recommended for Cu.

Regarding As, data below 1 ppm ww were reported in Malaysian Indian mackerel and Sri Lankan skipjack (see online supplementary material Table S2) which is in accordance with those measured in the present study. [START_REF] Storelli | Accumulation of mercury, cadmium, lead and arsenic in swordfish and bluefin tuna from the Mediterranean Sea: A comparative study[END_REF] reported wide range of As concentrations in the muscle of Mediterranean swordfish and bluefin tuna reaching 7 ppm ww as observed in rough triggerfish and silky shark from the western-central Indian Ocean. However fish accumulating mainly the non-toxic organic form of As, the concentrations reported in fish flesh in the present study do not pose any issue for health human through fish consumption (Codex Alimentarius Commission, 2011).

Se is the essential element that has been the most investigated in marine species mainly due its major role in counteracting Hg toxicity (see section 4.4). Dolphinfish and swordfish exhibited similar muscle Se concentrations in different parts of the western Indian Ocean, i.e. Seychelles, Reunion Island and Mozambique Channel (Kojadinovic et al., 2007). Our results also fit with those reported in Hawaii for blue marlin, wahoo and dolphinfish [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF][START_REF] Shultz | The distribution of total and organic mercury in seven tissues of the Pacific blue marlin, Makaira nigricans[END_REF][START_REF] Shultz | Mercury and selenium in blue marlin, Makaira nigricans, from the Hawaiian Islands[END_REF]. Swordfish from the western Indian Ocean, however, contained almost 2 times more Se in its flesh compared to specimens inhabiting the Eastern Atlantic Ocean (Azores and equatorial region; [START_REF] Branco | Mercury and selenium in blue shark (Prionace glauca, L. 1758) and swordfish (Xiphias gladius, L. 1758) from two areas of the Atlantic Ocean[END_REF]), the northern Pacific Ocean (Hawaii and Japan; [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF][START_REF] Shultz | Mercury and selenium in blue marlin, Makaira nigricans, from the Hawaiian Islands[END_REF][START_REF] Yamashita | Selenoneine, total selenium, and total mercury content in the muscle of fishes[END_REF], and the Mediterranean Sea [START_REF] Plessi | Mercury and Selenium Content in Selected Seafood[END_REF].

Influence of biological processes on trace elements in oceanic pelagic communities

Fish size, trophic position (nitrogen isotopic signatures, δ 15 N) and diet sources (carbon isotopic signature, δ 13 C) had variable effects on the bioaccumulation of trace elements in oceanic pelagic fish communities depending on the element considered. For instance, a positive exponential relationship between Hg concentrations and trophic position was observed likely reflecting Hg biomagnification through the oceanic pelagic food web. This process occurs because consumers feeding at higher trophic levels eat larger prey with higher body burdens that smaller ones. However, our results also revealed that pelagic species may share the same trophic level but different Hg bioaccumulation patterns. It is the case for example of the rough triggerfish and skipjack tuna, both with a mean  15 N value of around 11.6‰ whereas Hg concentrations were 7-fold higher in skipjack tuna. Similarly, rainbow runner exhibited  15 N in the same range as bigeye, albacore and yellowfin tunas but had lower Hg content by a factor 5 to 7 compared to the three tuna species. This can be related to the differences of lifespan between species as suggested by the positive relationship obtained between fish size (proxy of fish age) and Hg muscle levels. Due to its slow elimination rates (half-life on the order of years), Hg accumulates in tissue as fish consume more food through time leading to higher Hg contamination of long-lived species such as tunas (e.g., maximum age of 13 and 16 years old for albacore and bigeye tunas, respectively; [START_REF] Farley | Age, growth and preliminary estimates of maturity of bigeye tuna, Thunnus obesus, in the Australian region[END_REF][START_REF] Wells | Age and growth of North Pacific albacore (Thunnus alalunga): Implications for stock assessment[END_REF] compared to the middle-lived rainbow runner (max. age = 5 years old; [START_REF] Iwasaki | Age and growth of rainbow runner[END_REF]. However, the observed difference of Hg contamination between skipjack tuna and triggerfish with similar lifespan (between 12-15 years; [START_REF] Burton | Age, growth, and mortality of gray triggerfish (Balistes capriscus) from the southeastern United States[END_REF][START_REF] Collette | Scombrids of the world. An annotated and illustrated catalogue of tunas, mackerels, bonitos and related species known to date[END_REF]) is most likely related to the difference of food sources observed between these two species. Indeed, C. maculata exhibited the lowest  13 C values (-17.70.2‰) of the oceanic pelagic food web indicating that it feeds on more pelagic-based prey compared to skipjack characterised by intermediate  13 C values (-16.90.3‰). In addition, the positive relationship between Hg concentrations and  13 C reflects the higher exposure of deep-feeding predators such as swordfish and wahoo compared to surface-feeding species [START_REF] Goutte | Trace elements in Antarctic fish species and the influence of foraging habitats and dietary habits on mercury levels[END_REF]. The influence of forage depth on the Hg body burden of predator species has been reported in different ecosystems including oceanic pelagic food web from waters surrounding Hawaii in the central North Pacific Ocean [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Chumchal | Habitat-Specific Differences in Mercury Concentration in a Top Predator from a Shallow Lake[END_REF][START_REF] Eagles-Smith | Spatial and temporal patterns of mercury concentrations in freshwater fish across the Western United States and Canada[END_REF][START_REF] Magalhães | Intra-and interspecific variability in total and methylmercury bioaccumulation by eight marine fish species from the Azores[END_REF]. Thus, our study confirmed the complex inter-related determinants that affect Hg bioaccumulation processes in pelagic ecosystems.

