Effect of Body Size, Feeding Ecology and Maternal Transfer on Mercury Accumulation of Vulnerable Silky Shark Carcharhinus Falciformis in the Eastern Tropical Pacific

Abstract

The silky shark Carcharhinus falciformis is a large pelagic shark species distributed in the global oceans and recently been listed as “Vulnerable” by the IUCN because of its decline in population due to overfishing. In addition, as an apex predator, the silky shark accumulates elevated quantities of mercury (Hg), posing risks to its remaining population. 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 the δ 15 N values of these tissue types were measured to conduct comparisons of Hg concentrations with its feeding ecology. The THg concentrations mainly accumulated in muscle and liver rather than dermis, RBC and plasma. The maternal THg transfer was observed in silky sharks and equaled 33.16% and 1.98% in muscle and liver comparing with their respective mothers, respectively, showing the early-stage embryos have been exposed to the toxic mercury. The potentially harmful THg concentrations in silky shark tissues may lead to healthy problems of sharks and consumers. The THg concentrations in all tissue types were significantly correlated with fork length and showed faster accumulation rates after maturity and more prominent in the liver. THg concentrations were negatively correlated with δ 15 N values for all tissues, indicating the δ 15 N values may be biased from the isotopic baselines to the feeding relationships. Positive correlations were observed among THg concentrations of all tissue types, suggesting potential applications of nonlethal tissues such as muscle and dermis for predicting mercury concentration of other internal tissues.

Keywords: silky shark, mercury, δ15N, ecotoxicology, nonlethal sampling