Although polychlorinated biphenyls (PCBs) and dichlorodiphenyltrichloroethane (DDT) were banned throughout much of the developed world in the 1970s, these and other organochlorine contaminants persist in the atmosphere and in the oceans. These chemicals, which were synthesized for use as pesticides and lubricants, accumulate in the lipids of animals, and biomagnify as they move up the food chain to apex predators. In addition, most organochlorines (OC) persist in the environment, and are transported with time to northern latitudes, where they precipitate out in cold air sinks. OC exposure has several well-documented effects on various animal species, and in seals and sea lions, these contaminants can suppress the immune system, act as endocrine disruptors, cause cancer, and interfere with reproduction and normal gestation. Two recent papers document OC levels in two pinniped species affected by population declines, the Steller Sea Lion and the Caspian Seal.
Caspian Seals (Pusa caspica) experienced an unusually high mortality rate beginning in spring 2000, the cause of which was determined to be infection with canine distemper virus. Since OC burden is known to increase susceptibility to viral diseases in mammals, Kajiwara and colleagues (2008 ) measured levels of these compounds in the blubber of Caspian seals that had been affected by the distemper virus, and stranded along the coasts of Iran, Azerbaijan, Dagestan Republic, Kazahkstan, and Turkmenistan between May 2000 and June 2001. In addition, OC levels in six prey species of fish for the seals were measured; compounds analyzed included PCBs, DDT, polychlorinated dibenzo-p-dioxins (PCDDs), and furans (PCDFs).
Steller Sea Lion, Aleutian Islands
Photo: Captain Harry Reed, NOAA Photo Library
The researchers found OC contaminants in the blubber of all 36 male and 17 female Caspian Seals stranded during the study period, with DDTs and PCBs as the predominant classes of OC. DDT levels were high enough to indicate heavy use, possibly illegal, and residues of this pesticide around the Caspian Sea. Although the six fish species examined showed regional differences in PCB/DDT contamination, the seals appeared to be ubiquitously exposed to OCs during their seasonal migration in the Caspian Sea. Kajiwara and colleagues found that high OC contamination levels were correlated with decreased blubber thickness, indicating that these contaminants are concentrated further as animals mobilize their lipid stores. They suggest that the period from spring to summer represents a “high risk season” for Caspian seals, during which the higher concentrations of OC contaminants in their blubber have immunosuppressive effects, and result in increased susceptibility to viral infections.
With a range from the California Channel Islands and along the North Pacific Rim to Japan, the Steller Sea Lion (Eumetopias jubatus) is a more widespread pinniped species than is the Caspian Seal, but no less susceptible to the detrimental effects of OC contaminants. Since 1976, Steller Sea Lion populations have decreased by 70-80%, with the most prominent declines in the eastern Aleutian/ western Gulf of Alaska portions of the range. To determine whether OC levels were correlated with population declines in western Alaska and the Russian Far East, Myers and colleagues (2008 ) measured PCBs and DDTs in the whole blood of Steller Sea Lion pups. Seventy six pups from western Alaskan rookeries, and 136 pups from the Russian Far East, were included in the study.
Steller Sea Lions, Alaska
Photo: Captain Bud Christman, NOAA Photo Library
Pups from the Russian rookeries exhibited higher OC contaminant levels in whole blood, than did conspecifics from Alaskan rookeries. The researchers suggest that these sea lions may be exposed to more point source pollution, or that atmospheric and oceanic transport patterns of OC contaminants may lead to higher exposures. Although OC levels appear to be higher in Steller Sea Lions than in other North Pacific pinniped species, direct comparisons are subject to confounding factors, and may be of limited use. PCB levels were higher than DDT levels in sea lions from both Russian and Alaskan rookeries, and circulating OC levels were higher in female, compared to male, pups, perhaps because of body mass differences. A significant number of the Russian Steller Sea Lion pups had blood OC levels sufficient to produce immunosuppressive effects in another pinniped species, the Harbor Seal (Phoca vitulina). Myers and colleagues concluded that OC contaminants in the Steller Sea Lion pups cannot be dismissed as a possible cause of population declines in the western Alaska and Russian Far East regions.
Kajiwara, N., Watanabe, M., Wilson, S., et al. (2008). Persistent organic pollutants (POPs) in caspian seals of unusual mortality event during 2000 and 2001. Environmental Pollution 152, 431-442.
MYERS, M., YLITALO, G., KRAHN, M., BOYD, D., CALKINS, D., BURKANOV, V., ATKINSON, S. (2008). Organochlorine contaminants in endangered Steller sea lion pups (Eumetopias jubatus) from western Alaska and the Russian Far East. Science of The Total Environment DOI: 10.1016/j.scitotenv.2008.02.008