Elemental analysis of otoliths and eye lenses in the assessment of Steller sea lion diets
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Steller sea lions (Eumetopias jubatus) have historically ranged along the North Pacific Rim from the coast of California to Japan, but the population has dramatically declined since the 1960s. Research has indicated that nutritional stress is likely to be the main cause of the decline. Scat analysis is the preferred technique for dietary analysis of Steller sea lions, and fish otoliths and eye lenses are routinely recovered from pinniped scat. Fisheries scientists use elemental analysis of otoliths and eye lenses to provide information on fish biology, but marine mammalogists have not incorporated this technique to study prey fish or foraging behavior. In this dissertation, I examined the use of elemental analysis of prey fish otoliths and eye lenses in dietary studies for Steller sea lions. I first examined the use of otoliths as indicators of total body burdens of metal contaminants in the fish. Then, I assessed the effects of Steller sea lion digestion on the microchemistry of otoliths. Third, I examined the microchemistry of fish eye lenses, the effects of digestion on eye lenses, and their potential use in dietary analysis. Concentrations of some metals, such as zinc and barium, in undigested otoliths are significantly correlated with concentrations found in homogenized tissues, but several factors affect this relationship, such as fish species, sampling site on the otolith, and the specific metal being analyzed. The degradation of an otolith in the sea lion digestive tract is also likely to affect correlations between otolith and tissue metal concentrations. Steller sea lion digestion has significant effects on otolith microchemistry. These effects do not prohibit the use of digested otoliths in species determination for dietary analysis, but they may preclude using otoliths recovered from sea lion scat for fish stock separation, determination of foraging locations, and fish life history analyses. Eye lenses appear to be resistant to sea lion digestion, and they form sequential growth layers that can be used to age fish. The fibrous structure of the layers may inhibit symmetrical distributions across the lens for some elements, but the distinct elemental distributions across the lens may be useful in distinguishing fish species, discriminating between fish stocks, and tracking fish movements and spatial locations.