Age- and sex-specific mortality and population structure in sea otters

We used 742 beach-cast carcasses to characterize age- and sex-specific sea otter mortality during the winter of 1990-1991 at Bering Island, Russia. We also examined 363 carcasses recovered after the 1989 grounding of the TN E xxon Valdez, to characterize age and sex composition in the living western Prince William Sound (WPWS) sea otter population. At Bering Island, mortali ty was male-biased (81%), and 75% were adults. The WPWS population was fema le-biased (59%) and most animals were subadult (79% of the males and 45% of the females). In the decade prior to 1990-1991 we found increasing sea ott er densities (particularly among males), declining prey resources, and decl ining weights in adult male sea otters at Bering Island. Our findings sugge st the increased mortality at Bering Island in 1990-1991 was a density-depe ndent population response. We propose male-maintained breeding territories and exclusion of juvenile females by adult females, providing a mechanism f or maintaining;densities in female areas below densities in male areas and for potentially moderating the effects of prey reductions on the female pop ulation. Increased adult male mortality at Bering Island in 1990-1991 likel y modified the sex and age class structure there toward that observed in Pr ince William Sound.