Stable Isotopic Variability Along Spiny Dogfish Spines: Implications for Retrospective Ecological Studies

Spiny dogfish Squalus acanthias are an abundant and commercially important species of fish off both the Atlantic and Pacific coasts of North America. They are opportunistic feeders and have a varied diet that can include many fish species, especially small forage fish such as herring, capelin, and sand lance as well as crustaceans, worms, euphausiids, gelatinous zooplankton, and cephalopods. The purpose of our present Study is to investigate the utility of the second dorsal spine in providing multi-year information on the feeding habits of dogfish using stable isotope (C and N) analysis. The outer dentine and enamel layers of a spine from three dogfish (caught off the coast of British Columbia) were analyzed to obtain their stable nitrogen and carbon isotope composition (delta N-15 and delta C-13, respectively). Each sample had annuli from multiple years, allowing possible seasonal migrations to be averaged over samples. The delta N-15 ranged from a low of 11.6 parts per thousand to a high of 14.9 parts per thousand over the three spines and delta C-13 ranged from -11.5 parts per thousand to -18.4 parts per thousand. The variable isotopic signatures along the spine indicate that the method may be used to assess ecological changes. Stable isotope measurements of dogfish spines could be a valuable means of determining long-term changes in habitat usage and feeding ecology.