Mercury in 16 demersal sharks from southeast Australia: Biotic and abiotic sources of variation and consumer health implications

Total mercury concentrations in Tasman Sea mesopelagic fish: Exploring biotic and abiotic drivers

Understanding mercury (Hg) concentrations in mesopelagic and mid-trophic fishes is important for assessing Hg accumulation in oceanic ecosystems and higher-order predators. This study measured total Hg (THg) concentrations in the whole body of 16 abundant mesopelagic fish species sampled in two distinct sites within the Tasman Sea. Across all species, total Hg concentrations ranged from 0.02 to 0.48 μg g-1 dry weight (0.01 to 0.15 μg g-1 wet weight). Total Hg concentrations varied with vertical migration patterns, with shallower migrators exhibiting higher THg. Females typically had statistically higher THg concentrations than males. Positive correlations between THg concentration and standard length were observed for some but not all species. At the community level, THg concentrations correlated positively with estimated trophic position and foraging habitat, as inferred by stable isotope values. These findings contribute to our understanding of Hg cycling in oceanic ecosystems and the potential for biomagnification in oceanic top-order predators.

Exceptionally high levels of total mercury in deep-sea sharks of the Southeastern Mediterranean sea over the last ∼ 40 years

Deep-sea habitats are currently recognized as a hot spot for mercury (Hg) accumulation from anthropogenic sources, resulting in elevated concentrations of total mercury (THg) in deep-sea megafauna. Among them, deep-sea sharks (Class Chondrichthyes) are characterized by high trophic position and extended longevity and are, therefore, at high risk for mercury contamination. Despite this, sharks are overexploited by fishing activity in increasingly deeper water, worldwide, imposing health risks to human consumption. While it is imperative to better understand long-term mercury contamination in deep-sea megafauna, few historical data sets exist to capture this process. Here we explore four decades (1985-2022) of THg accumulation in five species of deep-sea sharks (G. melastomus, E. spinax, S. rostratus, C. granulosus, and D. licha) of the ultra-oligotrophic Southeastern Mediterranean Sea (SEMS) sampled during 19 research cruises. We exhibited exceptionally high THg levels (per length/weight), the highest as 16.6 μg g-1 (wet wt.), almost entirely (98.9 %; n = 298 specimens) exceeding the limit for safe consumption (0.3-0.5 μg THg g-1 wet wt.). The maximal THg levels of the long-lived species D. licha and C. granulosus in the SEMS were enriched by a factor of ∼ 7 and >10 compared to counterpart species from other oceanic areas, respectively. We attribute this to the ultra-oligotrophic conditions of the SEMS, which cause slower growth rates and dwarfism in deep-sea sharks, resulting in an extended exposure time to mercury contamination. In the long-lived species, C. granulosus and D. licha, a temporal increase of average THg levels of ∼ 80 % was recorded between 1987-1999 and 2021-2022. This likely reflects the long-term accumulation of historical anthropogenic Hg in deep-sea environments, which is further amplified in marginal seas such as the Mediterranean, impacted by global air pollution crossroads and surrounded by land-based pollution sources. Future consumption of products from deep-sea sharks is potentially high risk to human health.

Mercury bioaccumulation and its relationship with trophic biomarkers in a Mediterranean elasmobranch mesopredator

Human activity has led to increased concentrations of mercury (Hg) in the world's oceans. Mercury can bioaccumulate and biomagnify in animal tissues via trophic transfer, thus, becoming most pronounced in larger and older predators. Here, we measured Hg concentrations and their relationship with stable isotopes-based proxies of trophic level (δ13C and δ15N values) in multiple tissues of Mustelus spp. from the Mediterranean Sea. We found higher Hg concentrations in muscle than in liver and fin tissues. The relationship between Hg concentrations and δ15N values in muscle suggested repeated foraging for low trophic level and Hg-poor prey, and biomagnification of Hg at higher trophic levels. Seasonal variations in δ13C values could indicate shifts in primary production sources and/or in local prey availability. The HBVSe index suggested no risk to human health, however the safe meal limit recommendations are 4.5 and 2.2 portions per month for adults and children, respectively.

Heavy metal concentrations in sharks, rays and chimaeras from the western Mediterranean Sea

The potential bioaccumulation of pollutants, such as heavy metals, may pose a threat to the western Mediterranean chondrichthyans and human consumers. Therefore, the first extensive assessment of cadmium (Cd), lead (Pb), and copper (Cu) concentrations in the muscle tissue of 17 species of sharks, rays, and chimaeras in this region was conducted via Microwave Assisted Extraction (MAE) and Graphite Furnace Atomic Absorption Spectrometry (GFAAS). Significant differences between species were observed, particularly related to the rabbit fish (Chimaera monstrosa) and the velvet belly lantern shark (Etmopterus spinax), which exceeded the European Union (EU) Commission Regulation 2023/915 threshold of Cd. Overall, heavy metal concentrations correlated negatively with size and trophic level but positively with depth. Although the consumption of these species may entail minimal risk to adult humans, caution is advised, especially for children. These findings are important due to the widespread consumption of chondrichthyans in many western Mediterranean regions.

Increase of blood mercury level with shark meat consumption: A repeated-measures study before and after Chuseok, Korean holiday

Sharks are known to contain high levels of mercury in their meat. However, few studies have directly assessed the changes in mercury concentration in the human body according to shark meat intake. One hundred and ninety-seven participants that traditionally consume shark meat during the Chuseok holiday were recruited from two areas of Gyeongsangbuk-do, South Korea to examine their blood mercury level before and after the holiday season. Blood mercury levels were measured before and after the holiday season. Characteristics such as the consumption of shark meat, intake amount, and the effect on mercury concentration were assessed during the survey. Univariable and multivariable analysis (Linear Mixed Model) were done for assessing the association between shark meat consumption of holiday season and blood mercury level. Among the total participants, 83 consumed shark meat during holiday. In the univariable analysis, a significant increase in blood mercury levels before and after Chuseok was observed only for the group that consumed shark meat during holiday. The multivariable analysis (adjusted for identified confounders that affect both exposure and outcome considering repeated measurements) showed that consuming shark meat was significantly associated with increased blood mercury levels by 3.56 μg/L (95% confidence interval [CI], 2.64-4.67 μg/L). In the model considering the amount consumed as two group, the level of increase was 2.61 μg/L (95% CI, 1.63-3.58 μg/L) for those consuming <100 g, and 6.20 μg/L (95% CI, 4.77-7.62 μg/L) for those consuming ≥100 g compared to group without consuming shark meat. Considering amount consumed as continuous value, 0.02 μg/L (95% CI, 0.01-0.02 μg/L) of blood mercury increase was significantly associated with consuming 1 g. Consumption of shark meat significantly elevated blood mercury levels, exceeding commonly suggested reference concentrations in less than 2 weeks. These findings suggest the need for public health warnings and regulations regarding shark meat consumption.

Lipid-extracted muscle and liver tissues: Can they reveal mercury exposure of pelagic sharks?

Pelagic sharks are apex predators in oceanic ecosystems and tend to accumulate high amounts of mercury (Hg). The conventional method for assessing Hg exposure in sharks involves analyzing tissue samples without any chemical treatment. However, a substantial number of chemically treated tissue samples are still being preserved in laboratories or museums. It is critical to maximize the utilization of existing samples to reduce the need for additional sampling of pelagic sharks, especially endangered species. Lipid extraction is a widely employed pretreatment process for carbon isotope analysis in shark trophic ecology, while its impact on Hg quantification remains uncertain. Here, we evaluated the feasibility of using lipid-free muscle and liver tissues for investigation of Hg exposure in four endangered pelagic sharks inhabiting the eastern Pacific, including bigeye thresher (Alopias superciliosus), pelagic thresher (A. pelagicus), blue shark (Prionace glauca) and silky shark (Carcharhinus falciformis). Results showed that total Hg concentrations (THg) differed between untreated (THgbulk) and lipid-free (THglipid-free) samples for each tissue type of each species. In addition, dichloromethane-methanol extractions significantly altered the amount of Hg. This may result from the removal of lipoprotein compounds that vary between tissues and species. The THgbulk can be calculated by THglipid-free using the following formulas, THgbulk = 1.14 × THglipid-free + 0.30 and THgbulk = 0.33 × THglipid-free + 0.18, for muscle and liver tissues, respectively. These findings emphasize the applications of lipid-free tissues in THg analysis. This study may have important implications for improving evaluation of Hg exposure in endangered pelagic sharks.

