Satiation and Compensation in Young Steller Sea Lions

The nutritional stress hypothesis suggests that young Steller sea lions may not be consuming sufficient prey to satisfy their nutritional needs. This might be because the quality or availability of prey is too low for sea lions to physically consume enough fish.

The theoretical amount of fish a sea lion needs to eat can be calculated mathematically. Bioenergetic models can estimate the amounts needed to sustain growth and reproduction based on prey type and availability. However, it is not known whether sea lions can actually physically consume the amounts predicted by the models.

In a recently published study, five captive yearling Steller sea lions participated in experiments designed to determine factors that influence the amount of fish that young sea lions eat. The goal was to understand how changes in availability and type of fish affected their total food intake.

The study found that sea lions adjusted the amounts of food they consumed when offered either high-energy herring or low-energy capelin. In both cases, however, the sea lions maintained similar energy intake (See adjoining Figure and compare the bars and dashed lines for daily herring and capelin consumption).

The sea lions also increased their intake when herring was only available every other day. However, the increase was not enough to match the amount of energy they were able to consume when it was available every day.

When low-energy capelin was offered every other day, the sea lions consumed slightly more compared to when they ate capelin everyday. But they were far from meeting their daily needs. In this case, the young sea lions appeared to reach an upper limit on how much fish they could consume and process.

The results suggest that younger Steller sea lions may have physical difficulty consuming sufficient quantities of lower energy prey, particularly when prey is not consumed on a daily basis. This limitation may be an important clue for understanding how changes in fish availability and species composition might have contributed to the decline of Steller sea lions in the North Pacific.
18 October 2004

Publication

Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus). Rosen, D.A.S. and Trites, A.W. 2004. Canadian Journal of Zoology pp. 1061-1069. abstract Foraging theory predicts that animals should proportionately increase their food intake to compensate for reduced 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 alternately offered 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 an equivalent gross energy intake. 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 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.