| Summary: | The core breeding population of northern fur seals (Callorhinus ursinus) in North America has declined significantly since the 1980s on St. Paul Island (one of the Pribilof Islands) while the smaller nearby population at Bogoslof Island (eastern Bering Sea) has increased exponentially. Further south, the population of northern fur seals on San Miguel Island off the coast of Southern California has fluctuated between exponential growth and catastrophic declines associated with re-occurring El Nino events. The goal of my thesis was to assess the physiological status of these three breeding populations of northern fur seals in North America to determine whether nutritional differences could explain the different population trajectories. I collected fecal samples (scats) in July 2009 from these three islands and measured the fecal metabolites of two hormones — a glucocorticoid associated with the stress response, and triiodothyronine (T3), a thyroid hormone associated with metabolic rate. I also assessed feeding conditions using diet and foraging data. I found that sub-adult males and lactating females on St. Paul Island experienced poorer feeding conditions (lower energy content food and longer feeding trips for lactating females) than at Bogoslof Island, but that only the females were nutritionally stressed. I also found that the San Miguel Island population differed physiologically compared to the northern populations in Alaska in terms of stress and nutritional status. The San Miguel fur seals were the most physiologically stressed of the North American fur seal populations (based on elevated levels of glucocorticoid metabolites). However, the stress was most likely related to heat stress and not food (based on low concentrations of T3 metabolites). The available hormone, diet, and foraging data from northern fur seals in North America suggest that lactating females were nutritionally stressed on St. Paul Island and heat stressed on San Miguel Island, and experienced better conditions on Bogoslof Island.
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