Rats and Steller Sea Lions- Marine Mammal Research Consortium

Steller Sea Rats?

Nutritional stress is the leading explanation for the decline of Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska. One theory is that the decline occurred as the sea lions switched their diet from fatty forage fishes (such as herring) to low fat fishes (such as pollock). Biologists noted that sea lion body growth was stunted during the decline, and that the sea lions had low pregnancy rates and a high mortality of juveniles.

Captive feeding studies should be able to test whether or not eating pollock could cause these changes. However, there is no aquarium in the world that could hold enough sea lions (>60) for a long enough time (>5 years) to fully test it. That is why Carolyn Donnelly, Andrew Trites and David Kitts decided to test this theory using another well-studied mammal—the laboratory rat.

Rats have long lived under fishing wharfs and have no problem eating fish. While they may not be as formidable as a Steller sea lion, they are a good starting point to understand some of the nutritional differences between different species of fish. What would take 5-6 years to accomplish with Steller sea lions can be done in as little as 5-6 months with rats.

Sixty recently weaned female rats were divided into 5 groups of 12. Each group was fed a diet consisting of either herring, pollock, pollock supplemented with herring oil, pollock supplemented with pollock oil, or rat chow.

The biggest rats were those that ate herring, while the smallest ones ate rat chow. The groups that ate pollock and rat chow consumed 10% more food than the other groups to make up for the lower number of calories in their diet. Mothers that ate pollock had significantly smaller young than those that were fed the other diets.

One of the biggest surprises was that the group that ate pollock supplemented with pollock oil did not do as well as the one that only ate pollock. It is not apparent why the fatty pollock group did not do better.

In general, the rats did well on pollock as long as they were able to consume more of it to make up for the lower caloric density of pollock. The question for Steller sea lions is whether young animals can physically consume the higher quantities of pollock they would require to achieve the same number of calories from herring or other fatty fishes.

For more information on this study, see Donnelly, Trites and Kitts (2003).

20 January 2003