Steller's sea lion  (Eumetopias jubatus)

The world population of Steller's sea lions has been undergoing a mysterious decline; since 1980, numbers have dropped from over 300,000 individuals to fewer than 100,000 (10). Despite this well-documented and worrying decline, the causes are still being debated; various hypotheses cite pollution, bycatch, parasites and disease, rookery disturbance and predation by killer whales (10). Research into dietary factors have revealed that Steller's sea lions in the northeast Pacific have suffered a decrease in the diversity and energy content of their diet since the mid 1970s, corresponding to changes in fish species available due to natural climatic changes (11). A diet dominated by low energy fish (such as pollock) can cause sea lions to lose condition, and can result in reduced pregnancy rates and increased susceptibility to disease or predation. This may be one of the major causes of the population decline (12).

Steller's sea lion was listed as Threatened on the United States Endangered Species Act in 1990 throughout its range; the western stock in Alaska was listed as Endangered in 1997 (6). The United States National Marine Fisheries Service (NOAA) has established a number of protection measures relating to fishing bans around major rookeries and feeding areas, in an attempt to slow the decline in population numbers (6). A consortium of North Pacific Universities is carrying out ongoing research into the causes of the perplexing population decline (10). The battle to understand the factors involved in the decline in Steller's sea lion numbers may also provide better understanding of the complex marine ecosystem and the effects of fish stock changes (by both natural and man-made causes) on other marine mammals and sea birds (10).