A non-lethal sampling method for stable carbon and nitrogen isotope studies of tropical fishes

Despite prior studies showing good agreement between fin and muscle isotope ratios in temperate fishes, the non-lethal method of fin sampling has yet to become a standard technique in isotopic food web studies, and the relationship between the two tissues has never been tested in the tropics. We hypothesized that fin and muscle d13C and d15N would be strongly correlated in tropical fishes, thus allowing non-lethal sampling of these species. To test this hypothesis, we analysed fin and muscle from 174 tropical fishes representing 27 species from the Mitchell River, Queensland, Australia. Fin tissue was a strong predictor of muscle tissue d13C (r2 = 0.89 for all species) and was slightly enriched in 13C (0.9–), consistent with studies on temperate species. Fin tissue was a poorer predictor of muscle tissue d15N (r2 = 0.56 for all species) but the mean difference between the tissues was small (<0.1–). Differences were smallest in the largest fish, possibly because the elemental composition (%N) of fin more closely resembled that of muscle. These measurements provide more impetus for increased use of fin tissue as a non-destructive means of answering food web questions about fishes using stable isotopes in the tropics and elsewhere.