Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean

Abstract

The non-essential metal mercury (Hg) can have deleterious effects on health of organisms, and tends to bioaccumulate with age in long-lived organisms and to biomagnify along food chains. Because sharks are fished for human consumption and their Hg levels are frequently above the maximum Hg concentration recommended for fish consumption, understanding the drivers of Hg concentration is of considerable interest. Total Hg concentrations were analysed in muscle tissues of 14 shark and 2 batoid species (n = 339 individuals) sampled across multiple habitats (coastal, oceanic and bathyal) in the southwestern Indian Ocean. Stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) were analysed to assess whether relative trophic position and foraging habitats affected Hg concentrations. Hg concentrations increased with δ 15 N and body length, highlighting the mechanisms of bioaccumulation and biomagnification in relation with the trophic position and size of the individuals. Habitats where elasmobranchs were collected also affected their Hg concentrations. Bathyal sharks had high Hg concentrations that were almost similar to those of oceanic species, despite their lower relative trophic position. Higher bioavailability of Hg due to its enhanced methylation in deeper waters was considered as the most likely explanation for this result. These results highlight that multiple factors contribute to mercury accumulation in elasmobranchs.

Mercury levels in Southern Ocean squid: Variability over the last decade

“…abyssicola, which is a deep-sea squid and occurs at a depth of 1000-1500 m (Roper and Young, 1975). Higher mercury levels were expected in species from deep habitats as increasing depth raises mercury concentrations in fish (Choy et al, 2009;Le Bourg et al, 2019;Monteiro et al, 1996). However, that was not the case in our study, as mercury concentrations in the tissues of the deep-sea squid B. abyssicola were within the same range as the majority of other analysed species (see Figure 4).…”

Main drivers of mercury levels in Southern Ocean lantern fish Myctophidae

“…Habitat use has a major effect on Hg accumulation, since longer exposure in more contaminated areas will result in higher concentrations of this element in tissues (Desta et al, 2008;Le Bourg et al, 2019). In this study, little evidence was found of regional variation in T-Hg concentrations from the 3 species that were caught concurrently at South Georgia and the South Orkneys (E. antarctica, G. braueri and G. nicholsi), with regional differences only apparent for E. antarctica.…”

“…Physiological and biological factors, such as sex and size, are known to influence Hg concentration in fish (Bastos et al, 2016;Dang and Wang, 2012;Gewurtz et al, 2011;Le Bourg et al, 2019). When assessing the effect of body size in T-Hg concentration in the muscle of Southern Ocean myctophids, we observed a general positive trend of increasing T-Hg with increasing size, except for E. antarctica females, for which this relationship was negative.…”

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

“…levels of Hg due to factors other than size and predatory habits (e.g. proximity to active volcanism, higher rates of Hg methylation and bio availability of organic monomethylmercury [MeHg] in the deep sea), pelagic sharks generally possess the highest Hg levels observed in marine fishes (Le Bourg et al 2019). For some species, relative concentrations can average as high as 3 mg kg −1 wet weight (WW) (e.g.…”

“…This is particularly true for pelagic or open ocean shark species, which have been shown to demonstrate muscle Hg concentrations that can be 2-to 3-fold higher than those ob served in large coastal sharks of similar size and trophic position (e.g. bull sharks Carcharhinus leucas, Le Bourg et al 2019). With the exception of some bathyal species, which may accumulate greater-than-anticipated ABSTRACT: 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.…”