Mercury (Hg) exposure poses a threat to both fish and human health. Sharks are known to bioaccumulate Hg, however, little is known regarding how Hg is distributed between different tissue groups (e.g. muscle regions, organs). Here we evaluated total mercury (THg) concentrations from eight muscle regions, four fins (first dorsal, left and right pectorals, caudal-from both the inner core and trailing margin of each fin), and five internal organs (liver, kidney, spleen, heart, epigonal organ) from two different shark species, bonnethead (Sphyrna tiburo) and silky shark (Carcharhinus falciformis) to determine the relationships of THg concentrations between and within tissue groups. Total Hg concentrations were highest in the eight muscle regions with no significant differences in THg concentrations between the different muscle regions and muscle types (red and white). Results from tissue collected from any muscle region would be representative of all muscle sample locations. Total Hg concentrations were lowest in samples taken from the fin inner core of the first dorsal, pectoral, and caudal (lower lobe) fins. Mercury concentrations for samples taken from the trailing margin of the dorsal, pectoral, and caudal fins (upper and lower lobe) were also not significantly different from each other for both species. Significant relationships were found between THg concentrations in dorsal axial muscle tissue and the fin inner core, liver, kidney, spleen and heart for both species as well as the THg concentrations between the dorsal fin trailing margin and the heart for the silky shark and all other sampled tissue types for the bonnethead shark. Our results suggest that biopsy sampling of dorsal muscle can provide data that can effectively estimate THg concentrations in specific organs without using more invasive, or lethal methods.

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.

Our understanding of global seagrass ecosystems comes largely from regions characterized by human impacts with limited data from habitats defined as notionally pristine. Seagrass assessments also largely focus on shallow-water coastal habitats with comparatively few studies on offshore deep-water seagrasses. We satellite tracked green turtles (Chelonia mydas), which are known to forage on seagrasses, to a remote, pristine deep-water environment in the Western Indian Ocean, the Great Chagos Bank, which lies in the heart of one of the world's largest marine protected areas (MPAs). Subsequently we used in-situ SCUBA and baited video surveys to survey the day-time sites occupied by turtles and discovered extensive monospecific seagrass meadows of Thalassodendron ciliatum. At three sites that extended over 128 km, mean seagrass cover was 74% (mean range 67–88% across the 3 sites at depths to 29 m. The mean species richness of fish in seagrass meadows was 11 species per site (mean range 8–14 across the 3 sites). High fish abundance (e.g. Siganus sutor: mean MaxN.site−1 = 38.0, SD = 53.7, n = 5) and large predatory shark (Carcharhinus amblyrhynchos) (mean MaxN.site−1 = 1.5, SD = 0.4, n = 5) were recorded at all sites. Such observations of seagrass meadows with large top predators, are limited in the literature. Given that the Great Chagos Bank extends over approximately 12,500 km2 and many other large deep submerged banks exist across the world's oceans, our results suggest that deep-water seagrass may be far more abundant than previously suspected.

The impact of marine ecotourism on reef predators is poorly understood and there is growing concern that overcrowding in Marine Protected Areas (MPAs) may disturb the species that these areas were established to protect. To improve our understanding of this issue, we used acoustic telemetry to examine the relationship between human activity at the Molokini Marine Life Conservation District (MLCD) and the habitat use of five reef-associated predators (Caranx melampygus, Caranx ignobilis, Triaenodon obesus, Carcharhinus amblyrhynchos, and Aprion virscens). During peak hours of human use, there was a negative relationship (R2=0.77, P<0.001) between the presence of bluefin trevally (Caranx melampygus) and vessels in subzone A. No other species showed strong evidence of this relationship. However, our results suggest that during this time, the natural ecosystem function that the reserve was established to protect may be compromised and overcrowding should be considered when managing MPAs.

Shark fisheries have expanded due to increased demand for shark products. As long-lived apex predators, sharks are susceptible to bioaccumulation of metals and metalloids, and biomagnification of some such as Hg, primarily through diet. This may have negative health implications for human consumers. Concentrations of Hg, As, Cd, Cu, Fe, Se and Zn were analysed in muscle, liver and fin fibres (ceratotrichia) from dusky Carcharhinus obscurus, sandbar Carcharhinus plumbeus, and white Carcharodon carcharias sharks from south-eastern Australian waters. Concentrations of analytes were generally higher in liver than in muscle and lowest in fin fibres. Muscle tissue concentrations of Hg were significantly correlated with total length, and >50% of sampled individuals had concentrations above Food Standards Australia New Zealand’s maximum limit (1mgkg−1ww). Arsenic concentrations were also of concern, particularly in fins. Results warrant further investigation to accurately assess health risks for regular consumption of shark products.