Associations between total mercury, trace minerals, and blood health markers in Northwest Atlantic white sharks (Carcharodon carcharias)

The ecology and life-histories of white sharks make this species susceptible to mercury bioaccumulation; however, the health consequences of mercury exposure are understudied. We measured muscle and plasma total mercury (THg), health markers, and trace minerals in Northwest Atlantic white sharks. THg in muscle tissue averaged 10.0 mg/kg dry weight, while THg in blood plasma averaged 533 μg/L. THg levels in plasma and muscle were positively correlated with shark precaudal length (153-419 cm), and THg was bioaccumulated proportionally in muscle and plasma. Nine sharks had selenium:mercury molar ratios in blood plasma >1.0, indicating that for certain individuals the potential protective effects of the trace mineral were diminished, whereas excess selenium may have protected other individuals. No relationships between plasma THg and any trace minerals or health markers were identified. Thus, we found no evidence of negative effects of Hg bioaccumulation, even in sharks with very high THg.

Mercury Concentrations in Whale Shark (Rhincodon typus) Embryo Muscle Tissue

Mercury (Hg) is known to be maternally transferred during embryonic development in sharks; however, Hg concentrations in embryos of filter feeding shark species have not previously been reported. This study measured the total Hg (THg) concentration in muscle tissue of 27 embryos taken from a pregnant whale shark (Rhincodon typus) landed in Taiwan in 1995 and the mean THg concentration compared to the mean muscle THg concentration in embryos from other shark species. The mean (± standard deviation) THg concentration in whale shark embryos was 0.0762 ± 0.0163 µg/g dry weight (0.0224 ± 0.0054 µg/g wet weight). There was no relationship between muscle THg concentration and body length and no significant difference in THg concentration between male and female embryos (p > 0.05). Whale shark embryos have the lowest reported muscle THg concentrations compared to literature values for muscle THg concentrations for embryos from other shark species.

Mercury bioaccumulation in thresher sharks from the eastern tropical Pacific: Influences of body size, maturation stage, and feeding habitat

Sharks, as top order predators, provide a guidance on how contaminants such as mercury bioaccumulate in marine environments. This study assessed the bioaccumulation of mercury (total mercury, THg) in the muscle, liver, red blood cells (RBC), and plasma of pelagic and bigeye thresher sharks (Alopias pelagicus and A. superciliosus) from eastern tropical Pacific. Additionally, the concentration of methylmercury (MeHg) in muscle was also determined to assess risks for human consumption. For both species, muscle THg concentrations (4.05 ± 2.15 and 4.12 ± 1.84 μg g^-1 dry weight for pelagic and bigeye thresher shark) were higher than that in other tissues. THg concentrations for all tissues were significantly correlated with precaudal length, with higher accumulation rates after maturity in pelagic than bigeye thresher sharks, suggesting an associated dietary shift at maturation. Correlations among tissues in both species suggested similar transportation and distribution patterns in internal tissues. The δ¹³C values in muscle, RBC and plasma suggested that habitat shifts influenced Hg accumulation, whereas trophic position, estimated by δ¹⁵N values, had limited effects on patterns of Hg bioaccumulation. Diet shifts towards prey more cephalopods that content higher Hg than small fishes (large fishes: 1.77 μg g^-1; cephalopods: 0.66 μg g^-1 and small fishes 0.48 μg g^-1, dry weight) increased Hg accumulation rates in adult pelagic thresher sharks. Concentrations of MeHg in the muscle of both thresher shark (3.42 ± 1.68 μg g^-1 in A. pelagicus and 3.78 ± 2.13 μg g^-1 in A. superciliosus) exceeded the recommended levels for human consumption. This research provides insight into the factors influencing mercury bioaccumulation in thresher sharks, which are essential for the management and conservation of these species.

Major, minor and trace element concentrations in the muscle and liver of a pregnant female Pacific sharpnose shark (Rhizoprionodon longurio) and its embryos

The Pacific sharpnose shark Rhizoprionodon longurio is an abundant shark consumed by human population in Mexico. This study aimed to determine and compare the concentrations of thirteen essential elements (K, S, P, Na, Ca, Mg, Fe, Zn, Se, Cu, Mn, Cr and Co) and eleven non-essential elements (As, Sr, Cd, V, Li, U, Tl, Ag, Sn, Sb and Pb) in the muscle and liver of a pregnant female, fished near the copper mine of Santa Rosalía, and their respective embryos. Major, minor and trace (essential and non-essential) elements were transferred during gestation to embryos. All major elements analyzed had significantly higher concentrations in embryos liver than the pregnant female, except for magnesium. Higher concentrations in embryo tissues than the pregnant female tissues were found for the non-essential trace elements of strontium, lithium, thallium and silver, which indicate an easy maternal transfer of these non-essential elements.

Understanding the role of ecological factors affecting mercury concentrations in the blue shark (Prionace glauca)

Human activities have increased environmental concentrations of pollutants in marine ecosystems, which can cause harmful effects on marine organisms. Top predators are particularly susceptible to bioaccumulation and biomagnification of pollutants through the food webs and are described as good sentinels for monitoring metal accumulation such mercury (Hg) in marine ecosystems. However, to be used as sentinels, it is important to understand the main ecological factors affecting the concentrations of pollutants in these organisms. In the present study, our main objective was to investigate the effect of body size, sex, trophic niche and geographic area on Hg concentrations in a top marine top predator, the blue shark (Prionace glauca). We analysed Hg in muscle samples from male and female blue sharks of different body sizes collected from the waters surrounding the Canary Islands and the South of Portugal, in the Atlantic Ocean, to waters of the north-western Mediterranean Sea. The results revealed that the sampling area was an important factor explaining Hg concentrations, showing higher values in the Mediterranean blue sharks. We also found a positive relationship between Hg concentrations and body size of blue sharks, indicating a bioaccumulation process of this pollutant in relation with body size. Moreover, we observed a relationship between Hg concentrations and δ¹³C values, a proxy of the use of inshore-offshore marine habitats. Individuals with depleted δ¹³C values that potentially foraged in offshore waters showed higher Hg values. Importantly, most of the analysed blue sharks presented Hg concentrations that exceeded the limits established by the European Union for human consumption.

Shark depredation: future directions in research and management

Shark depredation is a complex social-ecological issue that affects a range of fisheries worldwide. Increasing concern about the impacts of shark depredation, and how it intersects with the broader context of fisheries management, has driven recent research in this area, especially in Australia and the United States. This review synthesises these recent advances and provides strategic guidance for researchers aiming to characterise the occurrence of depredation, identify the shark species responsible, and test deterrent and management approaches to reduce its impacts. Specifically, the review covers the application of social science approaches, as well as advances in video camera and genetic methods for identifying depredating species. The practicalities and considerations for testing magnetic, electrical, and acoustic deterrent devices are discussed in light of recent research. Key concepts for the management of shark depredation are reviewed, with recommendations made to guide future research and policy development. Specific management responses to address shark depredation are lacking, and this review emphasizes that a "silver bullet" approach for mitigating depredation does not yet exist. Rather, future efforts to manage shark depredation must rely on a diverse range of integrated approaches involving those in the fishery (fishers, scientists and fishery managers), social scientists, educators, and other stakeholders.

