Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus)

Foraging theory predicts that animals should proportionately increase their food intake to compensate for reduced food energy content and (or) prey availability. However, the theoretical intake levels will, at some point, exceed the digestive capacity of the predator. We tested the ability of Steller sea lions, Eumetopias jubatus (Schreber, 1776), to compensate for short-term changes in prey energy density and availability, and quantified the maximum amount of food a young sea lion could consume. Five 1-2-year-old captive Steller sea lions were offered either herring (high energy) or capelin (low energy) each day or every second day. When prey were available on a daily basis, the sea lions compensated for differences in the energy content of herring and capelin by consuming sufficient quantities of each (8.3 vs. 14.0 kg.d(-1), respectively) to maintain equivalent gross energy intakes. When herring was available only on alternate days, the sea lions increased their consumption by 52% to 11.5 kg.d(-1), which was not sufficient to maintain an average gross intake equal to that maintained when herring was available every day. When capelin was available only on alternate days, some animals increased their intake for a few days, but average intake (15.2 kg.d(-1)) was far below levels observed during daily feeding. Generally, the sea lions appeared to reach their digestive limit at a level equivalent to 14%-16% of their body mass. Our findings suggest that Steller sea lions can alter their food intake in response to short-term changes in prey quality or availability, but that these variables can quickly combine to necessitate food intake levels that exceed the physiological digestive capacities of young animals.