Prey competition between sympatric Steller sea lions (Eumetopias jubatus) and northern fur seals (Callorhinus ursinus) on Lovushki Island, Russia

Abstract

Approximately 1 000 Steller sea lions ( Eumetopias jubatus (Schreber, 1776); SSL) and 14 000 northern fur seals ( Callorhinus ursinus (L., 1758); NFS) breed sympatrically on Lovushki Island in the Russian Far East, creating the potential for interspecific competition for prey. An additional 13 000 – 14 000 juvenile NFS are present during the breeding season. The diets of breeding SSL and both breeding and juvenile NFS were examined through analysis of scats and spews collected during the breeding seasons of 2003, 2005, and 2007–2008. There were significant overlaps in the prey species and size selection of SSL and juvenile NFS. There were significant differences between the diets of SSL and breeding NFS. SSL and juvenile NFS fed primarily on Atka mackerel ( Pleurogrammus monopterygius (Pallas, 1810)), while breeding NFS fed on cephalopods, salmon (genus Oncorhynchus Suckley, 1861), Atka mackerel, and northern smoothtongue ( Leuroglossus schmidti Rass, 1955). The partitioning of resources between breeding animals has allowed both species to coexist within the same region and likely reflected differences in foraging abilities and provisioning strategies of the adults and the fasting abilities of their pups. However, continued growth of the NFS population may lead to the exclusion of SSL owing to interspecific competition for prey.

Influence of environment, morphology, and instrument size on lactating northern fur seal Callorhinus ursinus foraging behavior on the Lovushki Islands, Russia

“…Most prominently, females dove deeper during periods of high nighttime moonlight. Similar to reports at other portions of the species' range, vertically migrating prey species appear to make up a substantial portion of female northern fur seal diet on Lovushki (Waite et al 2012). Therefore, the simplest explanation for this effect was that females were tracking changes in the distribution of their prey (Croxall et al 1985, Horning & Trillmich 1999, Ream et al 2005, Lea et al 2010.…”

Environmental and biological factors influencing maternal attendance patterns of Steller sea lions (Eumetopias jubatus) in Russia

“…), pollock (Theragra chalcogramma), and Atka mackerel (Pleurogrammus monopterygius), with Pacific herring (Clupea pallasii) and northern smoothtongue (Leuroglossus schmidti) within the top 3 prey items on some islands (Table 4). Considerable variability existed among the islands, with Atka mackerel predominating on Lovushki (Waite 2010;Waite and Burkanov 2006) and, with a smaller margin, on Brat Chirpoev. Pollock was the main prey item by a wide margin on Yamsky.…”

Foraging behavior of lactating northern fur seals (Callorhinus ursinus) in the Commander Islands, Russia

“…Blokhin at Kamchatka Research Institute of Fisheries and Oceanography identified prey species to the lowest possible taxonomic group. Finally, for each prey type, we calculated the frequency of occurrence as FO = (N i /N t ) 9 100, where N i is the number of samples containing prey type i and N t is the total number of samples (Waite et al 2012).…”