An opposite pattern has been noticed for Mn compared to Hg. Indeed, Mn concentrations in oceanic pelagic communities were negatively correlated to fish size, trophic position and diet sources suggesting a biodilution process. Some studies also reported decreasing Mn concentrations with increasing trophic level in fish from various ecosystems [START_REF] Cheng | Trophic relationships and health risk assessments of trace metals in the aquaculture pond ecosystem of Pearl River Delta, China[END_REF][START_REF] Ikemoto | Analysis of biomagnification of persistent organic pollutants in the aquatic food web of the Mekong Delta, South Vietnam using stable carbon and nitrogen isotopes[END_REF]. The biodilution of Mn in oceanic pelagic food web suggests a lack of the necessary regulatory and detoxification mechanisms for this essential element in lower trophic levels and smaller-sized species. However, the negative relationship between Mn and  13 C also raised the hypothesis of the influence of diet source on Mn exposure for the studied fish, with higher accumulation potential in pelagic-feeding compared to benthic-feeding species [START_REF] Petkevich | Elemental composition of bony tissues of plankton-feeding and benthos-feeding fish from the Northwest part of the Black Sea[END_REF].

Apart from Hg and Mn, few elements appeared influenced by biological processes. Only Cd and Cu tend to increase and decrease, respectively, with fish length, both metals being significantly correlated with Hg.

Finally, the lack of correlation between the other different elements in the same samples indicates that the possible detoxification process might operate independently for each metal in the selected oceanic pelagic fish.

