Linking Pinniped Foraging Ecology to Ephemeral and Seasonal Aggregations of Forage Fish

Forage fish are of high energetic quality, densely aggregated, and often predictable in space and time.  As such the foraging ecology of some pinnipeds and ultimately their fitness are thought to be directly coupled with the seasonal availability of forage fish.  Life-history strategies vary along phylogenetic lines within the pinnipeds and may influence the numerical and functional responses of pinniped species to changes in the abundance and distribution of forage fish prey. Otariids (eared seals and sea lions) are income breeders that rely upon intermittent foraging during an extended lactation period (up to 3 years) and as a result are highly dependent upon local prey availability given the extended parental care period.  In contrast, phocids (earless seals) are capital breeders that rely primarily upon stored energy to nourish young during a brief lactation period (~4-45 days) and are less-constrained than otariids during the non-breeding phase of their annual cycle.  We compare the life-history strategies of two sympatric pinnipeds, harbor seals (Phoca vitulina richardii) (phocid) and Steller sea lions (Eumetopias jubatus) (otariid), and describe individual and population-level responses of these two species to ephemeral and seasonal aggregations of forage fish in southeastern Alaska.  Seasonal attendance patterns and diets of Steller sea lions were associated with the seasonal availability of prey throughout the year, particularly aggregations of over-wintering herring and spring-spawning eulachon, herring, and capelin.  Satellite-linked telemetry of individual animals revealed that prior to the breeding season, in April and May when energy demands are highest, both harbor seals and Steller sea lions were associated with ephemeral aggregations of spring-spawning forage fish.  Ephemeral and seasonally predictable aggregations of forage fish may be of substantial significance to the nutrition and energy budgets of pinnipeds when energy demands are at a peak during the annual cycle.  Ultimately, it is important to consider how life-history constraints differ among pinniped species when attempting to predict how individuals may respond to changes in forage fish availability.