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Quantifying metabolic rates and the factors that influence them is key to wildlife conservation efforts because anthropogenic activities and habitat alteration can disrupt energy balance, which is critical for reproduction and survival. We investigated the effect of diving behaviour, diet and season on field metabolic rates (FMR) and foraging success of lactating northern fur seals (Callorhinus ursinus) from the Pribilof Islands during a period of population decline. Variation in at-sea FMR was in part explained by season and trip duration, with values that ranged from 5.18 to 9.68 W kg-1 (n = 48). Fur seals experienced a 7.2% increase in at-sea FMR from summer to fall and a 1.9% decrease in at-sea FMR for each additional day spent at sea. There was no effect of foraging effort, dive depth or diet on at-sea FMR. Mass gains increased with trip duration and were greater in the fall compared with summer, but were unrelated to at-sea FMR, diving behaviour and diet. Seasonal increases in at-sea FMR may have been due to costs associated with the annual molt but did not appear to adversely impact the ability of females to gain mass on foraging trips. The overall high metabolic rates in conjunction with the lack of any diet-related effects on at-sea FMR suggests that northern fur seals may have reached a metabolic ceiling early in the population decline. This provides indirect evidence that food limitation may be contributing to the low pup growth rates observed in the Pribilof Islands, as a high metabolic overhead likely results in less available energy for lactation. The limited ability of female fur seals to cope with changes in prey availability through physiological mechanisms is particularly concerning given the recent and unprecedented environmental changes in the Bering Sea that are predicted to have ecosystem-level impacts.

Stable isotope analysis (SIA) of whiskers is increasingly used to investigate the foraging ecology of pinnipeds. An understanding of whisker growth dynamics is lacking for most species yet is necessary for study design and interpretation of isotope data. Here we present measurements of whisker growth obtained using photogrammetry in 5 California sea lions Zalophus californianus and 2 spotted seals Phoca largha. Data were collected from captive individuals for at least 1 yr, resulting in serial measurements of 321 sea lion and 153 spotted seal whiskers. The sea lion whiskers exhibited linear growth, with growth rates that ranged from

Pinniped have evolved a range of reproductive and foraging patterns to deal with the constraint of terrestrial parturition and marine feeding. To understand the link between these constraints it is necessary to understand both reproductive periods on land and foraging at sea. I examined the foraging behavior of a phocid (northern elephant seal, Mirounga angustirostris) and otariid (California sea lion, Zalophus californianus) species to provide insight into the divergent life histories of these families. To understand foraging behavior it is necessary to know when and where animals find prey. Using stomach temperature telemetry, I conducted rigorous feeding experiments with both species to determine whether measures of stomach temperature can be used to identify and quantify prey consumption. Feedings were identified with high accuracy and the equations to estimate mass consumed were not significantly different between species.Until recently, studies on the foraging behavior of marine predators have relied on indirect measures of feeding, such as changes in diving behavior or movement patterns. When combined with measures of at sea behavior, stomach temperature telemetry can provide information about when and where animals uccessfully capture prey. I deployed stomach temperature telemeters along with time-depth recorders and satellite transmitters to examine the foraging behavior of northern elephant seals. This resulted in the first measures of feeding behavior and foraging success in elephant seals, as well as provided a validation of indirect indices of foraging behavior.Finally, I examined the foraging behavior of free-ranging California sea lions and documented how their behavior changed annually in response to environmental variation that occurred over three years. Although stomach temperature telemeters were deployed on sea lions, I was only able to maintain the telemeters in the animals for long enough to measure at sea feeding in one female. Therefore, foraging behavior was examined based on indirect measures of feeding, such as diving behavior and at sea distribution. I found females alter movement patterns in response to even modest environmental variation. Such detailed information on the foraging behavior of these species will make it possible to examine the link between reproductive pattern and at sea foraging behavior.