The decline of shark populations in the world ocean is affecting ecosystem structure and function in an unpredictable way and new ecological information is today needed to better understand the role of sharks in their habitats. In particular, the characterization of foraging patterns is crucial to understand and foresee the evolution of dynamics between sharks and their prey. Many shark species use the mesopelagic area as a major foraging ground but the degree to which different pelagic sharks rely on this habitat remains overlooked. In order to depict the vertical dimension of their trophic ecology, we used mercury stable isotopes in the muscle of three pelagic shark species (the blue shark Prionace glauca, the shortfin mako shark Isurus oxyrinchus and the smooth hammerhead shark Sphyrna zygaena) from the northeastern Pacific region. The Δ¹⁹⁹Hg values, ranging from 1.40 to 2.13‰ in sharks, suggested a diet mostly based on mesopelagic prey in oceanic habitats. We additionally used carbon and nitrogen stable isotopes (δ¹³C, δ¹⁵N) alone or in combination with Δ¹⁹⁹Hg values, to assess resource partitioning between the three shark species. Adding Δ¹⁹⁹Hg resulted in a decrease in trophic overlap estimates compared to those based on δ¹³C/δ¹⁵N alone, demonstrating that multi-isotope modeling is needed for accurate trophic description of the three species. Mainly, it reveals that they forage at different average depths and that resource partitioning is mostly expressed through the vertical dimension within pelagic shark assemblages. Concomitantly, muscle total mercury concentration (THg) differed between species and increased with feeding depth. Overall, this study highlights the key role of the mesopelagic zone for shark species foraging among important depth gradients and reports new ecological information on trophic competition using mercury isotopes. It also suggests that foraging depth may play a pivotal role in the differences between muscle THg from co-occurring high trophic level shark species.

Focusing on 27 rare filter-feeding megamouth sharks (Megachasma pelagios) captured as a by-catch of drift gillnet fishery in the Pacific Ocean to the east of Taiwan, this study analyzes the concentrations of 24 elements in their muscle, discusses the bioaccumulation of each element and the correlation between different elements, and assesses the potential health risks of consuming megamouth shark muscle. Among the 24 elements, mean concentrations of Ga, Ag, Li, Bi, Hg, Co, and Cd were relatively low ranging from 10⁻³ to 10⁻¹ mg/kg, those of Pb, Ba, Mn, Ni, As, Cr, B, Sr, Cu, and Zn ranged from 10⁻¹–10¹ mg/kg, and those of Fe, Ca, Al, K, Mg, Ti, and Na were relatively high ranging from 10¹ to 10³ mg/kg. The toxic element content index was most significantly correlated with the concentration of Cu. Hence, this study recommends that the concentration of Cu could be used as an indicator of metal accumulation in megamouth shark muscle. The log bioconcentration factor (BCF) ranged from less than 0 to 7.85 in shark muscle. For elements with a concentration of less than 100 μg/L in seawater, the log BCF was inversely proportional to their concentration in seawater. According to the correlation analysis, the accumulation of elements in muscle of megamouth sharks is primarily affected by the concentrations of dissolved elements in seawater, except that the accumulation of Hg, As, Cu, Ti, Al, and Fe appears to be mainly affected by feeding behaviors. The assessment of the health risk of consuming megamouth shark muscle showed that its total hazard index was greater than 1. This suggests that the long-term or high-frequency consumption of megamouth shark muscle may cause health hazards due to the accumulation of trace elements, particularly those with a large contribution of health risk, including As, Hg, and Cu.

One of the most landed sharks in Portuguese fisheries is the lesser-spotted dogfish (Scyliorhinus canicula), which is ever-present in Portuguese fish markets and consumed as cheap fish protein source. The focus of this study was to evaluate element contamination in consumed tissues of Atlantic S. canicula, with the intent of safeguarding possible public health issues. A total of 74 specimens were analysed for metals and metalloids in the deeper white muscle and skin. Arsenic, zinc, iron and aluminium were the elements with higher mean values. There was a tendency for higher levels in the skin, with differences between life-stage and gender. Many individuals surpassed stipulated guideline limits for mercury and arsenic, posing a risk for human consumption (according to the health risk assessment performed for the average Portuguese fish consumption) or even for use in feed production. Besides the public health concern, this study also evidences troubling signs on marine contamination status.

Contamination by metals is among the most pervasive anthropogenic threats to the environment. Despite the ecological importance of marine apex predators, the potential negative impacts of metal bioaccumulation and biomagnification on the health of higher trophic level species remains unclear. To date, most toxicology studies in sharks have focused on measuring metal concentrations in muscle tissues associating human consumption and food safety, without further investigating potential impacts on shark health. To help address this knowledge gap, the present study evaluated metal concentrations in the gills, muscle, liver and rectal gland of coastal sharks opportunistically sampled from Brazilian waters and tested for potential relationships between metal bioaccumulation and general shark health and homeostatic balance metrics. Results revealed high metal concentrations in all four tissue types, with levels varying in relation to size, sex, and life-stage. Metal concentrations were also associated with serum biomarkers (urea, lactate, ALT, triglycerides, alkaline phosphatase, and phosphorus) and body condition, suggesting potential negative impacts on organismal health.

