Nonlethal Fin Sampling of North American Freshwater Fishes for Food Web Studies Using Stable Isotopes

Food web investigations using stable isotope analyses inform management strategies by quantitatively describing interactions among species. Fin tissue is a nonlethal alternative to white muscle tissue for obtaining stable isotope ratios of fish. However, the relationship between fin and muscle values must be evaluated before fin tissue can be substituted for muscle tissue. We examined the relationships between fin and muscle carbon (delta C-13) and nitrogen (delta N-15) isotope ratios in 15 species of North American freshwater fish using linear regression for multispecies models and species-specific models, and compared the performance of our models against models developed for European and Australian freshwater fishes. Fin and muscle isotope ratios differed but covaried with good explanatory power ( R2 delta 13C = 0.74-0.99; R2 delta 15N= 0.45-0.99). Useful conversions of fin isotope values were provided by our species-specific models (delta C-13(error) = 0.27 parts per thousand; delta N-15(error) = 0.22 parts per thousand), and to a lesser extent our multispecies models (delta C-13(error) = 0.51 parts per thousand; delta N-15(error) = 0.35 parts per thousand) and the European models (delta C-13(error) = 0.57 parts per thousand; delta N-15(error) = 0.42 parts per thousand). Models developed for Australian fishes gave less precise conversions when applied to North American freshwater species (delta C-13(error) = 0.78 parts per thousand; delta N-15(error) = 0.82 parts per thousand). We recommend using species-specific conversion models when available, but in their absence, multispecies models may be adequate. Our study contributes to a growing literature seeking fin-muscle conversion models, which should serve to reduce the lethal take of individuals.