Isotopic evidence of structuring killer whale groups along the Southwestern South Atlantic Ocean

LOIZAGA DE CASTRO, ROCÍO; VALES, DAMIÁN G.; GARCÍA, NÉSTOR ANÍBAL; CRESPO, ENRIQUE ALBERTO

Viña del Mar

Conferencia; 11th ISOECOL, International Conference on Applications of Stable Isotope Techniques to Ecological Studies; 2018

Universidad Andrés Bello

Killer whales (Orcinus orca) are large marine top predators that inhabit all oceans and are highly mobile. Despite being the most conspicuous top predator in the Southwestern Atlantic Ocean (SWAO), little is known about their ecology in the region. Here, we used carbon (δ13C) and nitrogen (δ15N) isotope analysis of bone collagen (n= 10) and dentine tooth (n= 3) from killer whales stranded between 28°S to 45°S during the period 1970-2014 to assess the trophic ecology of this species over the Patagonian shelf. Muscle and skin of potential prey from the region were also isotopically analyzed. Isotopic values of paired bone and tooth samples from 4 individuals were not significantly different, hence samples were pooled. After correcting δ13C values to account for the Suess effect, Discriminant Analysis of Principal Components identified three distinct groups. Group 1 (Patagonian group, n= 8) included animals with high δ13C and δ15N values that after correcting for trophic discrimination factor (δ13C= 4.5, δ15N= 5) fitted with isotopic ratios of their potential prey over the Patagonian shelf, mainly pinnipeds, cetaceans, sharks and large bony fishes. In contrast, the two remaining groups had δ13C and δ15N values that do not correspond with the potential prey from the Patagonian shelf, suggesting that they were foraging mainly in adjacent ecosystems. Group 2 (Subtropical group, n= 3) is characterized by individuals with low δ15N and high δ13C values that corresponded with the isotopic values reported for killer whales stranded in southern Brazil. Group 3 (Subantarctic/Antarctic group, n= 2) were individuals with low δ13C and δ15N values, which are typical from high latitude waters. The presence of killer whales trophic groups over the Patagonian shelf suggests the existence of structuring subpopulations exploiting specific habitats. Moreover, one of the individuals from the Patagonian group shares haplotype with conspecific from Subantactic/Antarctic waters, revealing some degree of connectivity between populations among these large marine ecosystems. Nonetheless, population genetic studies are needed to confirm the population structure and migration along SWAO. Our results further the scarce ecological knowledge for the species in the region, while presents the basis to infer more complex ecological hypothesis.