STABLE-ISOTOPE ANALYSES OF TOOTH ANNULI REVEAL TEMPORAL DIETARY RECORDS - AN EXAMPLE USING STELLER SEA LIONS

Stable isotope analysis of teeth of marine mammals can provide valuabl e information on trophic level and source of feeding. However, the iso topic analysis of whole teeth presents only an average dietary estimat e for individuals across the period of growth of that tooth. While suc h analyses can be valuable, particularly in the case of fossil materia l, in contrast, isotopic analysis of individual annuli of teeth can pr ovide dietary information for each year of tooth growth, in some cases representing the whole of the animal's life. We measured stable-carbo n isotope ratios (C-13/C-12) in the inorganic (hydroxyapatite) and sta ble-nitrogen isotope ratios (N-15/N-14) in the organic (primarily coll agenous) components of individual tooth annuli of 18 male Steller sea lions (Eumetopias jubatus) obtained from archived collections from the Bering Sea and Gulf of Alaska and from single northern fur seals (Cal lorhinus ursinus) and northern elephant seals (Mirounga angustirostris ) from the central Aleutian Islands and eastern Gulf of Alaska, respec tively. In several individuals, we detected considerable variation in stable isotope values among annuli, up to 6.1 parts per thousand for d elta(15)N and 5.1 parts per thousand for delta(13)C values. Enrichment in delta(15)N and depletion of delta(13)C values in the first annulus may correspond to dietary inputs from mother's milk during the period of suckling. Other variations among years may be caused by dietary ch anges or movements of individuals between regions differing in isotopi c signatures of foodweb primary production. Our study indicates that t he isotopic analysis of individual tooth annuli represents a fine-scal e cool for dietary reconstructions involving marine mammals, and cauti ons against the use of whole-tooth material averaged over several annu li.