HOOPES, L.A.*; REA, L.D.; WORTHY, G.A.J.; Texas A&M University; University of Alaska, Fairbanks; University of Central Florida: Born free: resting metabolic rate in free-ranging Steller sea lions (Eumetopias jubatus).

The prevalent hypothesis for the >80% decline of Steller sea lions (SSLs) in western Alaska is that changing prey quality and/or quantity have prevented sea lions from meeting their energetic demands, thus creating a deficit, or nutritional stress. This may be particularly true in younger animals which experience the high energetic demands associated with growth. When energy intake is insufficient to meet daily energetic demands, some animals are able to compensate by limiting their energy expenditures. The most common physiological response to limited food intake is a lowering of resting metabolic rate (RMR) known as metabolic depression. Given observed reductions in RMR in captive SSLs fed a low energy diet, the potential significance of metabolic depression in juvenile free-ranging SSLs was examined via open flow respirometry. Free-ranging animals were captured from western stocks in Prince William Sound (PWS, n =30) and the Central Aleutian Islands (AL, n=16) and from the eastern stock in Southeast Alaska (SE, n=43). RMR was measured in animals aged 2, 5, 8, 11, 14, 20 and 26 months at ambient air temperatures (-6-18 °C). Standard morphometrics, blubber depth, and total body fat stores were measured for an estimate of body condition at each location. Mean RMR ranged from 11.0 to 32.3 MJ d^-1 across all aged animals, and as expected, increased with increasing body mass. Although air temperatures varied between capture sites, similar temperatures existed between PWS and SE locations, allowing direct comparisons of RMR, blubber depth, and total body fat stores between similarly aged animals. Our data indicate no evidence of metabolic depression in western stock animals.