Fasting affects the surface and diving metabolic rates of Steller sea lions Eumetopias jubatus

Changes in metabolic rates were measured in 3 captive female Steller sea lions Eumetopias jubatus that experienced fasts during summer and winter. We measured metabolic rates (via O-2 consumption) before (MRs, surface) and after (DMR, dive + surface interval) the sea lions dove to 10-50 m depths. Measurements were obtained prior to and immediately after 9 to 10 d fasts, and during a 14 d recovery period. The sea lions lost significantly more body mass (M-b) during the winter fast (10.6%), compared with the summer (9.5%). Mass-corrected dive metabolic rate (cDMR = DMR x M-b(-0.714)) was not affected by dive depth or duration, but increased significantly following the winter fasts (13.5 +/- 8.1%), but did not change during summer (-1.1 +/- 3.2%). However, mass-corrected surface metabolic rate (cMRs) decreased significantly after both the summer (-16.4 +/- 4.7%) and winter (-8.0 +/- 9.0%) fasts. Consequently, the ratio between cDMR and cMRs was significantly higher in winter, suggestive of an increased thermal challenge and convective heat loss while diving. Increased cMRs following the fast indicated that digestion began during foraging and was not deferred, implying that access to ingested energy was of higher priority than optimizing diving ability. cDMR was elevated throughout the recovery period, independent of season, resulting in a 12% increase in foraging cost in winter and a 3% increase in summer. Our data suggest that Steller sea lions are more sensitive to changes in body condition due to food shortages in the winter compared with the summer.