Abstract
In the present study, Xyrichtys novacula (Labridae) were sampled at five locations around the islands of Ibiza and Formentera (western Mediterranean Sea). Isotopic signatures of δ 13 C, δ 15 N and the C:N ratio were analysed in relation to locality, sex and size differences. δ 13 C and δ 15 N partitioning was also studied in the reproductive spawning period. There were significant differences in the δ 13 C signature between localities for both sexes, but not for δ 15 N. Sex differences were also found with a mean ± s.e . value of −17·38 ± 0·06‰δ 13 C and 8·36 ± 0·05‰δ 15 N for females and −17·17 ± 0·07‰δ 13 C and 8·80 ± 0·06‰δ 15 N for males. Increasing total length in both sexes was positively correlated with δ 15 N enrichment and a significant positive linear regression was established for both variables. During the reproductive spawning period, there were changes in δ 13 C fractioning with enrichment in postspawning females and males (with respect to prespawning and spawning periods) and δ 15 N impoverishment in postspawning females (with respect to prespawning and spawning periods). Xyrichtys novacula uses local food sources, as confirmed by δ 13 C and δ 15 N, and females and males use different food sources, thus avoiding intraspecific competition. This was confirmed by δ 15 N enrichment as size increased. Spawning leads to special requirements for gonad maturation, which is reflected in the isotopic signatures for both sexes.