Toxic metal and metalloid contamination in seafood from an eutrophic Brazilian estuary and associated public health risks

Guanabara Bay (GB) is a highly contaminated estuarine system and an important fishing area in Southeastern Brazil. In this regard, knowledge concerning the association of certain contaminants in seafood to abiotic factors and human health risk assessments is still understudied. Therefore, this study aimed to quantify nine toxic elements in highly consumed crabs, shrimp, and squid, and associate the results with abiotic factors. A human health risk assessment was also performed. Our findings indicate that crabs are the main bioaccumulators. Transparency and depth were noteworthy for all three taxonomic groups. In general, contaminant concentrations were below the limits established by different international agencies, except for As, which was higher than the Brazilian limit (1 mg kg^-1). However, the Hazard Index identified risks to consumer health for the ingestion of seafood. This study emphasizes the importance of jointly evaluating different toxic elements, for a more accurate health risk assessment.

Trace elements in the silky shark Carcharhinus falciformis in the Central Pacific Mexican Shelf

Mercury (Hg), cadmium (Cd) and copper (Cu) concentrations were determined in muscle, liver and fin of the silky shark (Carcharhinus falciformis) caught in the Central Pacific Mexican Shelf. Liver tissue presented the highest concentrations of Cd (2.83 μg g^-1 wet weight (ww)) and Cu (2.34 μg g^-1 ww). For muscle and fin, Cu presented the highest concentrations (0.97 and 1.80 μg g^-1 ww, respectively). Liver concentrations were influenced by the maturation stages for the three elements. Immature organisms exhibited lower trace element concentrations than adults. No significant differences were found between sexes, except for Cu concentrations in muscle, where adult females present higher levels than adult males. One muscle sample (2.3 %) exceeded the Mexican Legislation for Hg (1.0 μg g^-1 ww) and five muscles samples (11.6 %) exceeded the Mexican limit for Cd (0.5 μg g^-1 ww). Meat consumption of this species can be a risk to human health if it is ingested regularly.

Total mercury concentrations in sharks, skates and rays along the South African coast

Global declines in elasmobranch populations resulting from several stressors raises conservation concern. Additionally, apex predators bioaccumulate high concentrations of total mercury (THg), due to biomagnification. Although South Africa is considered one of the top ten contributors of Hg emissions globally, information on Hg concentrations in elasmobranchs is limited. The aim of this study was to evaluate the THg concentrations in 22 species of elasmobranchs along the South African coastline. Concentrations ranged between 0.22 and 5.8 mg/kg in Haploblepharus pictus (dark shysharks) and Rostroraja alba (white skates) on the south coast, respectively. Along the east coast it ranged between 0.21 and 17.8 mg/kg in Mobula kuhlii (shortfin devil rays) and Sphyrna lewini (scalloped hammerheads), respectively. Mercury concentrations on the east coast were in the same range or higher compared to the same species sampled between 2005-10 from the same region, with generally higher concentrations compared to the same species sampled globally.

Effect of body size, feeding ecology and maternal transfer on mercury accumulation of vulnerable silky shark Carcharhinus falciformis in the eastern tropical pacific

The silky shark Carcharhinus falciformis is a large pelagic species distributed in the global oceans and was recently listed as "Vulnerable" by the IUCN because of its decline in population due to overfishing. As an apex predator, the silky shark can accumulate elevated quantities of mercury (Hg), posing a potential risk to its remaining population. In this study, total Hg (THg) concentrations were determined in silky shark muscle, liver, dermis, red blood cells (RBC) and plasma sampled from the eastern tropical Pacific, and δ¹⁵N values were measured to explore the influence of feeding ecology on Hg accumulation. The highest THg concentrations were in muscle (7.81 ± 6.70 μg g^-1 dry weight (dw) or 2.14 ± 1.83 μg g^-1 wet weight (ww)) and liver (7.88 ± 10.22 μg g^-1 dw or 4.66 ± 6.04 μg g^-1 ww) rather than dermis, RBC and plasma. The THg concentrations in all tissue types were significantly correlated with fork length and showed faster accumulation rates after maturity. Maternal THg transfer was observed in silky sharks with embryos having 33.16% and 1.98% in muscle and liver compared with their respective mothers. The potentially harmful THg concentrations in silky shark tissues and embryos may lead to health problems of sharks and consumers. THg concentrations were negatively correlated with δ¹⁵N values for all tissues, indicating likely baseline variations in δ¹⁵N values that reflect changes in the foraging habitats or regions of silky sharks with size or age. Lastly, strong correlations were observed among THg concentrations of all tissue types, indicating that nonlethal sampling of muscle and dermis tissue can be used effectively to quantify THg concentration of other internal tissues.

Influence of life history variation and habitat on mercury bioaccumulation in a high-order predatory fish in tropical Australia

Mercury distribution and bioaccumulation in aquatic ecosystems of tropical Australia is poorly characterised. Barramundi (Lates calcarifer), a widespread high-order predator in both fresh and coastal marine waters of the region, fulfils requirements for a bio-indicator of mercury contamination. In a study of the Mary River system of the Northern Territory, total mercury in the muscle tissue of 300 specimens gathered over four years (2013-2017, across both wet and dry seasons) was determined by direct combustion-atomic absorption spectrometry. Source of nutrition and trophic position of barramundi in the food web was also estimated via carbon and nitrogen isotopes (δ¹³C and δ¹⁵N), respectively, in tissue by stable isotope mass spectrometry, and determination of strontium isotopes (⁸⁷Sr/⁸⁶Sr) in otoliths by laser ablation-ICPMS differentiated between freshwater and saltwater residence. Results showed that fish moving into freshwater floodplain wetlands concentrated mercury in muscle tissue at approximately twice the level of those that remained in saline habitats. Resolving life histories through otolith analyses demonstrated diversity in mercury bioaccumulation for individual fish of the same migratory contingent on the floodplains. Although trophic level (δ¹⁵N), capture location, source of nutrition (δ¹³C), and age or size partly predicted mercury concentrations in barramundi, our results suggest that individual variability in diets, migration patterns and potentially metabolism are also influential. Using a migratory fish as a bio-indicator, and tracking its life history and use of resources, proved valuable as a tool to discern hot spots in a coastal waterway for a contaminant, such as mercury.

Quantifying maternal transfer of trace elements and stable isotopes in the endangered pelagic thresher shark (Alopias pelagicus)

To quantify maternal provisioning of nutrients in the pelagic thresher shark (Alopias pelagicus) and the potential for negative impacts, the concentrations of trace elements (essential: Co, Cr, Cu, Mn, Ni, Se, and Zn; nonessential: As, Ba, Cd, Hg, and Pb) and fractionation of stable isotopes (¹³C and ¹⁵N) were analyzed in the muscle and liver of 10 pregnant females and 18 associated embryos. Essential trace elements were observed to be offloaded at higher concentrations to embryos, with the exception of Zn and Ni in liver, while nonessential trace elements were unevenly distributed between maternal-embryo tissues. Observed Hg concentrations were at levels considered toxic in A. pelagicus, but the Se: Hg molar ratios in all embryonic tissues were all greater than one. A negative correlation was observed between transfer ratios and concentrations of all elements in maternal tissue, indicating the existence of a regulatory mechanism in maternal ovaries of A. pelagicus. Compared with maternal specimens, associated embryos had higher δ¹³C and δ¹⁵N values in muscle and liver tissue. Negative correlations were observed between δ¹³C, δ¹⁵N, and Δδ¹³C values and precaudal length in embryonic muscle tissue potentially reflecting either a dietary-habitat shift in pregnant females during the latter period of gestation or a physiological change modifying fractionation. Higher concentrations of essential elements are linked to potential benefits for embryos during early development, levels of Hg suggested a degree of anthropogenic impact with unknown consequences while the directionality of isotopic fractionation could suggest a potential reproductive migration as a protective mechanism for birthing.

