Sexual foraging segregation in South American sea lions increases during the pre-breeding period in the Río de la Plata plume

Abstract

Stable carbon and nitrogen isotopes in skin and bone of South American sea lions from Brazil and Uruguay were analysed to test the hypothesis that trophic overlap between the sexes is lower during the pre-breeding season than throughout the rest of the year. The isotopic values of skin and bone were used to infer the trophic relationships between the sexes during the pre-breeding period and year round, respectively. Prey species were also analysed to establish a baseline necessary for interpreting the stable isotope ratios of skin and bone. Standard ellipse areas, estimated using Bayesian inference in the SIBER routine of the SIAR package in R, suggested that males and females used a wide diversity of foraging strategies throughout the year and that no differences existed between the sexes. However, the diversity of foraging strategies was largely reduced during the pre-breeding period, with all the individuals of each sex adopting similar strategies, but with the two sexes differing considerably in stable isotope values and the ellipse areas of males and females not overlapping at all. Nevertheless, the results revealed a general increase in the consumption of pelagic prey by both sexes during the pre-breeding period. The progressive crowding of individuals in the areas surrounding the breeding rookeries during the pre-breeding period could lead to an increase in the local population density, which could explain the above reported changes.

Isotopic niche partitioning between two apex predators over time

“…; Drago et al . ). It should be noted that fishes from southern Brazil have usually lower δ 13 C ratios and higher δ 15 N ratios when compared with the same species from the Río de la Plata estuary (Vales et al .…”

“…; Drago et al . ). Nevertheless, demersal fishes of both regions have higher δ 13 C ratios than any other group of potential prey, and small pelagic fishes from both regions have lower δ 15 N ratios than any other group (Vales et al .…”

“…; Drago et al . ). Accordingly, a general increase in the consumption of demersal fish should produce a simultaneous increase in both the δ 13 C and δ 15 N values of eared seals.…”

“…Anchovies are pelagic, with a total length of less than 15 cm, and they forage on plankton; whereas striped weakfish, which can reach 50 cm in length, are demersal and forage on both pelagic and demersal fishes (Cousseau & Perrotta 2000). As a result, anchovies have lower d 13 C and d 15 N values than striped weakfish across the estuary and in southern Brazil Drago et al 2015). It should be noted that fishes from southern Brazil have usually lower d 13 C ratios and higher d 15 N ratios when compared with the same species from the R ıo de la Plata estuary Drago et al 2015).…”

Lipid extraction in stable isotope analyses of juvenile sea turtle skin and muscle

“…seabirds, Cherel et al 2005; sea turtles, Arthur et al 2008;sharks, Carlisle et al 2015; marine mammals, Drago et al 2015). The method is based on the principle that the isotopic composition in the tissues of consumers reflects the mixture of the isotopic composition present in the different dietary items consumed (DeNiro & Epstein 1978, 1981.…”