Hg-Se antagonism in oceanic pelagic communities

The mercury:selenium molar ratio (MHg:MSe) and Selenium Health Benefit Value (HBVSe) have been proposed as proxies to better appreciate Se-specific nutritional benefits in relation to Hg exposure risk [START_REF] Kaneko | Selenium and Mercury in Pelagic Fish in the Central North Pacific Near Hawaii[END_REF][START_REF] Ralston | Selenium Health Benefit Values: Updated Criteria for Mercury Risk Assessments[END_REF]. According to our results, Se was in molar excess of Hg in almost all species evaluated, indicating a general healthy profile of the fish analyzed. Only one specimen of wahoo from the 73 pelagic fish analysed for both Hg and Se displayed a slightly unbalanced Hg:Se molar ratio (1.01) and consequently negative Se-HBV (-0.89). Protective effects of Se against Hg bioaccumulation and toxicity have been demonstrated in a range of organisms [START_REF] Belzile | The effect of selenium on mercury assimilation by freshwater organisms[END_REF][START_REF] Chen | Antagonistic effect of selenium on mercury assimilation by fish populations near Sudbury metal smelters?[END_REF][START_REF] Storelli | Total and methylmercury residues in cartilaginous fish from Mediterranean Sea[END_REF] suggesting that a universal Se-Hg antagonism may exist. These protective effects, however, depend on the relative concentration and bioavailability of Hg and Se, the sensitivity of the species and tissue considered, as well as, for a given species, the individual life history traits as shown in the present study and others (e.g., influence of age, growth, diet, habitat). Further investigations on Hg biomagnification and exposure risk assessment in organisms and food webs together with Se availability and other nutrients have thus to be considered in different ecosystems to improve our understanding on the variability of Hg-Se interaction and its protective effects. Such research field is of primary importance for ecosystem and human health, and for the economy of countries such as the Republic of Seychelles which focus on a sustainable ocean-based development strategy. Table 1. Method quantification limits (MQL; mg.kg -1 dw), and recovery rate (%) for trace metal concentrations (mg.kg-1 dw) in the certified reference materials NRCC-DOLT-4 and NRCC-TORT-3 (n = 17). Data are given as mean±standard deviation. ICP-OES = inductively coupled plasma atomic emission spectrometry; ICP-MS = inductively coupled plasma mass spectrometry; nd = not determined. Table S1. Meanstandard deviation of trace metal levels (ppm ww) detected in the muscle of eight oceanic pelagic fish species caught the western-central Indian Ocean. Kruskal-Wallis results (statistic H and p-value) of tests of difference in elements' content between species are also given.

Fig. 1 .

 1 Fig. 1. Mean location of the oceanic pelagic fish species caught by industrial and semi-industrial fishing

Fig. 2 .

 2 Fig. 2. Boxplot of Hg levels (ppm ww) in the muscle of 13 oceanic pelagic fish species caught in the westerncentral Indian Ocean. ALB = Albacore tuna, BET = Bigeye tuna, BUM = Blue marlin, CNT = Rough triggerfish, DOL = Dolphinfish, EEN = Giant grouper, FAL = Silky shark, RAG = Indian mackerel, RRU = Rainbow runner, SKJ = Skipjack tuna, SWO = Swordfish, WAH = Wahoo, YFT = Yellowfin tuna. Letters indicate significant differences between species.

Fig. 3 .

 3 Fig. 3. Boxplots of trace element levels (ppm ww) in the muscle of eight oceanic pelagic fish species caught in the western-central Indian Ocean. BUM = Blue marlin, CNT = Rough triggerfish, DOL = Dolphinfish, FAL = Silky shark, RAG = Indian mackerel, RRU = Rainbow runner, SWO = Swordfish, WAH = Wahoo. Letters indicate significant differences between species.

Fig. 4 .

 4 Fig. 4. Boxplots of (A) Hg:Se molar ratios (MHg:MSe) and (B) Selenium Health Benefit Values (HBVSe) in the muscle of eight oceanic pelagic fish species caught in the western-central Indian Ocean. BUM = Blue marlin, CNT = Rough triggerfish, DOL = Dolphinfish, FAL = Silky shark, RAG = Indian mackerel, RRU = Rainbow runner, SWO = Swordfish, WAH = Wahoo. Letters indicate significant differences between species.

Fig. S1 .

 S1 Fig. S1. Plots of mean±standard deviation of (A) carbon and nitrogen stable isotope ratios ( 13 C and  15 N), and (B) fish size and  15 N for oceanic pelagic fish species caught in the western-central Indian Ocean. ALB = Albacore tuna, BET = Bigeye tuna, BUM = Blue marlin, CNT = Rough triggerfish, DOL = Dolphinfish, EEN = Giant grouper, FAL = Silky shark, RAG = Indian mackerel, RRU = Rainbow runner, SKJ = Skipjack tuna, SWO = Swordfish, WAH = Wahoo, YFT = Yellowfin tuna.

Fig. S2 .

