Tracking a Silent Killer: Is Disease Responsible for the Steller Sea Lion Decline?

The precipitous decline of Alaska’s Steller sea lion populations has sparked a concerted research effort to understand its causes. While many potential avenues are being explored – including malnutrition, predation, climate changes, pollution, and human interference – a recent study investigated the prevalence of disease as a possible factor in the Steller sea lion decline.

Dr. Kathy A. Burek, of Alaska Veterinary Pathology Services, led a team of researchers from Canada and the U.S. to examine the patterns of disease in wild Steller sea lion populations.

They note that "the spatial and temporal patterns of the rapid initial decline are consistent with a disease outbreak, but no sea lion carcasses were noted or recovered. ” The authors also write that “failure to find carcasses may have been due to the remoteness of the breeding (rookery) and resting (haul-out) sites and the enormous expanse of ocean occupied by Steller sea lions, or to the removal of sick animals by predators and carcasses by sinking or scavengers.”

In order to conduct this research without any hard evidence of a disease outbreak, the researchers had to rely on more circumstantial evidence: the prevalence of antibodies in the blood of wild Steller sea lions. The study involved a review of antibody prevalence in both published and unpublished historical data, as well as through the analysis of blood samples collected between 1997 and 2000.

The researchers organized their collected data by geography, which allowed them to compare and contrast the declining western Alaskan population of Steller sea lions with increasing populations in Alaska’s southeast. But this approach presented considerable hurdles and prevented them from drawing firm conclusions about whether disease had become any more or less prevalent in the declining population. In some cases, historical data were not available in all decades and from all regions, or samples were not tested for the same antibodies in different decades. In other cases, inconsistent test techniques prevented the researchers from comparing more recent data.

Nevertheless, the study did provide some valuable insight. The data revealed that Steller sea lion populations had not been significantly exposed to morbilliviruses, Brucella spp., canine parvovirus or Leptospira interrogans. Furthermore, the researchers found that both increasing and decreasing sea lion populations had been exposed to a phocid herpesvirus, caliciviruses, canine adenovirus, and Chlamydophila psittaci. But the fact that antibodies to these disease agents were found at comparable levels in both increasing and declining populations indicates that they were probably not a factor in the decline of the western Alaskan population.

If anything, this study served to generate more questions about how disease has impacted Steller sea lion populations. Could diseases have caused undetected mortality? Could they have impacted the ability of mature adults to reproduce? Could disease agents have subtly affected the body condition of animals that were already under other stresses? Answering these questions – and more – will require further monitoring for disease agents in order to determine their role in the continued decline of Steller sea lions.

Publication:
Infectious disease and the decline of Steller sea lions (Eumetopias jubatus) in Alaska: insights from serology data. Burek, K.A., F.M.D. Gulland, G. Sheffield, K.B. Beckman, E. Keyes, T.R. Spraker, A.W. Smith, D.E. Skilling, J.E. Evermann, J.L. Stott, J.T. Saliki and A.W. Trites. 2005. Journal of Wildlife Diseases 41(3):512-524. abstract Serology data were examined to determine whether infectious disease may have played a role in the decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and Aleutian Islands. Available published data, historical unpublished data, and recent collections (1997-2000) were compared and reviewed. Data was stratified by geography in order to compare the declining western Alaska population in the Aleutian Islands regions through eastern Prince William Sound to the increasing population in Southeast Alaska.  Prevalences of antibodies from the 1970s to early 1990s were noted for Leptospira interrogans, Chlamydophila psittaci, Brucella spp., phocid herpesvirus 1, and canine parvovirus.  Serum samples collected and analyzed from 1997?2000 were tested for antibodies to these agents as well as to caliciviruses, marine mammal morbilliviruses, and canine adenoviruses 1 and 2.  Conclusions could not be drawn about changes in the prevalence of exposure to disease agents during the decline of Steller sea lions because data were not comparable either because of inconsistencies in test techniques, or because the samples were either not collected in all decades from all regions or were not tested for antibodies to the same disease agents in different decades.  Despite these shortcomings, the available data contained no convincing evidence of significant exposure of Steller sea lions to morbilliviruses, B. spp., canine parvovirus or L. interrogans.  Steller sea lions have been exposed to a phocid herpesvirus, caliciviruses, canine adenovirus, and C. psittaci or to cross reactive organisms in regions of both increasing and decreasing sea lion abundance.  These disease agents are not likely to have been the primary cause of the decline because they are found at comparable levels in both the increasing and the decreasing populations.  However they may have contributed to the decline or impeded recovery of the Steller sea lion population due to undetected mortality and morbidity, or reduction of fecundity and body condition in animals under other stresses.  Systematic monitoring for disease agents and their effects is needed to determine whether infectious disease is currently playing a role in the decline and lack of recovery of Steller sea lions.

30 November 2005