Potential effects of short-term prey changes on sea lion physiology

Abstract

Changes in the proximate composition of prey can result in a nutritional imbalance in individual animals, regardless of total energy intake. This mechanism has been hypothesized to have contributed to the decline of Steller sea lions (Eumetopias jubatus). Yet little is known about how otariids react physiologically to short-term changes in prey quality and availability. A series of studies with young captive Steller sea lions tested several potential links between prey quality and sea lion health. Body composition (fat to total mass ratio) of animals fed constant, maintenance-level, isocaloric diets of highor low-lipid prey changed with season, but overall was not affected by prey composition. The sea lions appeared to prioritize maintaining core growth rates even when energy was limited, electing to deplete lipid reserves to fulfill energy deficits, resulting in changes in relative body condition. In contrast, sea lions subject to short-term, sub-maintenance diets of highor low-lipid prey utilized a greater portion of their lipid reserves when losing body mass on low lipid prey. Experiments with different ad libitum feeding regimes indicated that sea lions are readily able to alter food intake levels to compensate for differences in prey energy content and, to a lesser degree, prey availability. However, the results also suggest that decreases in prey quality and/or foraging opportunities can readily combine to require food intake levels that are greater than the digestive capacity of the individual. This is particularly true for young animals that may already be living “on the edge” energetically. Sea Lions of the World 103 Alaska Sea Grant College Program • AK-SG-06-01, 2006