Hard Parts and Steller Sea Lions- Marine Mammal Research Consortium

Hard Parts...
The decline of Steller sea lions in the Gulf of Alaska and Bering Sea has prompted researchers to monitor sea lion diets and explore potential links with changes in prey abundance and availability. This is being increasingly done by identifying the bones found in fecal remains (scats). Unfortunately, little is known about passage rates and degree of digestion of bones from different species of prey.

Paul Cottrell and Dr. Andrew Trites addressed this problem by feeding six captive young sea lions held at the Vancouver Aquarium and Marine Science Center, Atka mackerel, Pacific herring, pink salmon, squid and walleye pollock. Over 20,000 fish and squid were fed during 30 experiments.

Cottrell and Trites recovered over 119,000 hard parts representing 56 skeletal elements. That's an average of about 6 parts for each prey. Not very many when you consider how many bones each fish has, and the fact that squid only have a beak, a pen and eye lenses!

In general, the region of the body that yielded the most bones varied with the type of fish eaten. Herring had fewer hard parts recovered per fish than pollock or salmon. However, more bones were recovered from a meal of herring compared to a meal of pollock or salmon, because herring are smaller and were consumed in greater numbers. One of the most abundant parts recovered from all species were eye lenses. With no sharp edges, they appear to roll through the digestive tract and can be counted to determine the minimum number of prey consumed.

Estimates of the recovery rates of different skeletal structures will be used to determine the key structures that should be used to identify which prey have been consumed. Knowing what percentage of parts passed through can also be used to estimate the total number that must have been consumed. Understanding such biases associated with different sizes of prey, sizes of meals and species of prey are important to ensure that biologists accurately determine what sea lions are eating.

See Cottrell and Trites (2002) for further details.

8 August 2002