 S2 Fig. S2. Plots of significant relationships between (A) nitrogen stable isotope values ( 15 N) and logtransformed Hg levels, (B)  15 N and log-transformed Fe levels, (C)  15 N and log-transformed Mn levels, (D) fish size (length, cm) and log-transformed Cu levels, and (E) fish size and log-transformed Cu levels, for oceanic pelagic fish species caught in the western-central Indian Ocean. ALB = Albacore tuna, BET = Bigeye tuna, BUM = Blue marlin, CNT = Rough triggerfish, DOL = Dolphinfish, EEN = Giant grouper, FAL = Silky shark, RAG = Indian mackerel, RRU = Rainbow runner, SKJ = Skipjack tuna, SWO = Swordfish, WAH = Wahoo, YFT = Yellowfin tuna.

Table 2 .

 2 Meanstandard deviation of fish size (cm), and water content (%), fat content (TLC, %), nitrogen and carbon isotope values ( 15 N and  13 C, ‰), and mercury levels on a wet weight basis (ppm ww) in the white muscle of 13 oceanic pelagic fish species caught in the western-central Indian Ocean. n = Number of fish; Length = Size of the fish in cm corresponding to low jaw fork length for blue marlin and swordfish and to fork length for others.

	Symbol Element Method MQL	DOLT4 Expected value Recovery Expected value Recovery TORT3
	Ag	Silver	ICP-MS 0.0100	0.97±0.11	81.3±0.0	nd	nd
	As	Arsenic	ICP-OES 0.8200	9.66±0.62	96.3±0.0	59.5±3.8	114.6±0.1
	Cd	Cadmium	ICP-MS 0.0100	24.3±0.8	98.8±0.1	42.3±1.8	96.7±0.0
	Co	Cobalt	ICP-MS 0.0100	nd	nd	1.06	96.9±0.1
	Cu	Copper	ICP-OES 0.0800	31.2±1.1	99.6±0.1	497±22	90.9±0.0
	Cr	Chromium ICP-OES 0.0900	nd	nd	1.95±0.24	91.6±0.1
	Fe	Iron	ICP-OES 3.9700	1833±75	91.6±0.0	179±8	90.1±0.0
	Mn	Manganese ICP-OES 0.0100	nd	nd	15.6±1	90.1±0.0
	Ni	Nickel	ICP-OES 0.0010	1.33±0.12	97.5±0.1	5.3±0.24	93.4±0.1
	Pb	Lead	ICP-MS 0.0001	0.16±0.04	82.8±0.1	0.225±0.018	80.2±0.1
	Se	Selenuim	ICP-MS 0.1000	2.58±0.22	101.9±0.1	10.9±1	102.5±0.1
	V	Vanadium ICP-MS 0.9400	nd	nd	9.1±0.4	91.6±0.0
	Zn	Zinc	ICP-OES 0.3600	8.3±1.3	105.1±0.0	136±6	100.5±0.0

Table 3 .

 3 Spearman correlation matrix between trace element concentrations, fish size (length), and isotopic values ( 13 C and  15 N) in oceanic pelagic fish caught in the western-central Indian Ocean. Values with absolute rho coefficient > 0.5 are in bold. ** p < 0.01; *** p < 0.001.

