Resource Partitioning Among Sympatric Stellar Sea Lions And Northern Fur Seals On Lovushki Island, Russia

Abstract

The competitive exclusion principle maintains that one of two non-interbreeding species occupying the same ecological niche and geographical territory will be displaced if population growth is not the same between species. Steller sea lions (Eumetopias jubatus; SSL) and northern fur seals (Callorhinus ursinus; NFS) breed sympatrically on four rookeries in the Russian Far East, creating the potential for inter-specific competition for limited prey resources. Approximately 1,000 SSL and 14,000 NFS breed on Lovushki Island in the Kuril Island chain. An additional 13,000--14,000 juvenile NFS are present during the breeding season. The partitioning of forage resources among breeding SSL and both breeding and non-breeding NFS from 2003--2008 was examined through analysis of prey remains recovered from scats and spews, stable isotope (SI) analysis of vibrissae, fatty acid (FA) analysis of blubber, and analysis of foraging behavior through satellite-linked telemetry. Analysis of prey remains indicated a biologically significant overlap in the prey species and size selection of SSL and juvenile NFS and significant differences between the diets of SSL and breeding NFS. SSL fed primarily on Atka mackerel, while breeding NFS fed primarily on cephalopods and northern smoothtongue. SI analysis indicated significant differences in the trophic level and relative foraging location. SSL foraged at a higher trophic level, nearshore, and benthically, while NFS foraged at a lower trophic level, offshore, and pelagically. Analysis of FA signatures also suggested significant differences in the relative diets of breeding NFS and SSL. Foraging behavior analysis also indicated that SSL foraged nearshore and benthically and breeding NFS foraged offshore and pelagically. The combination of these four methodologies suggests breeding NFS and SSL partition their forage resources by prey type and prey size, as well as spatially. This partitioning of resources between breeding animals currently allows both species to coexist within the same geographical region and likely reflected the differences in foraging abilities and provisioning strategies of the adults, as well as the fasting abilities of their pups. However, continued growth of the non-breeding NFS population on Lovushki Island may lead to the competitive exclusion of SSL due to inter-specific competition for food resources.