Trophic niche comparison between two predators in northern Rio de Janeiro State, Brazil: a stable isotopes approach

The trophic niche of the sympatric predators Pontoporia blainvillei (franciscana dolphin) and Trichiurus lepturus (ribbonfish) was compared by stable isotope (delta N-15 and delta C-13) ratios in hepatic and muscular tissues to understand how they co-exist in the northern Rio de Janeiro State (21 degrees 30'S-22 degrees 15'S), south-eastern Brazil. Dolphin specimens were incidentally captured through commercial gillnet fisheries, while fish specimens were the target of these fisheries. The predators had similar delta N-15 values in the liver (P. blainvillei: 14.6 +/- 1.0 parts per thousand; T lepturus: 14.4 +/- 0.6 parts per thousand), which indicates similar trophic position in the recent food intake. However, P blainvillei showed lighter delta N-15 values in muscle (13.8 +/- 1.0 parts per thousand) than T. lepturus (15.0 +/- 0.4 parts per thousand), revealing differences in the long term diet that could be related with the prey size ingested. The delta C-13 signatures showed a preferential inshore and benthic food chain for P. blainvillei (liver:-15.2 +/- 0.6 parts per thousand; muscle: -16.0 +/- 0.5 parts per thousand) and a more offshore and pelagic food chain for T lepturus (liver: -17.2 +/- 0.6 parts per thousand; muscle: -16.8 +/- 0.3 parts per thousand). The isotopic variances of liver and muscle revealed a broader niche width to P. blainvillei in relation to T. lepturus, with a greater use of available food resources in coastal waters. In the area of study, the differences in habitat use and exploitation of food resources are favoring the predators' coexistence.