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.

Although shark meat is consumed worldwide, elevated arsenic (As) concentrations have been increasingly reported. The Caribbean sharpnose shark (Rhizoprionodon porosus) is a widely consumed fishing resource in Brazil, with scarce information on As burdens to date. Herein, commercial-sized juvenile Caribbean sharpnose sharks from Rio de Janeiro (Brazil) were assessed in this regard, presenting significantly higher hepatic As concentrations in males (8.24 ± 1.20 mg kg⁻¹ wet weight; n = 12) compared to females (6.59 ± 1.87 mg kg⁻¹ w.w.; n = 8), and a positive correlation (r = 0.74) was noted between female muscle and liver As concentrations, indicating systemic inter-tissue transport not evidenced in males. Arsenic concentrations were over the maximum permissible As limit established in Brazilian legislation for seafood and calculated Target Hazard Quotients suggest consumption risks, although cancer risks were not evidenced. Therefore, Public Health concerns with regard to Caribbean sharpnose shark As burdens deserve careful attention.

A study on mercury (Hg) and selenium (Se) concentrations in the liver and muscle of brown smooth-hound shark Mustelus henlei and its principal prey items, was conducted in the western coast of Baja California Sur, Mexico. Average Hg concentrations were found to be high in the muscle than in the liver; however, Hg concentrations were below the maximum permissible limits, and hence, the consumption of this species does not constitute a risk to human health. The mean Se concentrations were higher in the liver than in the muscle. The results of Hg: Se molar ratio revealed that Se counteracts the toxicity of Hg in hepatic tissues, whereas the contrary occurs in the muscle. Significant differences in Hg and Se accumulation were observed between females and males. Biomagnification factor values >1 demonstrate a biomagnification process from its principal prey species (i.e., red crab, Pleuroncodes planipes and Pacific mackerel, Scomber japonicus).

A thresher shark poached by fishermen in the stretch of sea between the island of Ponza and the coast of Terracina (Lazio, Central Italy, Mediterranean Sea) and confiscated by the Italian health authorities was evaluated for the presence of chemical contaminants. Hg mean concentration of 2.1 mg kg⁻¹ was found in muscle; speciation analysis confirmed it was almost in the toxic organic form of MeHg (2.0 mg kg⁻¹). The Se:Hg molar ratio was <1 not protective against the toxic effect of Hg. ¹³⁷Cs was detected at a mean concentration of 0.53 Bq kg⁻¹, and ⁴⁰K, radioisotope of natural origin was found at 91 Bq kg⁻¹. Dioxins and PCBs were found in the common thresher shark at mean concentrations of 0.012 and 0.088 pg TEQ-WHO g⁻¹ w.w. while NDL PCBs at 4.5 ng g⁻¹ w.w., lower than limits set by European Regulation. The major concern is constituted by the presence of Hg twice the maximum limit set by EU Regulation, then the consumption of thresher shark flesh should be strongly discouraged.

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.

The Gulf of California represents an important hotspot for whale shark (Rhincodon typus) aggregation. Anthropogenic activities and natural sources could expose sharks to high levels of trace elements (TEs). To determinate these levels in this endangered species, concentrations of As, Cd, Cu, Pb and Zn (in ng/g ww) were measured in 130 whale shark skin biopsies and 44 zooplankton samples collected from two areas of the Gulf of California, Bahía de Los Angeles (BLA) and Bahía de La Paz (LAP) during 2016–2018. For biopsies, Zn exhibited highest concentrations in BLA (2016–2017, 298 ± 406; 2017–2018, 1959 ± 2545) and at LAP (in 2016–2017, 595 ± 554; in 2017–2018, 2642 ± 1261). On the other hand, Cd (BLA 2016–2017, 3 ± 3; LAP 2016–2017, 4 ± 3; BLA 2017–2018, 17 ± 14; LAP 2017–2018, 13 ± 10) and Pb (BLA 2016–2017, 7 ± 7; LAP 2016–2017, 15 ± 32; BLA 2017–2018, 69 ± 76; LAP 2017–2018, 7 ± 5) showed lowest concentrations. Significant differences in TE concentrations between sites and periods occurred. Arsenic found in shark biopsies from La Paz suggested enrichment and/or increased bioavailability in this area. Sex alone was not a significant factor in TE concentration; nevertheless, a sex-dependent difference in correlation of TE concentration and size was noted (negative in males, positive in females). This indicates feeding strategies of whale shark may be sex and size segregated. During 2017–2018, zooplankton and sharks showed enrichment in all TEs. Essential elements were not biomagnified by sharks. Lead was biomagnified through zooplankton. Strong positive correlation between selected elements indicates that Zn, Cd and Pb follow the same metabolic route in the sharks’ body.