Estimation of seasonal energy content of Steller sea lion (Eumetopias jubatus) diet

Estimation of seasonal energy content of Steller sea lion (Eumetopias jubatus) diet

J. Vollenweider, J. Womble, and R. Heintz

Estimation of seasonal energy content of Steller sea lion (Eumetopias jubatus) dietThis is part of Sea Lions of the World
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We estimated the energy consumption of Steller sea lions (Eumetopias jubatus) in southeastern Alaska by integrating seasonal scat collection data with seasonal energy content of prey species of Steller sea lions. During 2001 and 2002, sea lion scat samples were collected quarterly at Benjamin Island in conjunction with quarterly collections of sea lion prey species from mid-water trawls near Benjamin Island. The biomass of prey species was reconstructed using biomass-variable (BV) and biomass-fixed (BF) techniques, and combined with prey energy content to estimate the amount of energy from each prey species during each season. Energy content of scats was variable across seasons and was lowest in February of both years and highest in December. A total of 41 prey species were identified from scat samples; however, the BV and BF models identified only five prey as constituting the majority of energy in sea lion scats. The five primary prey include salmon (Oncorhynchus sp.), skate (Rajidae), Pacific cod (Gadus macrocephalus), Pacific herring (Clupea pallasii), and walleye pollock (Theragra chalcogramma). These prey accounted for 91% of the total energy in the BV model and 84% in the BF model. Both models depicted similar seasonal trends in prey-derived energy, likely related to ephemeral prey aggregations associated with spawning or overwintering. In the BV model the primary prey species contributed relatively equal proportions of energy, with the exception of mature pollock. In contrast, herring was the predominant energy source in the BF model. The relative importance of prey types resulting from BV and BF models were more similar to each other than to raw biomass estimates or frequency of occurrence. Likely the true prey-derived energy is intermediate to the two models due to opposing inherent biases of each model.

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