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Alaska Ecosystems Program

Winter Behavior Studies of Young Northern Fur Seals

see caption
Figure 1. Northern fur seal pup from St. Paul Island with satellite tag used for tracking attached to its back. The tags are attached with marine epoxy and fall off during the annual molt.

In fall 2006, the Alaska Ecosystems Program embarked on the second year of a study examining the winter migration patterns of northern fur seal pups from four breeding sites in Alaska and California. The Eastern Pacific stock of northern fur seals is continuing to decline and is listed as depleted under the Marine Mammal Protection Act (MMPA). However this decline is not ubiquitous among all Alaska rookeries. While the population on the Pribilof Islands has shown a downward trend, the small rookery at Bogoslof Island has increased at approximately 12% each year for almost 10 years.

In October-November 2006, 47 satellite tags and 20 satellite-dive tags were deployed on pups prior to weaning in order to assess interannual variation in winter migratory routes and foraging behavior (Fig. 1). There are three fur seal rookeries in Alaska: St. Paul and St. George Islands in the Pribilof Islands and Bogoslof Island in the Aleutian Islands chain. Twenty-five pups were equipped with satellite transmitters at St. Paul, 18 at St. George and 15 at Bogoslof Island. As part of collaborative work with NMML’s California Current Ecosystem Program, nine pups were also equipped at San Miguel Island, California. (See California Current Ecosystem Program report below.) Pups captured from Alaskan populations had average body masses of 14.7 kg (Bogoslof, Oct.), 17.3 kg (St. Paul, Nov.) and 17.0 kg (St. George, Nov). Pups at San Miguel Island tagged in November, however, exhibited considerably smaller body masses at an average 12.6 kg. The satellite tags have been programmed to transmit for up to 400 days to cover the return to breeding sites next fall.

In addition to the pup work, a new study was initiated to also investigate winter migration routes of juvenile age (1 and 2 year-old) northern fur seals from the Eastern Pacific Stock. Typically, fur seals do not return to the breeding rookeries in large numbers until they are sexually mature. Therefore, little is known about the behavior of the population segments that have survived their first year at sea but are not yet breeders. Thirty satellite transmitters were deployed between 30 September and 13 October 2006. Ten transmitters were deployed on Bogoslof Island (5 males and 5 females), and 20 transmitters were deployed on St. Paul Island (7 males and 13 females). Twenty-five of the thirty transmitters were still transmitting as of 10 January 2007. All of the animals are currently south of the Aleutian chain, and several are as far south as lat. 40˚N.

As with the pups, this information will help us understand the habitat preferences and influence of oceanographic and environmental variables on animal behavior.

By Mary-Anne Lea, Tonya Zeppelin and Tom Gelatt


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