Mercury maternal transfer in two placental sharks and a yolk-sac ray from Baja California Sur, Mexico

Total mercury (THg) concentrations were measured in muscle and liver of two placental viviparous sharks, the Pacific sharpnose shark (Rhizoprionodon longurio) and the brown smooth-hound (Mustelus henlei); as well as in the muscle, liver, and yolk of the yolk-sac viviparous speckled guitarfish (Pseudobatos glaucostigmus) in Baja California Sur. The aim was to determine which factors could be involved in maternal transfer and resultant maternal and embryonic THg concentration. Higher THg concentrations were found in pregnant females compared to embryos paired tissues. THg concentrations of embryo tissues decreased with total length (T_L), except for the muscle of the Pacific sharpnose shark. THg concentrations of embryo muscle was positively related to THg concentration in the muscle of pregnant females. Embryos T_L, muscle THg concentration of pregnant females, percentage of THg concentration in embryos, along with the reproductive strategy are relevant factors required to improve our understanding of THg concentration in embryo tissues.

Cadmium and lead levels in muscle tissue of blue shark (Prionace glauca) in the Southeastern Pacific Waters

Prionace glauca is a top predator, prone to bioaccumulate trace elements, representing the most captured elasmobranch species for human consumption in Peru. Concentrations of cadmium and lead in the edible muscle tissue of the blue shark captured in the south Peruvian coastal waters and offshore north-central Chile were determined. The Cd and Pb levels varied between 0.004 and 0.014 and 0.004 and 0.03 mg kg^-1, respectively. We found direct correlation of Cd concentration with the total length, nevertheless there was no clear relationship regarding sex, seasons, and capture areas. Both metals were below the safety limits for human consumption. The target hazard quotient (THQ) values suggest that its consumption does not represent a risk to the human health. In Peru, this is only the second paper that reports Cd and Pb in sharks. We recommend increasing the sampling by including larger sharks and in other regions of the Peruvian coastline.

Foraging plasticity diversifies mercury exposure sources and bioaccumulation patterns in the world's largest predatory fish

Large marine predators exhibit high concentrations of mercury (Hg) as neurotoxic methylmercury, and the potential impacts of global change on Hg contamination in these species remain highly debated. Current contaminant model predictions do not account for intraspecific variability in Hg exposure and may fail to reflect the diversity of future Hg levels among conspecific populations or individuals, especially for top predators displaying a wide range of ecological traits. Here, we used Hg isotopic compositions to show that Hg exposure sources varied significantly between and within three populations of white sharks (Carcharodon carcharias) with contrasting ecology: the north-eastern Pacific, eastern Australasian, and south-western Australasian populations. Through Δ²⁰⁰Hg signatures in shark tissues, we found that atmospheric Hg deposition pathways to the marine environment differed between coastal and offshore habitats. Discrepancies in δ²⁰²Hg and Δ¹⁹⁹Hg signatures among white sharks provided evidence for intraspecific exposure to distinct sources of marine methylmercury, attributed to population and ontogenetic shifts in foraging habitat and prey composition. We finally observed a strong divergence in Hg accumulation rates between populations, leading to three times higher Hg concentrations in large Australasian sharks compared to north-eastern Pacific sharks, and likely due to different trophic strategies adopted by adult sharks across populations. This study illustrates the variety of Hg exposure sources and bioaccumulation patterns that can be found within a single species and suggests that intraspecific variability needs to be considered when assessing future trajectories of Hg levels in marine predators.

Ecophysiological effects of mercury bioaccumulation and biochemical stress in the deep-water mesopredator Etmopterus spinax (Elasmobranchii; Etmopteridae)

Mercury (Hg) is a non-essential metal that can have toxic effects on the fitness of organisms and tends to bioaccumulate with age and to biomagnify in higher trophic levels. Few studies have assessed oxidative stress and neurotoxicity in deep-water sharks. This study evaluated early ontogenetic changes and physiological effects (antioxidant defences, oxidative damage, aerobic metabolism and neurotransmission functions) of Hg accumulation in the white muscle and brain tissues of the velvet belly lantern shark Etmopterus spinax from the southern Iberian coast (NE Atlantic). Results suggested that the low mercury concentrations observed may induce acute effects in E. spinax before they reach sexual maturity. We found different Hg concentrations in E. spinax: [Hg] males > [Hg] females; [Hg] muscle > [Hg] brain. Females appeared to have higher redox capability translated into higher activities and levels of antioxidant defences than males. However, higher levels of oxidative damage were also observed in females. Whilst the mechanisms underlying these effects remain unknown, these results suggest differences in mercury accumulation between tissues and sex, and potentially deleterious effects on oxidative stress status and neurophysiology of E. spinax, potentially impairing swimming performance and reproduction, which could subsequently impact on the health of both individuals and population.

Assessing multigenerational exposure to metals in elasmobranchs: Maternal transfer of contaminants in a yolk-sac viviparous species

This study aimed to assess the maternal offloading of metals in the Brazilian guitarfishPseudobatos horkelii through determining essential (Cr, Cu, Fe) and non-essential (Cd, Hg, Pb) metal concentrations along two generations of this species: pregnant females and offspring. The maximum transfer capacity (ECER) and offspring/mother ratio were calculated to estimate the extent of offloading, as well as the proportion of contaminants presented in offspring related to the maternal concentrations. Transfer efficiency was element-dependent. Chromium had the highest ECER (99.7%), followed by Hg (67.7%). Other essential metals were less transferred (9.6-35.6%) and Cd and Pb were not detected in uterine content samples. The relationships between maternal length and concentration, as well as transfer capacity were not significant, indicating that females might be continuously exposed, and that offloading might not excrete metals efficiently. On the other hand, embryos are exposed to these elements which could impair embryonic development.

Mercury, selenium and cadmium in juvenile blue (Prionace glauca) and smooth hammerhead (Sphyrna zygaena) sharks from the Northwest Mexican Pacific coast

Cadmium, selenium, and mercury concentrations were measured in muscle and liver of juvenile blue (Prionace glauca) and smooth hammerhead (Sphyrna zygaena) sharks caught on the west coast of Baja California Sur, Mexico, to evaluate the human health risk associated with its consumption. Cd and Hg were lower than the maximum allowable limit for human consumption established by the Mexican government (Hg = 1.0 μg g^-1 and Cd = 0.50 μg g^-1). Interspecific differences in trace elements accumulation denoted diet variations and physiological requirements of each shark species. Calculated biomagnification factor (BMF) values confirmed a prey-predator trophic transfer of elements. Not significant results of Selenium health benefit Index value (P. glauca = -0.46; S. zygaena = -0.02) signify no potential risks for human health. However, calculated Hazard Index values displayed possible health hazards to the children who consume blue shark meat regularly. The local population is advised regarding the ingestion rates of shark.

Pollutant Pb burden in Mediterranean Centroscymnus coelolepis deep-sea sharks

We report lead (Pb) analyses in juvenile (n = 37; mean length = 24.7 ± 2.3 cm) and adult (n = 16; mean length = 52.3 ± 9.3 cm) Centroscymnus coelolepis Mediterranean deep-sea sharks that are compared to Pb content in bathy-demersal, pelagic and shallow coastal sharks. Median Pb concentrations of C. coelolepis muscle (0.009-0.056 wet ppm) and liver (0.023-0.061 wet ppm) are among the lowest encountered in shark records. Stable Pb isotope imprints in adult C. coelolepis muscles highlight that most of Pb in C. coelolepis is from human origin. Lead isotopes reveal the persistence of gasoline Pb emitted in the 1970s in low-turnover adult shark's muscle while associated liver imprints are in equilibrium with recent pollutant Pb signatures suggesting an efficient pollutant Pb turnover metabolism. The comparison of Pb distribution between adult and juvenile cohorts suggests the role of dietary exposure and possible maternal offloading of Pb during gestation, likely associated to vitellogenesis in this aplacental viviparous deep-sea shark.

