LAUSR

The field of stable isotope ecology is moving away from lethal sampling (internal organs, muscle) towards non-lethal sampling (fins, scales, epidermal mucous). Lethally and non-lethally sampled tissues often differ in their stable isotope ratios due to differences in metabolic turnover rate and isotopic routing. If not accounted for when using non-lethal tissues, these differences may result in inaccurate estimates of resource use and trophic position derived from stable isotopes. To address this, we tested whether tissue type, season, and their interaction influence the carbon and nitrogen stable isotope ratios of fishes, and whether estimates of species trophic position and resource use are affected by tissue type, season, and their interaction. We developed linear conversion relationships between two fin types and dorsal muscle, accounting for seasonal variation. We focused on three common temperate freshwater fishes: northern pike Esox lucius, yellow perch Perca flavescens, and lake whitefish Coregonus clupeaformis. We found that fins were enriched in 13 C and depleted in 15 N compared to muscle in all three species, but the effect of season and the interaction between tissue type and season was species and isotope dependent. Estimates of littoral resource use based on fin isotope ratios were between 13% and 36% greater than estimates based on muscle across species. Season affected this difference for some species, suggesting the potential importance of using season-specific conversions when working with non-lethal tissues. Fin and muscle stable isotopes produced similar estimates of trophic position for northern pike and yellow perch, but fin-based estimates were 0.2-0.4 trophic positions higher than muscle-based estimates for lake whitefish. The effect of season was negligible for estimates of trophic position in all species. Strong correlations existed between fin and muscle δ13 C and δ15 N values for all three species, thus linear conversion relationships were developed. The results of this study support the use of non-lethal sampling in stable isotope studies of fishes. We suggest researchers use tissue conversion relationships and account for seasonal variation in these relationships when differences between non-lethal tissues and muscle, and seasonal effects on those differences, are large relative to the scale of isotope values under investigation and/or the trophic discrimination factors under use. This article is protected by copyright. All rights reserved.