Variation in relationships of δ13C and δ15N between lethal and nonlethal samples in fishes

Objective We summarized variation in muscle-fin isotope relationships in studies of freshwater fishes and evaluated whether season and body condition explained variation in muscle-fin relationships in a field study.Methods We reviewed published relationships of delta 13C and delta 15N values from muscle and fin tissue samples in fishes and compared them to relationships in a large sample of muscle and fin tissue samples that we collected from two small-bodied fishes, juvenile Brown Trout Salmo trutta and Mottled Sculpin Cottus bairdii. We evaluated the importance of season and body condition (dry matter content and C:N) in our muscle-fin relationships using multiple regression with model selection.Results Correlations between muscle and fin delta 13C and 15N values within studies were high, but the variance in relationships across studies was large, even for the same species. Muscle and fin delta 13C and delta 15N values in our field study were also strongly positively correlated (r >= 0.89), with minor muscle-fin differences for Brown Trout (Delta delta 13C = -0.71 +/- 0.42 parts per thousand, Delta delta 15N = 0.13 +/- 0.38 parts per thousand) and Mottled Sculpin (Delta delta 13C = -0.73 +/- 0.29 parts per thousand, Delta delta 15N = -0.21 +/- 0.31 parts per thousand). Slopes of our muscle-fin relationships for delta 13C (beta 1 = 0.796-0.911) and delta 15N (beta 1 = 0.826-0.872) were similar to average literature values (delta 13C: beta<overline>1 = 0.824; delta 15N: beta<overline>1 = 0.875), but again, the variance in published relationships was large. Incorporating season and body condition in models improved muscle-fin relationships.Conclusions High variance in muscle-fin isotopic relationships makes it difficult to rely on previously established models. We found that body condition improved the fit of muscle-fin relationships, suggesting that the large variance among the previous studies might be due to unmeasured predictors. Incorporating additional predictors, such as energy status, could reduce variation in muscle-fin relationships and increase their applicability across systems. We showed that the relationship between fish muscle and fin isotope signatures is often strong but variable through time and space. Incorporating information on fish energy status can improve muscle-fin relationships and may increase the applicability of models that predict whole-body isotopic signatures from nonlethal samples such as fin clips.