Radioactive impacts of the Fukushima Dai-ichi Nuclear Power Plant Accident on blue sharks in the Northwest Pacific

The Fukushima Dai-ichi Nuclear Power Plant Accident (FDNPPA) derived ¹³⁴Cs, ¹³⁷Cs and ^110mAg in blue sharks captured in the Northwest Pacific during 2011-2018 were assessed for the first time in the aspects of radioactive contamination, temporal variation, maternal-to-fetus transfer, tissue distribution and radiation dose, to demonstrate the impacts of the FDNPPA on blue sharks. The contribution of the FDNPPA derived radiocesium in blue sharks (>52%) was estimated based on ¹³⁴Cs/¹³⁷Cs_{activity ratios}. The effective and ecological half-lives of the FDNPPA derived ¹³⁴Cs (270 d, 410 d), and ¹³⁷Cs (430 d, 450 d) were calculated. These contaminations decreased with time and returned to the level before the FDNPPA during the period of Sep. 2017-Sep. 2018.¹³⁴Cs and ¹³⁷Cs tended to distribute in muscles, while ^110mAg mainly distribute in their guts. ¹³⁴Cs and ¹³⁷Cs were also transferred to fetuses and the activities were up to ~30% of the maternal activities. Dose assessment demonstrated that the highest FDNPPA derived dose rate in blue sharks (~0.42 nGy/h) was far below the ERICA ecosystem screening benchmark of 10 μGy/h and the committed effective dose in humans from ingesting blue shark meat (0.06-0.90 μSv) was far less than that from annual consumption of food and water. It was far from causing radiation harm to blue sharks and humans, suggesting that the impacts of the FDNPPA on blue sharks were not significant.

Mercury concentrations and trophic relations in sharks of the Pacific Ocean of Colombia

Sharks are fished for human consumption in Colombia, and fins are exported illegally to international markets. The goal was to identify differences in total mercury (THg) concentrations in fins and muscles of shark species seized in the Buenaventura port (Colombian Pacific), and to assess potential human health risks related to shark consumption. Seven species were considered in this study: Pelagic Thresher (Alopias pelagicus), Pacific Smalltail Shark (Carcharhinus cerdale), Brown Smoothhound (Mustelus henlei), Sicklefin Smoothhound (Mustelus lunulatus), Scalloped Bonnethead (Sphyrna corona), Scalloped Hammerhead (Sphyrna lewini), and Bonnethead Shark (Sphyrna tiburo), and THg was analyzed in shark tissues. 24% muscle samples concentrations were above international recommended limits for human consumption, especially for A. pelagicus and S. lewini. Stable isotopes (δ¹³C and δ¹⁵N) analysis showed a foraging overlapping in coastal habitats, where overfishing usually occurs. This study provides useful scientific information to develop management plans for sharks in Colombia and neighboring countries.

Lifelong mercury bioaccumulation in Atlantic horse mackerel (Trachurus trachurus) and the potential risks to human consumption

Atlantic horse mackerel is one of the most commercially important species in Europe. It can reach a longevity of 30 years, with potential implications in lifespan mercury contamination. This study conducted along the Portuguese coast aimed at evaluating the total Hg content and tissue distribution, to determine the annual mercury bioaccumulation patterns and the associated risk for consumption. The T-Hg accumulation patterns observed followed the order: muscle (0.34) > liver (0.28) > heart (0.19) > gills (0.11) > brain (0.041 mg kg^-1). Significant differences between tissues reflect the role of the different tissues in storage and redistribution. Significant relationships observed between age and T-Hg for all tissues highlight the continuous nature of the bioaccumulation process. European food safety guidelines signalled significant risk of consumption in about 30% of the samples. Still, there was an overall low risk from the consumption of this species, which can be further minimized through consumer options to avoid health issues.

Nurse sharks, space rockets and cargo ships: Metals and oxidative stress in a benthic, resident and large-sized mesopredator, Ginglymostoma cirratum

It is widely recognized that apex predators, such as large sharks with highly migratory behavior, are particularly vulnerable to pollution, mainly due to biomagnification processes. However, in highly impacted areas, mesopredator sharks with resident behavior can be as vulnerable as apex sharks. In this context, this study evaluated cadmium (Cd), mercury (Hg), lead (Pb), and rubidium (Rb) concentrations, as well as the potentially protective effects of selenium (Se) and the behavior of two non-enzymatic biomarkers, metallothionein (MT) and reduced glutathione (GSH), employing the Atlantic nurse shark Ginglymostoma cirratum as a study model and compared the results with other resident benthic sharks, as well as highly mobile apex sharks. Muscle tissue samples from 28 nurse sharks opportunistically sampled from the Brazilian Amazon Coast were analyzed. Lower metal concentrations were observed for Pb, Rb and Se in the rainy season, while statistically significant correlations between metals were observed only between Hg and Cd and Pb and Se. Molar ratio calculations indicate potential protective Se effects against Pb, but not against Cd and Hg. No associations between MT and the determined metals were observed, indicating a lack of detoxification processes via the MT detoxification route. The same was noted for GSH, indicating no induction of this primary cellular antioxidant defense. Our results indicate that benthic/mesopredator sharks with resident behavior are, in fact, as impacted as highly mobile apex predators, with the traditional detoxification pathways seemingly inefficient for the investigated species. Moreover, considering the studied population and other literature data, pollution should be listed as a threat to the species in future risk assessments.

Bioaccumulation and biomagnification in elasmobranchs: A concurrent assessment of trophic transfer of trace elements in 12 species from the Indian Ocean

We provided the first multi-species study investigating the presence and organotropism of trace elements in three tissues of 12 elasmobranch species. Shark species showed comparable TE loads, although milk sharks and juvenile scalloped hammerhead sharks exhibited the highest Cd and Hg levels, respectively. Fins accumulated higher levels of Pb, Co, and Cr; muscles higher V, As, and Hg; livers higher Se and Cd levels. The organotropism of TEs calls for cautious when choosing a tissue to be sampled since certain tissues, like fin clips, do not provide reliable surrogate for the internal loads of some TEs. Strong correlations between essential and toxic TEs indicated detoxification mechanisms, while the TMF provided evidence for Hg, As and Se biomagnification along the food-web. Considering the difficulties in assessing elasmobranchs contamination from different areas, the proposed multi-species approach represents a valuable way to estimate the species-specific accumulation and transfer of pollutants in sharks.

Differences in macroelements, trace elements and toxic metals between wild and captive-reared greater amberjack (Seriola dumerili) from the Mediterranean Sea

Despite its legislative regulation and control, the quality and safety of aquatic products is somewhat questioned due to the potential bioaccumulation of pollutants. The elements (Al, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, Pb, Sr, V and Zn) were determined in the liver and muscle of wild and captive-reared Seriola dumerili with the aim of studying possible differences between origins, and sex-related variations. Additionally, the dietary intake of these elements derived from its consumption was also evaluated. Most of the elements and metals analyzed were accumulated to a higher extent in the liver of wild specimens whereas lower differences were observed in the muscle. Overall, the elements and metal composition of wild females strongly differed from that of captive-reared specimens probably related to the mobilization of nutrients for the spawning season in wild mature females, which were greater than their captive-reared counterparts.