Polychlorinated biphenyls (PCBs) are ubiquitous pollutants in the marine environment that are known to accumulate in apex predators such as sharks. Liver samples from dusky Carcharhinus obscurus, sandbar Carcharhinus plumbeus, and white Carcharodon carcharias sharks from south-eastern Australian waters were analysed for the seven indicator PCBs 28, 52, 101, 118, 138, 153 and 180. Median ∑PCBs were significantly higher in white than sandbar sharks (3.35 and 0.36μgg−1 lipid, respectively, p=0.05) but there were no significant differences between dusky sharks (1.31μgg−1 lipid) and the other two species. Congener concentrations were also significantly higher in white sharks. Significant differences in PCB concentrations between mature and immature dusky (3.78 and 0.76μgg−1 lipid, respectively) and sandbar (1.94 and 0.18μgg−1 lipid, respectively) sharks indicated that PCB concentrations in these species increased with age/growth. Higher-chlorinated congeners (hexa and heptachlorobiphenyls) dominated results, accounting for ~90% of ∑PCBs.

This study assessed the concentrations of nine trace metals from juvenile C. falciformis caught from Indian Ocean. This study also discussed the metal pollution index (MPI) and bioconcentration factor (BCF) of each element, and their correlations. Further, the potential health risks of consuming shark muscles (THI) were evaluated. Results showed the mean concentrations of 9 elements as follows: Cu (0.36 ± 0.17), Zn (5.19 ± 16.6), Pb (0.12 ± 0.23), Cd (0.17 ± 0.21), Cr (0.57 ± 1.61), Ni (0.086 ± 0.51), As (1.36 ± 0.83), Co (0.000073 ± 0.0074), and V (0.0024 ± 0.0094) mg/kg ww. The BCF values of the elements were higher than 1, with Co and V being the lowest indicating their bioaccumulative behavior. Correlation analysis showed MPI to be highly correlated with Cu, indicating its greater contributions to the total pollution load. Principal components analysis explained 81.0% of the variability in biometric characteristics and metal concentrations. Health risk assessment for consuming shark muscle in Taiwanese male and female adults suggests potential chronic non-carcinogenic health hazards.

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.

Cadmium concentrations were determined in the tissues of muscle and liver of Carcharhinus falciformis (silky shark) sampled in Todos Santos, Baja California South, Mexico. This is one of the main shark species for human consumption in Mexico. Results indicate that accumulation of Cd varied in both sexes, based on its metabolism, sex, maturity and other biological characteristics. High Cd values were observed in the liver of adults of male (529.61μgg−1) and female (457.43μgg−1), whereas, in muscular tissues it was low (0.37μgg−1) than the prescribed permissible limits for seafood (0.5μgg−1). Substantial correlations were observed between body length and Cd values in adults except young male due to faster growth rate and its metabolism. The study indicated the impact of environmental conditions in the accumulation of Cd and its risk to the food web structure in the marine environment and health hazard for humans.

Given their predatory characteristics, long life, and high trophic levels, sharks can have a remarkable ability to bioaccumulate and/or biomagnify trace elements (TE). In the present study, 13 TE (Hg, Pb, Cd, Cu, As, Se, Zn, Cr, V, Fe, Ni, Co, and Mn) were analyzed in the muscle tissue of three shark species (Prionace glauca, Carcharhinus falciformis, and Alopias pelagicus) using high-resolution mass spectrometry inductively coupled to a plasma generator (HR-ICP-MS); 30 shark specimens of each specie were obtained during commercial fish landings in the port of Manzanillo, Mexico. The morphometric characteristics and sex of the sharks were recorded. Shark A. pelagicus accumulated higher concentrations of TE, especially for Hg and Cd, than the other species. Significant differences in the TE levels were detected between sexes in P. glauca and A. pelagicus; in all cases, the highest concentrations were found in female muscle tissue. The ability of sharks to bioaccumulate ETs depends of the metals (essentials higher than non-essential) but is explained mainly by feeding habits associated to sexes (population segregation) and size (ontogeny). The association between TE in shark specimens was related to the essentiality, their antagonistic action, and origin. The Se/Hg ratio was significantly higher than 1, evidence of the protective role of dietary Se against Hg uptake and toxicity in all specimens of the three shark species. Also, the elevate inverse correlation of As and Cd versus Se could indicate a protective action of Se against these toxic metals, but the mechanism must be investigated.