Ontogenetic changes in isotopic signatures of an omnivorous fish Cultrichthys erythropterus in East Lake Taihu, China

The relationship between body size and stable isotopic signatures of the omnivorous Redfin Culter (Cultrichthys erythropterus), commonly found in East Lake Taihu, was investigated. Previous analyses of C. erythropterus stomach contents have shown that this species undergoes a diet switch from being predominantly zooplanktivorous to piscivorous during its life history. This was confirmed by stable carbon isotopic signature (delta C-13) in this study, in which delta C-13 was positively correlated with both standard length and weight. The importance of littoral-benthic resources in supporting C. erythropterus during its lifespan was also demonstrated using a two-source mixing model, the results of which showed a significant increasing trend in the contribution of littoral-benthic energy. However, the stable nitrogen isotopic signature (delta N-15) exhibited an unusual pattern compared with previous studies. The delta N-15 of C. erythropterus showed no relationship with body size, even though dietary changes were observed. This indicated that delta N-15 alone cannot fully reflect a diet shift in a species and possible variability in isotopic signatures over its life history. This should be considered when using stable isotopic signatures to investigate intra-specific variations and the timing of life-history events, such as estimating the trophic positions of fish species.