Trophic transfer of heavy metals in the marine food web based on tissue residuals

Trophic transfer of metals has been well researched in aquatic food webs; however, most studies have examined the presence of metal residuals in the entire body of marine organisms and but not in specific tissues. In this study, we determined the concentrations of Cu, Cr, Pb, Zn, Cd, and Ni in various organs of 17 marine species, including crustaceans, gastropods, bivalves, and fishes, with different trophic levels (TLs), which were collected from the Liaodong Bay, China, in July 2019. Results showed that the liver, gill, and muscle tissues of marine species are ideal indicators for analyzing Cu, Cr, Pb, Zn, Cd, and Ni contamination in marine environments. When the entire bodies of these marine species were considered, a bio-dilution in Cu, Cr, Pb, Zn, Cd, and Ni was observed in the studied food web. In contrast, the metal tissue-specific bio-magnification in the entire studied food web showed different results. In the liver and gill tissues, negative correlations were found between the concentrations of cadmium and TLs, while copper bio-dilution was also observed in gill tissue. In the muscle tissues, Cu, Pb, and Ni showed bio-dilution and trophic magnification factors of Cu, Pb, and Ni ranged from 0.44 to 0.73. This study highlights the importance of tissue-specific considerations to obtain further accurate information on metal trophodynamics and trophic transfers in marine food webs, thereby enhancing the risk assessment of many elements in wildlife and human health.

Concentrations and stable isotopes of mercury in sharks of the Galapagos Marine Reserve: Human health concerns and feeding patterns

The human ingestion of mercury (Hg) from sea food is of big concern worldwide due to adverse health effects, and more specifically if shark consumption constitutes a regular part of the human diet. In this study, the total mercury (THg) concentration in muscle tissue were determined in six sympatric shark species found in a fishing vessel seized in the Galapagos Marine Reserve in 2017. The THg concentrations in shark muscle samples (n = 73) varied from 0.73 mg kg^-1 in bigeye thresher sharks (Alopias superciliosus) to 8.29 mg kg^-1 in silky sharks (Carcharhinus falciformis). A typical pattern of Hg bioaccumulation was observed for all shark species, with significant correlation between THg concentration and shark size for bigeye thresher sharks, pelagic thresher sharks (Alopias pelagicus) and silky sharks. Regarding human health concerns, the THg mean concentration exceeded the maximum weekly intake fish serving in all the studied species. Mass-Dependent Fractionation (MDF, δ²⁰²Hg values) and Mass-Independent Fractionation (MIF, Δ¹⁹⁹Hg values) of Hg in whitetip sharks (Carcharhinus longimanus) and silky sharks, ranged from 0.70‰ to 1.08‰, and from 1.97‰ to 2.89‰, respectively. These high values suggest that both species are feeding in the epipelagic zone (i.e. upper 200 m of the water column). While, blue sharks (Prionace glauca), scalloped hammerhead sharks (Shyrna lewini) and thresher sharks were characterized by lower Δ¹⁹⁹Hg and δ²⁰²Hg values, indicating that these species may focus their foraging behavior on prey of mesopelagic zone (i.e. between 200 and 1000 m depth). In conclusion, the determination of THg concentration provides straight-forward evidence of the human health risks associated with shark consumption, while mercury isotopic compositions constitute a powerful tool to trace the foraging strategies of these marine predators. CAPSULE: A double approach combining Hg concentrations with stable isotopes ratios allowed to assess ontogeny in common shark species in the area of the Galapagos Marine Reserve and the human health risks concern associated to their consumption.

Distribution of mercury species in different tissues and trophic levels of commonly consumed fish species from the south Bay of Biscay (France)

Mercury (Hg) is a contaminant of global concern in marine ecosystems, notably due to its ability to accumulate and concentrate in food webs. Concentrations of total mercury (THg), methylmercury (MeHg) and inorganic mercury (IHg) were assessed and compared in different tissues (liver, muscle, and gonads) of three common fish species (hake Merluccius merluccius, red mullet Mullus surmuletus, and sole Solea solea) from the continental shelf from the southern part of the Bay of Biscay. Several studies investigated Hg concentration in fish muscle, but few assessed concentrations in other organs, despite the importance of such data to understand contaminant organotropism and metabolization. Results showed that trophic position and feeding habitat are required to understand the variability of Hg concentration in muscle between fish species. In addition, high MeHg/THg ratio in muscle could be explained by the predatory behavior of the studied fish species and the biomagnification of this Hg species within the food web, MeHg. Despite differences between species, Hg concentration was always higher in muscle (from 118 ± 64 to 338 ± 101 ng g^-1 w.w.) and liver (from 122 ± 108 to 271 ± 95 ng g^-1 w.w.). These results can be related to physiological processes especially the MeHg detoxification strategies.

Mercury in neonatal and juvenile blacktip sharks (Carcharhinus limbatus). Part I: exposure assessment

Elasmobranchs are exposed to mercury (Hg) through a variety of pathways in the environment. This study assessed maternal offloading and diet-based Hg exposure for neonatal and juvenile blacktip sharks (Carcharhinus limbatus) from Charlotte Harbor located along southwest Florida's coast, a recognized Hg hotspot. Neonates (n = 57) had highest total Hg (THg) concentrations in the kidney (0.56 ± 0.26 mg kg^-1; n = 38) and muscle (0.53 ± 0.17 mg kg^-1; n = 57), followed by liver (0.31 ± 0.11 mg kg^-1; n = 38), and blood (0.05 ± 0.033 mg kg^-1; n = 57). Juveniles (n = 13) exhibited a different distribution with highest THg in the liver (0.868 ± 0.54 mg kg^-1; n = 6), followed by the muscle (0.84 ± 0.28 mg kg^-1; n = 13), kidney (0.55 ± 0.22 mg kg^-1; n = 6), and blood (0.11 ± 0.04 mg kg^-1; n = 11). The distribution of THg among tissues and liver-to-muscle ratios indicated that Hg originated primarily from maternal offloading in neonates, whereas juveniles continued to accumulate Hg through dietary exposure post-parturition. Additionally, comparisons between results of the present study and previous Florida blacktip shark surveys suggested that Hg levels have not declined in southwest Florida estuaries for over two decades.

The Twilight Zone as a Major Foraging Habitat and Mercury Source for the Great White Shark

The twilight zone contains the largest biomass of the world's ocean. Identifying its role in the trophic supply and contaminant exposure of marine megafauna constitutes a critical challenge in the context of global change. The white shark (Carcharodon carcharias) is a threatened species with some of the highest concentrations of neurotoxin methylmercury (MeHg) among marine top predators. Large white sharks migrate seasonally from coastal habitats, where they primarily forage on pinnipeds, to oceanic offshore habitats. Tagging studies suggest that while offshore, white sharks may forage at depth on mesopelagic species, yet no biochemical evidence exists. Here, we used mercury isotopic composition to assess the dietary origin of MeHg contamination in white sharks from the Northeast Pacific Ocean. We estimated that a minimum of 72% of the MeHg accumulated by white sharks originates from the consumption of mesopelagic prey, while a maximum of 25% derives from pinnipeds. In addition to highlighting the potential of mercury isotopes to decipher the complex ecological cycle of marine predators, our study provides evidence that the twilight zone constitutes a crucial foraging habitat for these large predators, which had been suspected for over a decade. Climate change is predicted to expand the production of mesopelagic MeHg and modify the mesopelagic biomass globally. Considering the pivotal role of the twilight zone is therefore essential to better predict both MeHg exposure and trophic supply to white sharks, and effectively protect these key vulnerable predators.

Elevated accumulation of the toxic metal mercury in the Critically Endangered oceanic whitetip shark Carcharhinus longimanus from the northwestern Atlantic Ocean

The oceanic whitetip shark Carcharhinus longimanus is a widely distributed large pelagic shark species once considered abundant in tropical and warm temperate waters, but recently listed as Critically Endangered by the IUCN due to drastic population declines associated with overfishing. In addition to risks posed to its populations due to overexploitation, oceanic whitetip sharks are also capable of accumulating elevated quantities of harmful environmental toxicants, placing them at special risk from anthropogenic pollution. Herein, we provide the first data on accumulation of the toxic, non-essential metal mercury (Hg) in northwest Atlantic (NWA) oceanic whitetip sharks, focusing on aggregations occurring at Cat Island, The Bahamas. Total Hg (THg) concentrations were measured in muscle of 26 oceanic whitetip sharks and compared with animal length and muscle δ15N to evaluate potential drivers of Hg accumulation. THg concentrations were also measured in fin and blood subcomponents (red blood cells and plasma) to determine their value as surrogates for assessing Hg burden. Muscle THg concentrations were among the highest ever reported for a shark species and correlated significantly with animal length, but not muscle δ15N. Fin, red blood cell, and plasma THg concentrations were significantly correlated with muscle THg. Fin THg content was best suited for use as a surrogate for estimating internal Hg burden because of its strong relationship with muscle THg levels, whereas blood THg levels may be better suited for characterizing recent Hg exposure. We conclude that Hg poses health risks to NWA oceanic whitetip sharks and human consumers of this species.

