δ15N and δ13C were determined for plants, invertebrates and fishes collected from 3 sites on the southern coast of the Mediterranean island of Mallorca, Spain. The sites were separated by distances of 1250 to 3750 m. The mean δ15N of plants was 1.1 to 4.1‰, benthic invertebrates 5.9 to 6.9‰, planktonic invertebrates 5.5 to 5.8‰ and fishes 84 to 13.8‰. δ15N became enriched with increasing trophic level. The mean δ13C of plants was -11.4 to -16.3‰, benthic invertebrates -14.8 to -16.8‰, planktonic invertebrates -19.3 to -19.8‰ and fishes -16.1 to -19.2‰. There were significant differences in the isotopic composition of individual species within the plant, invertebrate or fish groupings at each site and there were significant differences in the isotopic composition of the same species at different sites. Depleted 13C was associated with benthic food chains and enriched 13C with planktonic chains. The data suggest that benthic food chains are important to the rocky reel associated fishes studied, as might be expected in a nutrient poor system where planktonic production is relatively low. However, the variance in δ13C composition between sites was such that the relative significance of the 2 pathways could not be determined. 15N measurements indicated that some of the fish species studied had adopted different feeding strategies at different sites and, as a result, individuals of the same species could sometimes be assigned to different trophic groups at different sites. The data suggest that these fishes exhibit plasticity in their feeding strategies and this may provide them with greater adaptive flexibility to respond to site-specific changes in food availability. Moreover, the data provide empirical support for current theories of food web dynamics which suggest that trophic 'levels' are dynamic rather than fixed and that 'multichannel omnivory' is an important feature of food webs.

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