Comparison of muscle and scale isotope signatures for Pacific Herring (Clupea pallasii), and potential application in regional life history studies

Carbon and nitrogen are found in two naturally occurring isotopic forms, differing in their number of neutrons: a light isotope (12C and 14N) and a heavy isotope (13Cand 15N). These stable C and N isotopes are taken up by the primary producers in a food web in ratios that reflect the interacting effects of temperature, elemental availability (e.g. nutrient concentrations), and productivity levels, among other factors. Subsequently, the baseline primary producer isotope ratio, or signature, is transferred through the food web with largely predictable increases between trophic levels resulting from organisms’ preferential metabolism of the lighter isotopes. The carbon and nitrogen isotope ratios of an organism’s tissues therefore provide a powerful source of information on the environmental and feeding conditions experienced by that organism, as well as its trophic level. The isotope ratios of any tissue reflect a time integration that corresponds with the turnover rates of that tissue. In the case of fish, muscle tissue is expected to have relatively high turnover rates and thus reflect dietary information for the few months prior to capture. Conversely, hard body parts, such as scales and otoliths, are expected to reflect dietary information incorporated at the time the tissue was laid down, providing an incremental record of an individual’s feeding biology over time. Here we present the results of a test study comparing muscle and scale C and N isotope ratios of Pacific herring from different management areas: the Strait of Georgia, West Coast of Vancouver Island, Central Coast, and Haida Gwaii. We specifically assess the utility of scale and muscle isotope data in reconstructing the life history feeding biology of Pacific Herring, and provide a preliminary comparison of regional feeding dynamics