Seasonal availability of abundant, energy-rich prey influences the abundance and diet of a marine predator, the Steller sea lion Eumetopias jubatus

Abstract

Steller sea lions Eumetopias jubatus are central-place foragers that forage in the marine environment while using terrestrial sites to rest and care for young. Some terrestrial sites are used seasonally; however, the reasons for doing so are not fully understood. We addressed the hypothesis that seasonal availability of prey influences seasonal abundance and diet of sea lions. We quantified monthly prey availability and sea lion abundance and quarterly diet composition at Benjamin Island in SE Alaska (2001)(2002)(2003)(2004). Large numbers of sea lions occupied Benjamin Island during the nonbreeding season from October to April when Pacific herring Clupea pallasii biomass was highest. Herring was the most common species in sea lion diet (frequency of occurrence [FO] = 90%) and comprised over 81% of the available pelagic prey biomass and 96% of the energy encountered during pelagic surveys. Walleye pollock Theragra chalcogramma accounted for 19% of the available prey biomass but was only slightly less common in sea lion diet (FO = 88%) than herring. Herring biomass was correlated with the number of sea lions; in contrast, there was no relationship between pollock biomass and number of sea lions. Several fish species were found in nearshore areas, but were uncommon in sea lion diet. Sea lions consumed the available pelagic prey but little of the available nearshore prey. The FO of herring and pollock in sea lion diet did not differ significantly between seasons; however, the FO of other seasonal prey species differed between seasons. Seasonal occupation of Benjamin Island by sea lions is influenced by seasonally available, densely aggregated, energy-rich prey.

Spatial and temporal variation in the diet of Steller sea lions in the Kodiak Archipelago, 1999 to 2005

“…Although a number of studies have attempted to determine the fine-scale availability of Steller sea lion prey in the vicinity of rookeries and haulouts (e.g. Mueter & Norcross 2000b, Womble & Sigler 2006, most data are limited in spatial scale or species sampled and confounded by the fact that little is still known of the foraging distribution of Steller sea lions.…”

“…Given their extremely high energy content (Anthony et al 2000) and local abundance, herring most likely provide an important seasonally predictable and energy-rich prey source in the nonbreeding season. Overwintering and spawning herring have also been noted as important prey for sea lions at a number of locations in southeast Alaska, where their distribution has been shown to influence the spatial distribution and abundance of Steller sea lions (Womble et al 2005, Womble & Sigler 2006. Fine-scale analysis of the spatial and temporal patterns in the diet of Steller sea lions in relation to trends in their use of terrestrial haulouts would greatly improve our understanding of the foraging behavior of this species and may assist in assessing what proportions of the population are utilizing particular prey resources.…”

“…However, due to insufficient data most models have assumed that diet remains constant throughout the year and is similar over broad geographic areas. Recent diet studies in southeast Alaska have indicated considerable spatial, seasonal, and interannual variability in diet over a relatively fine scale (Womble & Sigler 2006, Trites et al 2007b). However, fine-scale dietary studies are lacking in western Alaska.…”

Exploring local fishery management through cooperative acoustic surveys in the Aleutian Islands

“…While considerable effort has been expended to understand the seasonal nutritional requirements for individual W-SSLs [22][23][24], less is known about the size of local, seasonal prey populations required to support the recovery of W-SSL populations. For instance, the abundance of pollock, Pacific cod, Atka mackerel, and other prey necessary for central Aleutian Islands W-SSLs during the winter and early spring [25,26] is not simply limited to the fish they consume, but also includes the much larger number of fish necessary in the environment to guarantee successful foraging [27][28][29]. Womble et al [27] and Sigler et al [29] concluded that the relationship between Steller sea lion abundance at seasonal haulouts and local prey biomass (e.g.…”

“…For instance, the abundance of pollock, Pacific cod, Atka mackerel, and other prey necessary for central Aleutian Islands W-SSLs during the winter and early spring [25,26] is not simply limited to the fish they consume, but also includes the much larger number of fish necessary in the environment to guarantee successful foraging [27][28][29]. Womble et al [27] and Sigler et al [29] concluded that the relationship between Steller sea lion abundance at seasonal haulouts and local prey biomass (e.g. temporary spawning aggregations) is exponential: more prey energy per sea lion is necessary as the number of sea lions increases as…”