Resource partitioning between common bottlenose dolphin (Tursiops truncatus) and the Israeli bottom trawl fishery? Assessment by stomach contents and tissue stable isotopes analysis

Common bottlenose dolphin (CBD) and bottom trawlers exploit the same ecological niche. The estimated CBD population along the Israeli coastline consumes roughly 1280 t of prey annually, similar to the mean annual trawl-fishery yield of 1300 t. In the ultra-oligotrophic Levantine Basin, the potential for competition for limited resources therefore exists. We aimed to examine whether the two consumers indeed harvest the same trophic level of the food web and the same members of that level. These questions were addressed by stomach content and stable isotope (delta N-15 and delta C-13) analyses. The database included 26 prey-containing stomachs collected between 1996 and 2008 and muscle tissue samples of 23 dolphins and of 27 species of commercial fish and invertebrates for stable isotopes analysis. The 59.3 kg combined stomach content included at least 754 prey items, 97.3% of which were fish, belonging to 22 identified species. About half (46.4%) of the prey mass had medium-to-high commercial value. The overall similarity of the composition of the pooled biomass and that of the average commercial catch was expressed by a Pianka index of 0.49. Sparidae (sea bream) was the only family important for both consumers. The mean delta N-15 value of CBD muscle was found to be relatively low compared to other geographical areas and the estimated mean delta N-15 of its diet fell below that of most commercial species tested. Our findings suggest that CBD and the fisheries only partly share resources, with CBD having an overall minor effect on the bottom trawl fishery, mainly on the Sparidae catch.