Trophic relationships in apex predators in an estuary system: A multiple-method approximation

The trophic level and ecological role of species in the ecosystem are determined by their feeding habits. While the trophic level and niche width are characteristic at population level, trophic overlap indicates the interaction between populations of a community. Three apex predators coexist in Uruguayan waters: Franciscana dolphin (Pontoporia blainvillei), South American sea lion (SASL, Otaria flavescens) and South American fur seal (SAFS, Arctocephalus australis). The two otariids consume different main prey species and have different isotopic niche, whereas feeding habits of Franciscana dolphin in Uruguayan waters remain practically unknown. This study aimed to determine Franciscana diet composition by stomach content and stable isotope (δ13C and δ15N) analyses, and to assess trophic overlap in diet composition as well as in isotopic niche space among the three apex predators. Diet composition of each species was estimated by Bayesian mixing models with prior information on the diet of predators. Moreover, Bayesian ellipses were used to define the isotopic niche space and overlap among the three species. Diets among predators varied: diet of SAFS was composed mainly by a few pelagic prey species, diet of SASL was more diverse and included pelagic and benthic preys, and for Franciscana dolphin the diet was mainly composed by coastal prey species. The SAFS had the largest isotopic niche area compared to the Franciscana and the SASL. The isotopic niche of the SASL did not overlap with those of the other two species. Although the Franciscana and the SAFS overlap in isotopic space, they do not overlap in feeding areas; this is because particulate organic matter δ13C values increase from inner Rio de la Plata to the estuary and the Atlantic Coast. Here, the relevance of using different sources of information to reach strong conclusions about feeding habits of marine predators and about co-use of resources is shown, mainly in environments where variation in δ13C can be related to diverse reasons.