Are concentrations of pollutants in sharks, rays and skates (Elasmobranchii) a cause for concern? A systematic review

This review represents a comprehensive analysis on pollutants in elasmobranchs including meta-analysis on the most studied pollutants: mercury, cadmium, PCBs and DDTs, in muscle and liver tissue. Elasmobranchs are particularly vulnerable to pollutant exposure which may pose a risk to the organism as well as humans that consume elasmobranch products. The highest concentrations of pollutants were found in sharks occupying top trophic levels (Carcharhiniformes and Lamniformes). A human health risk assessment identified that children and adults consuming shark once a week are exposed to over three times more mercury than is recommended by the US EPA. This poses a risk to local fishing communities and international consumers of shark-based products, as well as those subject to the widespread mislabelling of elasmobranch products. Wider screening studies are recommended to determine the risk to elasmobranchs from emerging pollutants and more robust studies are recommended to assess the risks to human health.

Mercury isotopes as tracers of ecology and metabolism in two sympatric shark species

In coastal ecosystems, top predators are exposed to a wide variety of nutrient and contaminant sources due to the diversity of trophic webs within inshore marine habitats. Mercury contamination could represent an additional threat to shark populations that are declining worldwide. Here we measured total mercury, carbon and nitrogen isotopes, as well as mercury isotopes, in two co-occurring shark species (the bull shark Carcharhinus leucas and the tiger shark Galeocerdo cuvier) and their potential prey from a coastal ecosystem of the western Indian Ocean (La Réunion Island). Our primary goals were to (i) determine the main trophic Hg sources for sharks and (ii) better characterize their diet composition and foraging habitat. Hg isotope signatures (Δ¹⁹⁹Hg and δ²⁰²Hg) of shark prey suggested that bull sharks were exposed to methylmercury (MeHg) produced in offshore epipelagic waters, while tiger sharks were exposed to offshore mesopelagic MeHg with additional microbial transformation in slope sediments. Δ¹⁹⁹Hg values efficiently traced the ecology of the two predators, demonstrating that bull sharks targeted coastal prey in shallow waters while tiger sharks were mainly foraging on mesopelagic species in the deeper waters of the island slope. Unexpectedly, we found a positive shift in δ²⁰²Hg (>1‰) between sharks and their prey, leading to high δ²⁰²Hg values in the two shark species (e.g. 1.91 ± 0.52‰ in bull sharks). This large shift in δ²⁰²Hg indicates that sharks may display strong MeHg demethylation abilities, possibly reflecting evolutionary pathways for mitigating their MeHg contamination.

Hg and Se composition in demersal deep-sea fish from the North-East Atlantic

It has been emphasized that seafood consumers may have a higher risk of mercury (Hg) exposure. Nevertheless, the co-occurrence of selenium (Se) in organisms may affect the toxicity and bioavailability of Hg. In this work, we aim to demonstrate the possible role of Se as a potential protective element against Hg in muscle and liver tissues of three demersal deep-sea fish species: common mora Mora moro, birdbeak dogfish Deania calcea, and smooth lanternshark Etmopterus pusillus. Comparing species, the birdbeak dogfish D. calcea showed the highest Hg concentrations, the lowest Se levels, the lowest mean Se:Hg molar ratio, a negative HBV-Se index, and no correlation between total length. On the other hand, smooth lanternshark E. pusillus showed the lowest Hg concentrations, the highest Se concentrations, the highest mean Se:Hg molar ratio, a positive HBV-Se index, and a significant positive correlation between total length and Se concentrations in muscle. Comparing tissues, the common mora Mora moro seems to accumulate more Hg and Se in liver than shark species D. calcea and E. pusillus that showed to accumulate Hg and Se preferentially in muscle. Our results indicate that these three species for having Hg concentrations near and above the EU regulatory thresholds and for presenting low Se:Hg ratios, and negative (or low positive) HBV-Se index may pose a real risk of Hg toxicity for the consumer.

Bioaccumulation and trophic transfer of potentially toxic elements in the pelagic thresher shark Alopias pelagicus in Baja California Sur, Mexico

Pelagic thresher shark (Alopias pelagicus) is a circumglobal species with high ecological and economic importance. Concentrations of mercury, selenium and cadmium in the muscle and liver tissues of A. pelagicus captured from Baja California Sur, Mexico were determined for assessing the potential human hazard. Results revealed that the average concentrations of Hg (0.76 mg kg^-1) and Cd (0.18 mg kg^-1) in muscle tissues were below the maximum permissible limits for human consumption. Se in the muscles were relatively low (mean: 0.30 mg kg^-1 -1) resulting in a molar excess of Hg over Se. Average levels of hepatic Cd were extremely higher than the maximum limit for consumption. Organotropism of Hg was muscle > liver, whereas Se and Cd presented an order of liver > muscle. Biomagnification Factor (BMF) emphasized the trophic transfer of elements. Selenium Health Benefit value was negative (-3.76) posing potential health risks demanding regular monitoring for health risks.

Mercury contamination in the recently described Brazilian white-tail dogfish Squalus albicaudus (Squalidae, Chondrichthyes)

The recently described Squalus albicaudus is a mesopredator shark and, as such, exposed to mercury biomagnification processes. Therefore, this study aimed to assess total Hg (THg) concentrations in S. albicaudus, a deep-water species, sampled off Southeastern Brazil and discuss ecological, reproductive, human consumption and conservation implications. Thirty-two individuals were sampled off the coast of Rio de Janeiro, including 13 gravid females carrying 34 embryos. Muscle THg concentrations were higher in all sex classes compared to liver, gonads and brain. The last three, in turn, presented THg concentrations above toxic biota thresholds. Significant correlations were observed between muscle and brain and liver, indicating systemic Hg contamination and inter-organ transport and distribution. In addition, correlations observed between organs strongly support efficient Hg blood-brain barrier crossing and maternal transfer. Maternal THg transfer was observed, with embryo THg also above toxic thresholds for fish. THg levels in muscle and liver, as well as embryos, were higher compared to other Squalus species worldwide. Hg contamination off the coast of Rio de Janeiro is of significant concern and should be further assessed. Potential human consumption risks are noted, as muscle THg concentrations were above maximum permissible levels set by regulatory agencies.

First concurrent assessment of elemental- and organic-contaminant loads in skin biopsies of whale sharks from Djibouti

The Gulf of Tadjoura (Djibouti) is an important site where the whale shark (Rhincodon typus Smith, 1828) aggregates seasonally. Because of the proximity to the port of Djibouti that is located along one of the busiest shipping areas in the world, whale sharks are potentially exposed to relatively high levels of trace elements and organochlorines. To assess their contamination status, concentrations of 15 trace elements, DDTs and PCBs were concurrently assessed for the first time in skin biopsies of 20 whale sharks. Additionally, 12 zooplankton samples were collected and analyzed for trace elements content. Concentration of As, Cu, Zn and Se were higher than in previous studies on this species. Whale shark samples exceeded the maximum allowable limits for foodstuffs for Cr, Pb Se, Cd and Zn. Results from this study suggests that Cr, Ni and Mo may biomagnify in this species. With regard to PCBs, the predominant congener were Tetra-CB, accounting for 41% of total PCBs and Penta-CB for 23%, while for DDTs, the predominant metabolite was DDE contributing for 51% of total compounds. The maximum residue limit for DDTs and for the ind-PCBs in fish set by U.S. EPA and by the EU regulation, respectively, was exceeded in 62% of whale sharks. Moreover, the p,p'DDE/ΣDDT ratios were higher than the critical value of 0.6, indicating possible recent inputs of technical DDTs in the area. This study suggests some concerns for the contamination status of whale sharks in Djibouti and, since major threats for this species include bycatch and illegal fisheries, highlights some level of risk from the exposure to elemental- and organic-contaminant via shark consumption.

