The diets of Steller sea lions have traditionally been identified from stomach contents and from the bony remains found in feces. However, this type of sampling only gives scientists a picture of what sea lions have eaten in the last few days. Fortunately, there is another way to gain insight into the diets of sea lions called Stable Isotope Analysis.

Trophic levels are a hierarchical way of classifying organisms according to their place in the food chain within an ecosystem. By convention, detritus and producers (such as phytoplankton and algae) are assigned a trophic level of one. Organisms that feed on the plants and detritus make up trophic level two. Higher order carnivores, such as most marine mammals, are assigned trophic levels ranging from three to five.

Isotopes are different forms of the same chemical (such as nitrogen-14 and nitrogen-15). Scientists can use the relative concentration of two isotopes in an animal to estimate what trophic level of the food chain sea lions have been preying on. This is because the ratio of the two isotopes shifts by a predictable amount as they are passed up the food chain.

Another useful stable isotope ratio is the relative concentration of carbon-13 and carbon-12. Carbon ratios are useful for assessing long-term changes in ocean productivity.

Amy Hirons, Don Schell and Bruce Finney recently measured the stable carbon and nitrogen isotope ratios in the bones of Steller sea lions, northern fur seals and harbor seals collected from the Bering Sea and Gulf of Alaska between 1951 and 1997. Hirons and her colleagues did not find any significant change in the nitrogen isotope ratios over the 47-year period, indicating that the trophic level of seals and sea lions did not change during this period.

However, they did find a significant decline in the ratio of stable carbon isotopes. A decrease in carbon isotope ratios with no accompanying change in nitrogen isotope ratios suggests a change occurred in the base of the food web (due to an environmental change) rather than a change in the trophic level being preyed upon.

Previous studies have suggested that the diet of Steller sea lions shifted from pelagic species (such as herring and sandlance) in the 1960s and 70s, to one now dominated by walleye pollock. Both species have a similar trophic level. A switch from herring to pollock would therefore not be detected by stable isotope analysis.

The change detected in carbon isotope ratios might reflect reduced growth rates of phytoplankton, and could indicate that the productivity of the North Pacific Ocean is reduced. As a result, sufficient quantities of optimal prey species may have fallen below threshold levels needed to sustain Steller sea lions, northern fur seals and harbor seals, particularly young individuals who have not yet developed adequate foraging skills.

Further details on the results of this study are contained in Hirons et al. (2001).

21 March 2002