Panning for Bones

Currently, the principal way to determine what sea lions eat is to identify the bones that are contained in their scats (feces). Many fish bones survive the acid conditions of a Steller sea lion's stomach, but bones of some prey are more resistant to digestion than bones from other species.

Differences in digestion rates need to be determined to accurately estimate the number of different types of prey consumed by Steller sea lions. Dr. Dom Tollit is determining this by feeding known numbers of pollock, herring, salmon and sandlance to capture Steller sea lions. He collects all the scats they produce, and identifies and counts the different bones recovered. By knowing how many bones went in one end, and how many came out the other, he can estimate the proportion of fish of each species that survived digestion.

One of the key elements of Dr. Tollit's experiment is simulating conditions of foraging sea lions in the wild (such as activity and feeding regime). He is working with animals that are trained to swim actively in an "aquatic" tread mill for their lunch!

Preliminary findings are coming in. One of the things that he has discovered is that bones from a single meal of fish may appear in scats anywhere from two hours to more than six days later, and may be spread across 1-6 different scats. Results also suggest that active animals pass more bones than inactive animals. However, active animals may also regurgitate larger bones. Significant differences in bone recovery were also found between types of fish, between individual sea lions, and even between identical experiments. Despite these differences identifiable remains were recovered from every meal fed. More trials are being conducted to interpret this observed variability and to investigate other factors influencing digestion.

18 February 2002