Sea Lions and Fur Seals in Decline
Two Marine Mammals Affected by Ocean Climate Shift Off Alaskan Coast
Northern fur seal and Steller sea lion populations are falling around the Aleutian and Pribilof Islands. Ecosystem changes in the Northern Pacific Ocean play a key role.
Steller sea lions (Eumetopias jubatus) are in serious decline in the Aleutian Islands and Gulf of Alaska. Northern fur seal (Callorhinus ursinus) pup production on the Pribilof Islands, where 70% of the breeding population is found, has been dropping since 1998. The reasons for these losses are not fully understood but a shift in North Pacific Ocean temperatures, which has resulted in changes in prey type and availability, is considered a significant underlying factor.
The Steller sea lion is listed as endangered according to the provisions of the US Endangered Species Act and the IUCN red list, with population declines of 80% since the mid-1970s. The northern fur seal is listed as depleted under the US Marine Mammal Protection Act and vulnerable on the IUCN red list. Their population has dropped by 33% in the 10 years between 1994 and 2004. Pup production has declined by 57% .
Reasons for Decline
In both cases, numerous possible causes for the declines have been put forth including increased predation by Orca, nutritional stress, increased contaminant levels from polluted food, entanglement in fishing gear and infectious disease. But underlying all this is a concern that a climate shift has resulted in increased sea temperatures, leading to alterations in the overall ecology of the area. This hurts the marine mammals in several ways.
Effects of Ocean Climate Shift
- When fish move far from the rookeries, nursing females have to travel longer distances to feed before returning to their pups.
- As commercially valuable species such as pollock move into the areas where the seals normally forage, fishing fleets may interfere with animals seeking food.
- When only less nutritionally dense fish are available for these marine mammals to feed on, the recently weaned young may be unable to consume enough food for growth and development.
- Nutritionally deficient females may not be able to reproduce.
- Animals spending more time at sea in search of food are at higher risk of predation.
Interestingly, there have been other periods of North Pacific Ocean warming that resulted in declines in Steller sea lion populations. It is harder to determine the effects those periods had on the northern fur seal, as the fur trade was heavily exploiting them at that time. Whether the climate shift that began in the 1970s is a result of normal weather progression or part of human induced global warming, it will be important to mitigate secondary problems such as those caused by contaminant exposure and overfishing, if Steller sea lions and Northern fur seals are to survive.
