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

• Published in: Canadian journal of zoology Vol. 78; no. 7; pp. 1243 - 1250
• Main Authors: Rosen, David AS, Trites, Andrew W
• Format: Journal Article
• Language: English
• Published: Ottawa, Canada NRC Research Press 01.07.2000; National Research Council of Canada; Canadian Science Publishing NRC Research Press
• Subjects: Animal and plant ecology; Animal behavior; Animal populations; Animal, plant and microbial ecology; Animals; Applied ecology; Aquatic mammals; Biological and medical sciences; Conservation, protection and management of environment and wildlife; Demecology; Food; Food supply; Fundamental and applied biological sciences. Psychology; Mammalia; Marine mammals; Parks, reserves, wildlife conservation. Endangered species: population survey and restocking; Pollock; Sea lions; Vertebrata; United States; Aleutian Islands; America; Alaska; Pacific Ocean; North America; Gulf of Alaska; North Pacific; Clupeidae; Availability; Carnivora; Unbalanced diet; Diversity; Weight loss; Energy input; Pinnipedia; Population decline; Feeding; Marine environment; Prey; Vertebrata; Mammalia; Clupea harengus; Pisces; Population dynamics; Predator; Animal conservation
• ISSN: 0008-4301; 1480-3283; 1480-3283; 0008-4301
• DOI: 10.1139/z00-060

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 fattier 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) only pollock or herring. All sea lions gained mass while eating herring. However, eating only pollock for short periods (11-23 d) caused the study animals 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 their food intake to compensate for the low energy they were receiving. The sea 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 the 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 pollock. Thus, our sea lions would have had to eat 35-80% more pollock than herring to maintain similar net energy intakes. Results from our captive-feeding studies are consistent with the junk-food hypothesis and have serious implications for Steller sea lions that have been eating primarily pollock in the Gulf of Alaska and the Aleutian Islands.