Body Size, Maturation and Feeding Habitat Drive Mercury Concentrations in Thresher Sharks from the Eastern Tropical Pacific

Abstract

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 thresher (Alopias pelagicus) and bigeye thresher shark (A. superciliosus) from eastern tropical Pacific, and methylmercury of muscle were also determined for human consumption risks. For both species, muscle THg concentrations (4.05 ± 2.15 and 4.12 ± 1.84 µg g-1 for pelagic and bigeye thresher shark, dry weight) were higher than that in other tissues. THg concentrations for all tissues were significantly correlated with precaudal length, with accumulation rates higher after maturity in pelagic than bigeye thresher sharks, indicating the diet shift effects on Hg burden. Correlations among tissues in both species suggested similar transportation and distribution patterns of internaltissues. 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 bioaccumulations. Diet shifts towards higher Hg content prey items increased Hg accumulation rates in adult pelagic thresher sharks. Concentrations of MeHg (3.42 ± 1.68 and 3.78 ± 2.13 µg g-1) in the muscle of both thresher shark species were above recommended levels for human consumption. These results highlighted that body size, diet shifts and habitat use contributed to the different patterns of Hg accumulations in both thresher sharks and assessed the nonlethal sampling methods in quantifying Hg contamination of the two thresher sharks.

Keywords: thresher shark, contaminants, multi-tissues, bioaccumulation, Stable Isotope, Health risk