Studying northern fur seals in the field
The decline in northern fur seals on St. Paul Island contrasts sharply with the increasing population on Bogoslof island in the Aleutian chain. One likely possibility for this is that productivity differs around the two islands. Mothers at St. Paul Island may make longer trips than mothers at Bogoslof Island to get enough energy to feed their pups so that they grow to a large enough size to survive the difficult first year on their own when they are learning to dive and forage for themselves. Foraging trips for St. Paul females appear to be twice as long as those from Bogoslof. Climate change may affect the distribution of fur seal prey and further compound the problems of finding enough to eat. We are exploring this problem in the field using biochemistry, oceanography and behavioral analysis.
Daily Diary tags are micro-computers that record acceleration and the earth’s magnetic field as well as temperature, depth and light. When carried by a fur seal, the magnetometers allow us to reconstruct the 3 dimensional dive profile to tell us exactly where the seals are going, while the accelerometers provide fine scale data on the movement of the animals, such as how many flipper strokes it takes for them to dive. We attached Daily Diary tags to nearly 50 animals on each island to compare the fine scale foraging habits of the seals from the two islands to help determine why the St. Paul Island animals must take longer foraging trips to “refuel” and how much more energy it takes.
An integral component of our field study is to describe the oceanographic conditions where fur seals are foraging. Fur seal prey species are sensitive to changes in water temperature so we expect temperature to impact the location and timing of seal foraging trips. We hope to identify any differences between the areas used by seals from St. Paul and Bogoslof and to see how these conditions change throughout the summer. To accomplish this task, we attached a fine-scale gps device with a sensitive thermistor to the same seals tagged with the Daily Diary tags. The seals became oceanographers as they collected ocean temperature data during their foraging trips. We have two goals for the data: 1) compare ocean temperature profiles collected by seals with traditional data measurements obtained by ships around each island; and 2) to identify temperature-specific regions, eddies, filaments, or thermoclines that might influence the at-sea distribution of foraging seals.
For northern fur seals and their pups, not getting enough food can have some unfortunate consequences. For instance, when a fur seal is unable to meet its energy requirements, it can either exert more energy and continue foraging, or rest in order to conserve energy. The latter situation leads to a reduction of the secretion of thyroid hormones (triiodothyronine or T3) that help regulate metabolism. A lack of food can also produce a physiological stress response, activating the hypothalamic-pituitary-axis (HPA) and triggering the release of glucocorticoids. All animals experience some sort of environmental stress, and activation of the HPA axis is essential for survival. However, “chronic stress” can destabilize the system and have potentially life-threatening consequences. We collected scat samples from northern fur seals on St. Paul Island where the population is decreasing, and on Bogoslof Island where the population is increasing, in order to investigate these hormones and how they differ in each location. If the disparity between populations is due to differences in productivity around the two islands, we expect to see higher levels of glucorcorticoids and lower levels of T3 in the St. Paul population versus the Bogoslof population, indicating that the St. Paul fur seals are possibly experiencing chronic stress due to sub-standard nutrition.
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