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

J Gelsleichter,La Howey,On Shipley,G Sparkman,Ej Brooks

doi:10.3354/esr01068

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. In addition to risks posed to its populations due to overexploitation, oceanic whitetip sharks are also capable of accumulating elevated quantities of harmful environmental toxicants, placing them at special risk from anthropogenic pollution. Herein, we provide the first data on accumulation of the toxic, non-essential metal mercury (Hg) in northwest Atlantic (NWA) oceanic whitetip sharks, focusing on aggregations occurring at Cat Island, The Bahamas. Total Hg (THg) concentrations were measured in muscle of 26 oceanic whitetip sharks and compared with animal length and muscle δ15N to evaluate potential drivers of Hg accumulation. THg concentrations were also measured in fin and blood subcomponents (red blood cells and plasma) to determine their value as surrogates for assessing Hg burden. Muscle THg concentrations were among the highest ever reported for a shark species and correlated significantly with animal length, but not muscle δ15N. Fin, red blood cell, and plasma THg concentrations were significantly correlated with muscle THg. Fin THg content was best suited for use as a surrogate for estimating internal Hg burden because of its strong relationship with muscle THg levels, whereas blood THg levels may be better suited for characterizing recent Hg exposure. We conclude that Hg poses health risks to NWA oceanic whitetip sharks and human consumers of this species.

Highlights

Large predatory sharks often accumulate higher quantities of the highly toxic, non-essential metal mercury (Hg) than most marine fishes due to their large size, long lifespan, and high trophic position (Zillioux 2015, Bosch et al 2016)
This hypothesis has been strengthened by observations of total Hg (THg) concentrations in muscle of oceanic whitetip sharks from the southwestern Indian Ocean, which averaged 7.14 ± 7.44 mg kg−1 dry weight (DW) (~2 mg kg−1 wet weight (WW) based on 70% moisture, Bergés-Tiznado et al 2015), exceeding levels in all other large pelagic shark species collected from the same location including the shortfin mako (5.96 ± 2.78 mg kg−1 DW, Kiszka et al 2015)
This is well illustrated by comparing muscle THg concentrations in Cat Island oceanic whitetip sharks (5.04 ± 2.52 mg kg−1 WW) with concentrations previously reported in other large pelagic sharks from northwestern Atlantic (NWA) waters, including shortfin makos, blue sharks Prionace glauca, common thresher sharks Alopias vulpinus, and porbeagle sharks Lamna nasus (Table 2)

Summary

Introduction

Large predatory sharks often accumulate higher quantities of the highly toxic, non-essential metal mercury (Hg) than most marine fishes due to their large size, long lifespan, and high trophic position (Zillioux 2015, Bosch et al 2016). It has been hypothesized that the oceanic whitetip shark Carcharhinus longimanus, one of the most critically threatened yet poorly studied pelagic sharks (Howey-Jordan et al 2013, Tolotti et al 2015), may be uniquely prone to accumulating elevated levels of Hg, putting it at special health risks from the potential effects of this toxicant This was initially suggested based on a single measurement of the highly toxic MeHg in oceanic whitetip fin tissue, which at 0.530 mg kg−1 dry weight (DW) was the fourth highest concentration observed out of 13 large shark species, including apex predator species such as shortfin mako sharks and white sharks Carcharodon carcharias (Nalluri et al 2014). It is important to determine whether Hg accumulation poses risks to oceanic whitetip shark populations in NWA waters, especially since significant population declines in this region, due primarily to high incidental bycatch in commercial fisheries, have resulted in the population being listed as Critically Endangered (Rigby et al 2019, Young & Carlson 2020)

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