Abstract

Harbor seals (Phoca vitulina) may be especially susceptible to bioaccumulation and biomagnification of toxicants because they are long-lived, upper trophic level consumers that often are in proximity to urbanized and/or industrialized regions. In California, harbor seals are present year-round in San Francisco Bay (SFB), an urbanized estuary that provides critical habitat for resting ashore, pupping, social interaction, and feeding. The number of harbor seals in SFB has not increased in the last 30+ years, which has prompted concern about how environmental contamination may affect the health of harbor seals in SFB.^1,2 It has been suggested that a proportion of harbor seals in SFB may suffer from chronic selenium (Se) toxicosis, and the primary objective of this study was to evaluate that hypothesis.² To accomplish this, we measured Se and total mercury (THg) concentrations in hair (n = 138) and blood (n = 73) of free-ranging harbor seals from three locations in central California (Elkhorn Slough, SFB, Tomales Bay). We determined how concentrations of Se, THg, and the Se:Hg molar ratio differed with location, and also the relationship between Se and THg concentrations in hair and blood, with the expectation that harbor seals from SFB would have greater Se concentrations and Se:Hg molar ratios than seals from the other two locations. Fine-scale locational differences in THg, Se, and Se:Hg were detected for both tissue types, with significantly greater THg and lesser Se concentrations, and lesser molar ratios of Se:Hg in seals from SFB and Tomales Bay than Elkhorn Slough. There was a weak, yet significant negative correlation between Se and THg concentrations in blood (r = -0.32), which appeared to be driven by seals from SFB. Results from this study do not support the chronic Se toxicosis hypothesis, and instead indicate that harbor seals from SFB have relatively low Se concentrations. In contrast, THg concentrations in seals from SFB and Tomales Bay were relatively high compared with seals from Elkhorn Slough and reported values for other pinnipeds worldwide, which was likely the result of historic gold and mercury mining in these two regions. Increases in THg did not result in corresponding increases in Se, which may indicate that the ability of Se to provide a protective effect against Hg toxicosis is limited. The decreased Se concentrations in harbor seals from Hg-contaminated regions may indicate an increased need for Se, or other nutrients, to mitigate the toxic effects of Hg, and further investigation into the Se status of seals from these regions is warranted. Results from this study indicate that Hg remains a potential health threat to harbor seals, and warrant future investigation into the role that Hg may play in the health and populations dynamics of harbor seals from central California.

Acknowledgements

This study would not have been possible without support from the National Parks Service (especially Sarah Allen and Ben Becker), US Fish and Wildlife Service, Don Edwards National Wildlife Refuge, Moss Landing Marine Laboratories, the Wildlife Toxicology Laboratory, USGS, and all the volunteers that assisted in harbor seal captures. Funding for this project was provided by the PADI Foundation, Packard Foundation, Harvey Fellowship, and the Early and Ethel Myers Oceanographic and Marine Biology Trust.

* Presenting author
+ Student presenter

Literature Cited

1.  Grigg EK, Allen SG, Green DE, Markowitz H. 2004. Harbor seal, Phoca vitulina richardii, population trends in the San Francisco Bay Estuary, 1970–2002. Calif Fish Game. 90:51–70.

2.  Kopec DA, Harvey JT. 1995. Toxic pollutants, health indices, and population dynamics of harbor seals in San Francisco Bay, 1989–1992. Moss Landing Marine Laboratories Technical Publication 96–4.