Abstract

Mercury accumulation in aquatic mammals can pose significant health threats because mercury disrupts physiological pathways and can be transferred to offspring, predisposing future generations to exposure.^1,2,5,6 Mercury moves globally and is detectable in marine ecosystems far from anthropogenic sources;³ therefore, contaminant impacts on organismal physiology are a concern not only in coastal regions but also in difficult-to-study open-ocean ecosystems. Elevated mercury concentrations were found in large fish at mesopelagic depths (200–1000 m) near Hawaii,⁴ suggesting that other mesopelagic predators may also have increased risk of mercury accumulation; however, relatively little is known about mercury in the mesopelagic. Northern elephant seals (Mirounga angustirostris) are mesopelagic marine predators that travel thousands of kilometers during their biannual migrations to forage in coastal and open-ocean regions of the North Pacific. As upper-trophic level predators, elephant seals bioaccumulate mercury and may be good sentinels of mesopelagic contamination. Additionally, to appropriately conduct contaminant monitoring and interpret tissue concentrations it is imperative to correctly understand relationships between tissues in order to design the most appropriate sampling methods around the physiology and ecology of the study species. Body tissues have varying turnover and equilibration rates, and it is possible the extensive fasting periods in elephant seals could decouple correlations in total mercury concentrations between tissues with different turnover rates. Elephant seals return to their colonies twice each year and fast entirely while they undergo breeding and molting, which makes them an excellent model system to examine how significant changes in body condition affect mercury concentrations. Known-age adult females from the Año Nuevo colony in California, USA were sampled for blood, muscle, and fur before and after foraging trips during 2010–2012 (N = 89 seals). Each female was equipped with a satellite tag and time-depth recorder to document at-sea movement and diving behavior. Adult males were also sampled for blood, muscle, and fur during 2010–2012 (N = 59) but were not satellite tracked. We examined whether correlations between tissues remained consistent across varying body conditions, and if females and males differed in their between-tissue correlations and tissue concentrations across varying body conditions. We used mixed effects models to test for significant correlations between tissues, as well as for any potential effect of the time of year when the sample was taken and the sex of the animal, with individual seal included as a random effect. Mercury concentrations in female muscle samples upon arrival to the breeding colony ranged from 2.59–8.80 µg/g dry weight (mean 5.39 ± 1.46) and male muscle samples ranged from 3.00–13.77 µg/g dry weight (mean 6.40 ± 3.14), which are among the highest muscle mercury concentrations observed in marine mammals and seabirds. Fur concentrations in some seals exceeded proposed toxicological thresholds for mercury in fur.² We found that male and female blood THg concentrations were significantly correlated with muscle THg at all time periods. However, our results suggest that ecological factors can alter mercury correlations among tissue types, and therefore the interpretation of mercury concentrations in marine mammals is highly complex.

Acknowledgements

The authors wish to thank M. Peterson, C. Goetsch, P. Robinson, P. Morris, the UCSC Año Nuevo field-crew, the Año Nuevo rangers, and R. Keister for field and laboratory assistance. The authors wish to thank the Office of Naval Research, Clairol, the Friends of Long Marine Lab, The Earl and Ethel Myers Oceanographic and Marine Biology Trust, and the UCSC Ecology and Evolutionary Biology Department for funding.

* Presenting author
+ Student presenter

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