Three Steller sea lions Eumetopias jubatus were trained to participate in free-swimming, open-ocean experiments designed to determine if activity can be used to estimate the energetic cost of finding prey at depth. Sea lions were trained to dive to fixed depths of 10 to 50 m, and to re-surface inside a floating dome to measure energy expenditure via gas exchange. A 3-axis accelerometer was attached to the sea lions during foraging. Acceleration data were used to determine the overall dynamic body acceleration (ODBA), a proxy for activity. Results showed that ODBA correlated well with the diving metabolic rate (dive + surface interval) and that the variability in the relationship (r 2 = 0.47, linear regression including Sea lion as a random factor) was similar to that reported for other studies that used heart rate to estimate metabolic rate for sea lions swimming underwater in a 2 m deep water channel. A multivariate analysis suggested that both ODBA and dive duration were important for predicting diving metabolic cost, but ODBA alone predicted foraging cost to within 7% between animals. Consequently, collecting 3-dimensional acceleration data is a simple technique to estimate field metabolic rate of wild Steller sea lions and other diving mammals and birds.

SUMMARYThe metabolic costs of foraging and the management of O 2 and CO 2 stores during breath-hold diving was investigated in three female Steller sea lions (Eumetopias jubatus) trained to dive between 10 and 50 m (N=1142 dives). Each trial consisted of two to eight dives separated by surface intervals that were determined by the sea lion (spontaneous trials) or by the researcher (conditioned trials). During conditioned trials, surface intervals were long enough for O 2 to return to pre-dive levels between each dive. The metabolic cost of each dive event (diveϩsurface interval; DMR) was measured using flow-through respirometry. The respiratory exchange ratio (V O 2/V CO 2) was significantly lower during spontaneous trials compared with conditioned trials. DMR was significantly higher during spontaneous trials and decreased exponentially with dive duration. A similar decrease in DMR was not as evident during conditioned trials. DMR could not be accurately estimated from the surface interval (SI) following individual dives that had short SIs (<50 s), but could be estimated on a dive by dive basis for longer SIs (>50 s). DMR decreased by 15%, but did not differ significantly from surface metabolic rates (MR S ) when dive duration increased from 1 to 7 min. Overall, these data suggest that DMR is almost the same as MR S , and that Steller sea lions incur an O 2 debt during spontaneous diving that is not repaid until the end of the dive bout. This has important consequences in differentiating between the actual and 'apparent' metabolic rate during diving, and may explain some of the differences in metabolic rates reported in pinniped species.

Three Steller sea lions Eumetopias jubatus, trained to dive voluntarily to depths ranging from 10 to 50 m, were used to determine whether the relationship between activity and metabolic rate during a diving interval (MR DI , dive + surface interval) was affected by fasting (9 d) during the breeding season (spring through summer). We subsequently used the relationship between activity and MR DI to partition the metabolic costs between underwater breath-holding activity and surface breathing activities. We estimated activity from overall dynamic body acceleration (ODBA) measured using a 3-axis accelerometer, and measured MR DI using flow-through respirometry. The relationship between ODBA-based activity and MR DI was not affected by fasting period, suggesting that ODBA can be used to predict energy expenditure regardless of nutritional state in the spring and summer. However, the relationship between ODBA and dive metabolic rate differs from the relationship between ODBA and the surface metabolic rate before diving. Partitioning MR DI into the metabolic cost of remaining at the surface versus swimming underwater suggests that the metabolic cost of diving for Steller sea lions is approximately 29% lower than when breathing at the surface. ODBA appears to be a reasonable proxy to estimate metabolic rate in marine mammals, but more detailed behavioral data may be required to accurately apply the method in the field.

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 (MR s , 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 ( c DMR = DMR × 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 ( c MR s ) decreased significantly after both the summer (-16.4 ± 4.7%) and winter (-8.0 ± 9.0%) fasts. Consequently, the ratio between c DMR and c MR s was significantly higher in winter, suggestive of an increased thermal challenge and convective heat loss while diving. Increased c MR s 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. c DMR 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.