| Summary: | Efficient extraction of energy from the environment is key to the survival and reproductive success of wild animals. Understanding the ratio of energy gained to energy spent of different foraging strategies (i.e., foraging efficiency) can shed light on how animals cope with environmental changes and how it affects population trajectories. I investigated how female foraging strategies during the breeding season impact the foraging efficiencies and reproductive successes of two fur seal species—one declining (NFS–northern fur seals, St. Paul Island, Alaska) and one increasing (AFS–Antarctic fur seals, Kerguelen Island, Southern Ocean). I also sought to develop new accelerometry-based methods to easily determine fine-scale energy expenditure at sea (VeDBA and flipper stroke metrics). Twenty lactating females of each species were captured and equipped with biologging tags to record GPS locations, depth and tri-axial acceleration. Energy expenditure for each foraging trip was measured using the doubly-labeled water method, and energy gained while foraging was determined from 1) diet composition (scat hard-parts and DNA) and blood stable isotope ratios; and 2) numbers of prey capture attempts (from head acceleration). Maternal investment in pups was determined from pup growth rates or from energy content of milk samples. Results showed acceleration metrics were only accurate at predicting energy expended by fur seals when time-activity budgets were taken into account (i.e., time spent performing different types of activity). Foraging strategies of AFS females resulted in efficiencies of ~3.4, with more efficient females producing bigger pups at weaning that had greater chances of survival. NFS females employed two foraging strategies with very different efficiencies (~1.4 vs ~3.0) that were associated with different foraging habitats and diet qualities. However, NFS with the more efficient strategy (3.0) undertook longer foraging trips than the other NFS (1.4) or AFS (3.4), and thus fed their pups ~20% less frequently. As a consequence, the declining NFS (unlike the increasing AFS) had to compromise between the rate of energy acquisition and the pup feeding frequency. Such reductions in energy intake and time allocated to nursing pups can ultimately lower juvenile survival, and may explain the population decline of NFS in Alaska. Science, Faculty of Zoology, Department of Graduate
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