	Common name	Scientific name	N Length	Water	TLC	 13 C	 15 N	Hg
				(cm)	(%)	(%dw)	(‰)	(‰)	(ppm ww)
	Rough triggerfish Canthidermis maculata 5 32±4 84.3±2.1 5.0±0.7 -17.7±0.2 11.6±0.3 0.028±0.013
	Indian mackerel Rastrelliger kanagurta	10 26±1 71.9±1.6 13.9±9.6 -17.3±0.2 12.0±0.3 0.038±0.008
	Rainbow runner Elagatis bipinnulata	5 75±16 82.5±2.3 6.4±2.2 -17.0±0.2 12.6±0.2 0.069±0.031
	Dolphinfish	Coryphaena hippurus	10 99±7 77.4±2.8 1.9±0.7 -16.6±0.1 13.5±0.4 0.177±0.043
	Silky shark	Carcharhinus falciformis 5 80±11 73.9±2.2 6.3±1.1 -16.5±0.1 13.3±0.5 0.198±0.076
	Skipjack tuna	Katsuwonus pelamis	13 57±10 71.9±2.2 1.5±0.2 -16.9±0.3 11.5±0.6 0.205±0.112
	Yellowfin tuna	Thunnus albacares	5 133±13 72.6±3.3 3.4±1.2 -16.6±0.2 12.3±0.5 0.375±0.166
	Bigeye tuna	Thunnus obesus	27 87±46 72.8±2.6 1.8±0.3 -17.2±0.3 12.6±0.7 0.339±0.291
	Albacore tuna	Thunnus alalunga	30 97±5 69.8±2.7 1.5±0.5 -17.4±0.3 12.5±0.4 0.478±0.140
	Giant grouper	Epinephelus lanceolatus 1	190	74.1	5.4	-15.4	14.5	0.823
	Blue marlin	Makaira nigricans	3 219±14 76.8±4.3 2.9±0.6 -16.2±0.4 13.8±0.1 0.747±0.865
	Swordfish	Xiphias gladius	33 150±31 67.1±8.7 26.2±18.9 -16.2±0.2 14.8±0.4 0.987±0.446
	Wahoo	Acanthocybium solandri 5 106±9 75.2±2.1 3.1±0.2 -16.2±0.2 14.1±0.5 1.329±0.695

  018±0.017 0.025±0.000 0.410±0.120 2.286±0.112 0.036±0.009 0.023±0.001 0.009±0.001 1.126±0.550 6.213±0.301 Rough triggerfish 7.009±4.769 0.003±0.001 0.025±0.000 0.538±0.240 2.587±0.738 0.111±0.034 0.003±0.006 0.000±0.000 0.617±0.111 5.609±1.744 Dolphinfish 2.258±1.280 0.004±0.001 0.025±0.000 0.580±0.169 2.298±0.511 0.064±0.012 0.002±0.005 0.001±0.002 0.647±0.116 5.853±0.601 Silky shark 7.191±2.468 0.017±0.009 0.025±0.000 0.500±0.056 1.685±0.514 0.060±0.004 0.002±0.005 0.002±0.002 1.040±0.423 4.558±0.398 Indian mackerel 0.648±0.159 0.008±0.002 0.106±0.066 0.705±0.136 7.095±1.546 0.064±0.011 0.015±0.015 0.003±0.003 0.600±0.070 5.253±0.831 Rainbow runner 0.838±0.378 0.012±0.003 0.031±0.009 0.558±0.127 5.100±2.691 0.063±0.014 0.021±0.005 0.003±0.003 0.395±0.043 4.311±0.712 Swordfish 0.939±0.468 0.059±0.048 0.044±0.058 0.342±0.175 4.124±2.782 0.036±0.014 0.020±0.025 0.010±0.017 0.763±0.284 7.480±3.128 Wahoo 1.503±0.449 0.004±0.001 0.025±0.000 0.379±0.124 1.586±0.648 0.044±0.007 0.019±0.003 0.008±0.001 0.842±0.170 4.

	Common name	As	Cd	Cr	Cu	Fe	Mn	Ni	Pb	Se	Zn
	Blue marlin	0.316±0.037 0.990±0.508
	H	47.40	58.44	34.70	29.41	38.79	47.22	26.16	30.85	25.64	30.77
	p-value	4.67E-08	3.09E-10	1.27E-05	1.22E-04	2.15E-06	5.05E-08	4.72E-04	6.61E-05	5.84E-04	6.86E-05

Acknowledgments

The authors wish to thank the fishermen and observers onboard the EU purse-seiner Belouve and the Seychelles longliner Albacore for their help with fish sampling, in particular Elvis Hoareau and Emmanuel Chassot. We are also very grateful to Emmanuel Chassot for drawing the map and editing the manuscript.

This study is part of the SEYFISH project co-funded by the Institute for Research and Development (IRD) and the Seychelles Fishing Authority (SFA).

Supplementary material

Trace elements in oceanic pelagic communities in the western-central Indian Ocean Nathalie Bodin 1,2 , Dora Lesperance 2 , Rona Albert 2 , Stephanie Hollanda 2 , Philippe Michaud 3 , Maxime (Kojadinovic et al., 2007)