Quantifying energy demands and prey consumption of marine mammals is important to understand the population dynamics of species and their ecological role in marine ecosystems. We developed a bioenergetic model to quantify the energy and prey requirements of adult female California sea lions Zalophus californianus, an abundant predator that has recently experienced several years of poor reproductive success presumably associated with oceanographic changes. We also examined how changes in at-sea field metabolic rates and the proportion of time at sea — 2 mechanisms that female otariids use to compensate for changes in prey availability — affected energy requirements. Mean gross energy requirements (±SD) ranged from 12 500 ± 1900 to 21000 ± 2200 MJ yr-1, depending on reproductive status, resulting in prey consumption estimates of 1700-3000 to 2900-5000 kg prey yr-1, respectively. California sea lions had energy requirements that were 3 to 4 times higher than phocid seals because of high metabolic rates and the considerable cost of lactation. Behavioral changes resulted in a 3 to 25% change in energy requirements, depending on the type and magnitude of the change, time period, and lactation status. Energy requirements may therefore increase during periods of reduced prey availability, assuming that a female does not reduce milk energy delivery to her pup. These results highlight the importance of considering behavioral responses to environmental conditions when developing and applying results from bioenergetic models to understand the relationships between energy requirements, prey availability, and reproductive success.
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