Disease

The pattern of decline of Steller sea lions that spread from the eastern Aleutians is consistent with the introduction of a disease agent into the population. Infectious diseases are known to degrade general health, reproductive capacity and survival of wildlife, and may have played a role in the decline and lack of recovery of Steller sea lions in Alaska.

Blood samples (serum) have been collected from Steller sea lions in Alaska since the 1970s. Some of this serum was analyzed close to the time of collection, and showed that Steller sea lions were potentially exposed to a variety of disease agents. Other serum samples were archived for extended periods of time. Dr. Kathy Burek and colleagues recently analyzed some of the archived samples and compiled all known disease records for Steller sea lions collected from 1975-2000.

Tests were done for the presence of 10 diseases. In most cases, conclusions could not be drawn about chronological changes in the prevalence of disease during the population decline because the samples were not collected from all regions in each time period; nor were they collected from sufficient numbers of animals in each age class. In addition, samples were not all analyzed by the same laboratories, were not stored under controlled conditions, were not tested for the same disease agents, and assays were not validated for Steller sea lions. What diseases were noted, were found in both increasing and decreasing sea lion populations.

It seems unlikely that any of the 10 diseases examined caused the population decline of sea lions by epidemic mortality. However, some pathogens are likely endemic and may be interacting with malnutrition or predation to decrease survival or reproduction—therefore preventing recovery of depleted populations. It seems likely that disease has been a contributing factor in the mysterious disappearance of Steller sea lions.

A systematic disease agent monitoring protocol should be initiated for Steller sea lions to adequately test for disease agents in different time periods and regions. Further studies should also be aimed at detecting infectious agents directly, and at determining their association with morbidity and mortality.

For further information see:

Disease agents in Steller sea lions in Alaska: A review and analysis of serology data from 1975-2000. Burek, K.A., F.M.D. Gulland, G. Sheffield, E. Keyes, T.R. Spraker, A.W. Smith, D.E. Skilling, J. Evermann, J.L. Stott and A.W. Trites. 2003. Fisheries Centre Research Reports, Vol. 11, No. 4, pp. 26.   3 September 2003

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