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USING STABLE ISOTOPES FOR FISH DIETARY ANALYSES: COPING WITH TOO MANY SOURCES
Citation:
Phillips, D L. AND J W. Gregg. USING STABLE ISOTOPES FOR FISH DIETARY ANALYSES: COPING WITH TOO MANY SOURCES. Presented at Oregon Chapter American Fisheries Society, Eugene, OR, February 26-28, 2003.
Description:
Stable isotope analysis can provide a useful tool for determining time-integrated measures of proportional food source contributions to fish diets. Ratios of stable (non-radioactive) isotopes of common elements (e.g., C,N,S) vary among food sources, and tissues of consumers (e.g., fish) reflect the isotopic ratios of their diet. For isotopic ratios of n elements, mixing models based on mass balance equations can be used to uniquely determine the contributions of n+1 different sources. Often, however, the number of potential food sources far exceeds the number of elements with meaningful variation in stable isotopic ratios, which prevents solving for unique source proportions. For these situations we propose a method to determine bounds for each source's contribution. All possible combinations of food source proportions are examined, and mass balance equations are used to predict the isotopic ratios in the consumer for each combination. Those that result in predicted consumer isotopic ratios within a small tolerance of the observed ratios are considered to be feasible solutions. For each source, the range of proportions in the set of feasible solutions gives the minimum and maximum dietary contribution to the consumer. We demonstrate this method on dietary analyses of coastal Alaska mink and two species of Micronesian inshore fish. Fisheries biologists should find this technique useful for quantifying utilization of specific food sources as well as broad patterns of allochthonous and autochthonous nutrient sources for fish food webs, such as marine derived nutrients from anadromous fish returns.