Mercury (Hg) and selenium (Se) content in the shark Mustelus henlei (Triakidae) in the northern Mexican Pacific

Mercury and selenium were assessed in Mustelus henlei, which is a carnivorous predatory shark that is important for the coastal communities of the northern Mexican Pacific (NMP). Sixty-two individuals were sampled; muscle and liver were isolated and analyzed by atomic absorption spectrophotometry. The mean Hg concentrations (wet weight) obtained for muscle (0.08 ± 0.10 μg g^-1) and liver (0.09 ± 0.26 μg g^-1) were below the allowed limits (< 1.0 μg g^-1 Hg). The average Se concentration was 0.03 ± 0.01 μg g^-1 in muscle and 0.13 ± 0.05 μg g^-1 in liver. The Se/Hg molar ratio of muscle was 1.83; however, the selenium health benefit value (HBV_Se) was of 0.08. We calculated that an adult man (70 kg), an adult woman (60 kg), and a child (16 kg) could consume 1595, 838, and 223 g/week of M. henlei muscle, respectively, without risks to health. In conclusion, the concentrations and molar ratio of Hg and Se in M. henlei muscle mean that consumption of this shark's meat does not represent neither a benefit nor a public health risk.

Mercury-selenium concentrations in silky sharks (Carcharhinus falciformis) and their toxicological concerns in the southern Mexican Pacific

Mercury- Selenium concentrations were determined in 136 samples of Carcharhinus falciformis (Silky shark) sampled from the Chiapas coast, Gulf of Tehuantepec, Mexico during August 2014 - January 2015. Average Hg concentrations in neonates and juveniles (all values in μg g^-1 w.w.) were found to be 0.115 and 0.129 respectively, below the maximum permissible limit of 1 μg g^-1 set by the Mexican government. However, excess Se values (all values in μg g^-1 d.w.) observed in neonates (5.366) and juveniles (2.815) prove to maintain antioxidant ability by inducing Hg excretion and reducing its toxicity. Calculated Biomagnification Factor (BMF_TL) denoted high values for the prey C.hippurus, signifying absolute magnification of Hg and Se along the food chain. This study provides key toxicological evidences of Hg- Se interaction and their effects in marine systems and human health.

Cadmium, mercury, and selenium in muscle of the scalloped hammerhead Sphyrna lewini from the tropical Eastern Pacific: Variation with age, molar ratios and human health risk

With the aim of assessing health risk to shark consumers, cadmium, mercury, and selenium were measured in muscle of Sphyrna lewini from four coastal states (Baja California Sur, Sinaloa, Nayarit, and Colima) in western Mexico. According to length of specimens, three age modes were found: juveniles and neonates (the majority of the individuals), preadults and adults. Average concentrations (μg g^-1 dry weight) in all the studied individuals followed the order cadmium (0.06), selenium (0.94), and mercury (1.56). The mean concentrations of cadmium and mercury increased significantly (p < 0.001) with mean length of specimens. Overall, hazard quotient and hazard index values were below one so there is no health risk to consumers. According to molar ratios of Hg and Se in the edible portion (muscle) of sharks, and depending on the areas of collection, individuals from Baja California Sur might not be beneficial to consumers.

Trace elements levels in muscle and liver of a rarely investigated large pelagic fish: The Mediterranean spearfish Tetrapturus belone (Rafinesque, 1810)

We determined levels of mercury, cadmium, lead and selenium in muscle and liver of 29 specimens of a large pelagic fish rarely investigated, the Mediterranean spearfish Tetrapturus belone (Raf., 1810). The following element concentration ranking (mean ± S.D.; espressed in mg/kg dry weight) was recorded in muscle: Hg (3.401 ± 1.908) > Se (1.727 ± 0.232) > Pb (0.532 ± 0.322) > Cd (0.019 ± 0.015), and Se (6.577 ± 1.789) > Cd (5.815 ± 3.038) > Hg (2.698 ± 2.214) > Pb (0.661 ± 1.334) in liver. Levels of Hg, Se and Cd were compared to those reported for other Istiophoridae from oceanic areas and for other large predators of Mediterranean Sea, like swordfish and tuna. Organotropism of trace elements and their relation to size was discussed. Ecophysiological considerations regarding the Se-Hg relationship as well as Se-Cd indicate a possible detoxification mechanism. The implications for human consumption are briefly discussed.

Mercury Accumulation and Effects in the Brain of the Atlantic Sharpnose Shark (Rhizoprionodon terraenovae)

Few published studies have examined whether the elevated concentrations of the nonessential toxic metal mercury (Hg) often observed in shark muscle also occur in the shark brain or whether Hg accumulation affects shark neurophysiology. Therefore, this study examined accumulation and distribution of Hg in the shark brain, as well as effects of Hg on oxidative stress in the shark central nervous system, with particular focus on the Atlantic sharpnose shark (Rhizoprionodon terraenovae). Sharks were collected along the southeastern U.S. coast throughout most of this species' U.S. geographical range. Total Hg (THg) concentrations were measured in and compared between shark muscle and brain, whereas known biomarkers of Hg-induced neurological effects, including glutathione depletion, lipid peroxidation, and concentrations of a protein marker of glial cell damage (S100b), were measured in shark cerebrospinal fluid. Brain THg concentrations were correlated with muscle THg levels but were significantly lower and did not exceed most published thresholds for neurological effects, suggesting limited potential for detrimental responses. Biomarker concentrations supported this premise, because these data were not correlated with brain THg levels. Hg speciation also was examined. Unlike muscle, methylmercury (MeHg) did not comprise a high percentage of THg in the brain, suggesting that differential uptake or loss of organic and inorganic Hg and/or demethylation of MeHg may occur in this organ. Although Hg accumulation in the shark brain generally fell below toxicity thresholds, higher THg levels were measured in the shark forebrain compared with the midbrain and hindbrain. Therefore, there is potential for selective effects on certain aspects of shark neurophysiology if brain Hg accumulation is increased.

Trophic Structure and Biomagnification of Total Mercury in Ray Species Within a Benthic Food Web

Stable isotopes of C (δ¹³C) and N (δ¹⁵N) were used to explore the trophic structure and evaluate mercury (Hg) biomagnification in the food web of muscle of three commercially important ray species from the Pacific coast of Baja California Sur (PCBCS): the shovelnose guitarfish (Pseudobatos productus), banded guitarfish (Zapteryx exasperata), and bat ray (Myliobatis californica). The food web of these ray species predominately consisted of zooplankton, three species of fish, and five species of invertebrates. Mean δ¹⁵N values in all species ranged from 10.54 ± 0.18‰ in zooplankton to 17.84 ± 0.81‰ in the shovelnose guitarfish. Mean δ¹³C values ranged from - 22.05 ± 0.75‰ in the red crab to - 15.93 ± 0.78‰ in the bat ray. Mean total Hg concentration ([THg]) in all species ranged from 0.0009 ± 0.0002 mg kg^-1 ww in zooplankton to 0.24 ± 0.19 mg kg^-1 ww in the banded guitarfish. The food web magnification factor was 6.38 and significantly greater than 1.0. The present study describes [THg] biomagnification in the benthic food web of three ray species of the PCBCS. This provides an important baseline knowledge of the biomagnification dynamics and pathways of Hg in this environment for these multiple interacting species.