Pollock and the decline of Steller sea lions: testing the junk-food hypothesis

The decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska and the Aleutian Islands may be the result of them eating too much pollock (a gadid fish) instead of a more balanced and diverse diet containing fatt ier fishes, such as herring or sandlance. We sought to test this junk-food hypothesis by feeding six captive Steller sea lions (ages 0.9-4.5 years) on ly pollock or herring. All sea lions gained mass while eating herring. Howe ver, eating only pollock for short periods (11-23 d) caused the study anima ls to lose an average of 6.5% of their initial body mass (0.6 kg/d) over an average feeding trial of 16 d (initial mass averaged 125 kg). The animals were allowed to eat as much pollock as they wanted but did not increase the ir food intake to compensate for the low energy they were receiving. The se a lions showed progressive metabolic depression while losing body mass on a pollock-only diet. The loss of body mass while eating pollock was due to t he lower gross energy content of pollock versus herring, the higher cost of digesting pollock, and the increased energy loss from digesting the larger quantity of fish needed to compensate for the lower energy content of poll ock. Thus, our sea lions would have had to eat 35-80% more pollock than her ring to maintain similar net energy intakes. Results from our captive-feedi ng studies are consistent with the junk-food hypothesis and have serious im plications for Steller sea lions that have been eating primarily pollock in the Gulf of Alaska and the Aleutian Islands.