Diet and potential feeding overlap between Trichiurus lepturus ( Osteichthyes : Perciformes ) and Pontoporia blainvillei ( Mammalia : Cetacea ) in northern Rio de Janeiro , Brazil

This study describes the diet and assesses potential overlap in the feeding habits of Trichiurus lepturus Linnaeus, 1758 and Pontoporia blainvilleiGervais & D'Orbigny, 1844 in northern Rio de Janeiro, southeastern Brazil. Fishes were numerically dominant in both diets, followed by cephalopods for P. blainvillei and crustaceans for T. lepturus. Both predators move along similar coastal feeding areas in northern Rio de Janeiro, but our results indicate differences in their resource exploitation, what allows for their coexistence.

This study compares the feeding habits of these predator species, since both coexist in northern Rio de Janeiro and could be trophic competitors.We endavour to contribute to a better understanding of the marine food chain along the Brazilian coastal waters.
All specimens analyzed were collected in northern Rio de Janeiro through gillnet fisheries carried out at Atafona fishing village, between 21º18'S and 22º25'S, from 0.02 to 42.1 nautical miles from shore and in waters 5-70 m deep.Between August 2004 and July 2006, 350 individuals of T. lepturus were collected, and 343 were included in the diet analysis.Only adult specimens, measuring between 100 and 163 cm of total length, were analyzed, due to their hightest likelyood of being potential trophic competitors of P. blainvillei (DI BENEDITTO et al. 2001).Ninety-nine specimens of P. blainvillei collected from September 1989 to December 2005 were analyzed.The total length of the P. blainvillei specimens ranged from 78.2 to 148.0 cm.
Stomachs were separated and their contents rinsed in a sieve of 400 µm to recover the contents.Undigested prey were measured and weighted.The Index of Relative Importance (IRI) (PINKAS et al. 1971) was applied to determine the representative prey species and considered teleosts, cephalopods, and crustaceans as independent prey to reduce under or overestimation of their importance (CLARKE 1986).A non-parametric descriptive statistic, with the median as a central tendency measure, was used for the size, biomass and density (number of individuals) comparison considering the main teleost and cephalopod items consumed by both carnivores.The crustaceans were not considered in our comparisons due to their scarcity in the diet of P. blainvillei.The diets of T. lepturus and P. blainvillei were also compared through the diversity and dominance indexes of Simpson and the Jaccard (qualitative), and Morisita-Horn (quantitative) similarity indexes, described in MAGURRAN (1988).
Trichiurus lepturus and P. blainvillei prey upon neritic species, both pelagic and demersal (Tabs I and II).Teleosts species of Scianidae, Engraulidae and Clupeidae are the most representative items in the diet of both species (Tabs I and II).Cephalopods are more frequent in the diet of P. blainvillei, while the crustaceans dominate in stomach contents of T. lepturus.All favoured fish (DI BENEDITTO et al. 2001), cephalopod (ROPER et al. 1984) and crustacean (DI BENEDITTO et al. 1998, COSTA et al. 2003) species occur all year round in northern Rio de Janeiro.
Results from the non-parametric descriptive statistics are presented on table III.In general, prey species ingested by T. lepturus are larger than those ingested by P. blainvillei.However, the total biomass and density recorded in the stomach contents are lower in T. lepturus.The greater prey selectivity by T. lepturus is possibly associated with its voracity, as reported    (2006).The values of biomass and density (number of individuals), in turn, may reflect differences in the digestion rate, the ability to fill the stomach and/or the nutritional needs of the predators.The diet of P. blainvillei is more diverse than that of T. lepturus.Although prey richness is similar, the high abundance of prey consumed for P. blainvillei may be responsible for this difference.The prey diversity is influenced by the equitable distribution of the fishes consumed by P. blainvillei, resulting in a lower dominance value.For T. lepturus, the opposite pattern is recorded.The two e species show 60% of similarity in their diet.However, the quantitative comparison (abundance) indicates a low diet similarity between them (Tab.IV). can under or over estimate their food preferences.
In northern Rio de Janeiro, the adult specimens of T. lepturus and P. blainvillei exploit coastal waters to obtain their food resources.However, the former feeds preferentially on pelagic species, while the latter feeds along the water column.In general, the prey consumed by both species in northern Rio de Janeiro have low commercial value or are treated as by-catch by the local fisheries.When the preys are commercially valuable, as I. parvipinnis and L. plei, the fisheries targets larger specimens than those consumed by the species studied (COSTA & HAIMOVICI 1990, DI BENEDITTO et al. 1998).This indicates that the prey species can be at pre-recruitment sizes of the fishery grounds.Despite some overlap in their feeding habits, quantitative variations in size, biomass and density of prey consumed indicate differences in their exploitation of resources, allowing for the coexistence of T. lepturus and P. blainvillei in the region.However, additional studies on their feeding habits are still needed in the areas where both species are sympatric in order to confirm this pattern.

Table I .
Prey species consumed by T. lepturus in northern Rio de Janeiro, with the Index of Relative Importance (IRI), numeric frequency (%FN), biomass (%W) and occurrence (%FO).

Table II .
Prey species consumed by P. blainvillei in northern Rio de Janeiro, with the Index of Relative Importance (IRI), numeric frequency (%FN), biomass (%W) and occurrence (%FO).