Sea lions in drag, fur seals incognito: Insights from the Otariid deviants

Sea lions in drag, fur seals incognito: Insights from the Otariid deviants

J.P.Y. Arnould, and D.P. Costa

Sea lions in drag, fur seals incognito: Insights from the Otariid deviantsThis is part of Sea Lions of the World
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Sea lions and fur seals have two broadly divergent foraging patterns. Lactating sea lions generally undertake short trips (1-2 days) foraging mostly on the benthos of continental shelf areas. In contrast, lactating fur seals generally undertake longer trips (4-23 days) foraging mostly on vertically migrating prey in oceanic frontal structures or continental shelf-edges with upwelling regions. Associated with the observed divergent trends of epipelagic and benthic foraging appear to be differences in the population dynamics of sea lions and fur seals. Populations of the various sea lion species have experienced little recovery since the sealing era, whereas fur seals have generally experienced rapid population recovery rates. The divergent patterns of foraging between the two otariid groups were originally thought to be due to the mode of insulation and diving ability. Subsequent studies, however, have shown that some fur seal species regularly forage at pelagic depths deeper and longer than some sea lions. Alternatively, the larger body size of sea lions may make foraging on small pelagic prey energy-inefficient and, hence, may explain why throughout most of their distribution sea lions have adopted the benthic foraging mode. Indeed, exceptions to the general fur seal and sea lion foraging patterns have been documented, which may be related to the productivity of their local marine habitat. California sea lions display epipelagic foraging behavior in the rich California Current, while Australian fur seals have been shown to forage exclusively over the shallow continental shelf of Bass Strait (southeast Australia), a region recognized as being an area of low oceanic productivity. Interestingly, these uncharacteristic foraging modes are associated with population dynamics uncharacteristic for the respective phylogenetic groups: California sea lions have been steadily increasing, while the Australian fur
seal has exhibited a very slow recovery in comparison to the conspecific cape fur seal which feeds epipelagically in the rich Benguela current and is now the most numerous otariid.

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