Utility of fish scales from stock assessment surveys in stable isotope analysis for initial assessments of trophic relationships in riverine fish communities

The utility of using fish scales collected during stock assessment exercises to assess the trophic relationships of riverine fishes using their stable isotopes of d(13)C and d(15)N was tested using three riverine fish communities in England (Rivers Great Ouse, Ivel and Goyt). In each river, European barbel Barbus barbus was an important species, with other cyprinid species, including chub Squalius cephalus, present. Stable isotope analyses was completed using relatively small sample sizes per species (<11) from fish samples collected in 2001, 2005 and 2006 when up to 5 scales were collected from each fish. The calculation of standard ellipse areas (as a measure of trophic niche size) revealed that relative to other fishes, B.barbus occupied high trophic positions with minimal overlap in their trophic niche with other species, especially S.cephalus. As the analysed fish samples comprised species of various length ranges and as length has strong ontogenetic consequences for fish diet composition, generalized linear models were developed in which length was the covariate; model outputs included length-adjusted mean C-13 and N-15 for each species. In each fish community, significant differences in C-13 and N-15 were apparent between B.barbus and S.cephalus, but were less apparent between B.barbus and other fishes. Thus, whilst the utility of using fish scales from stock assessments in stable isotope analyses are limited due to the differing length ranges of the sampled fishes, they can be useful in identifying trophic differences between species when methods such as stomach content analyses are unavailable.