Fin clips have been proposed as a non‐lethal and non‐invasive alternative to dorsal muscle samples in stable isotope analysis. However, potential differences in elemental composition and turnover rates can bias inferences when different tissues are combined. Here, we tested the average difference and correlation of the isotopic signature of δ13C and δ15N between muscle and fin samples in two arge‐bodied fishes: Longnose Gar (Lepisosteus osseus ) and Smallmouth Buffalo (Ictiobus bubalus ). We found that δ15N signatures of muscle and fin tissues were strongly and positively correlated for both species, although the muscle tissue for Smallmouth Buffalo was slightly more enriched in δ15N. δ13C signatures of both tissues were significatively different and not strongly correlated for Longnose Gar, but similar and strongly correlated for Smallmouth Buffalo. Our results suggest that fin and muscle tissue can be combined for analyses of δ15N, but correction for higher enrichment of muscle tissue may be necessary for Smallmouth Buffalo. Conversely, combining fin and muscle tissue for analysis of δ13C requires more caution due to their weaker correlation and dependence of species identity.