Steller sea lion foraging response to seasonal changes in prey availability

Abstract

We hypothesized that: (1) Steller sea lion Eumetopias jubatus diet choice is a function of prey availability, (2) sea lions move to take advantage of times and locations of seasonal prey concentrations and (3) the number present depends on the amount of prey available (numerical response). Over 3 yr, typically on a quarterly basis, in Frederick Sound, SE Alaska, multiple measurements were taken of Steller sea lion abundance (aerial surveys), diet (scats), dive behavior (satellite telemetry) and prey availability and caloric density (nearshore, pelagic and demersal fish surveys). We found that Steller sea lions shifted diet composition in response to changes in prey availability of pollock Theragra chalcogramma, hake Merluccius productus, herring Clupea pallasi and salmon Oncorhynchus spp. They selected intermediate-sized fish and avoided small (<10 cm) and large (> 60 cm) fish, and moved between areas as prey became available seasonally. The number of sea lions present depended on the amount of prey available; a standing biomass of 500 to 1700 t of prey in a nonbreeding area such as Frederick Sound, depending on species composition, can attract and sustain about 500 sea lions. Pollock was more frequent in sea lion diet in inside waters of SE Alaska -including Frederick Sound, Stephens Passage and Lynn Canal -than anywhere else in Alaska and contributed ~1⁄ 3 of the dietary energy in Frederick Sound. This finding implies that a diet with substantial year-round contributions from less nutritious, but abundant prey such as pollock can form part of a healthy diet as long as more nutritious prey such as herring, salmon or eulachon Thaleichthys pacificus also are consumed. Our study supports the conclusion that the Steller sea lion is an opportunistic marine predator with a flexible foraging strategy that selects abundant, accessible prey and shifts among seasonally available species.

Year-round algal toxin exposure in free-ranging sea lions

“…Sea lions change their diet based on seasonally and locally abundant prey and may migrate in response to seasonal prey fluctuations (Sigler et al 2009). The diets of both Steller and California sea lions, composed of a variety of bony and cartilaginous fishes and cephalopods, are often dominated by only a few species (Sinclair & Zeppelin 2002, Orr et al 2011, although as many as 25 different prey items have been found in a single scat (Riemer et al 2011).…”

Contrasting effects of food quality and quantity on a marine top predator

“…The 'junk-food hypothesis' predicts neg-ative impacts on predators from reduced prey quality and was originally suggested as an explanation for the decline of Steller sea lion Eumetopias jubatus in the Gulf of Alaska (Alverson 1992). The hypothesis has been questioned, but is discussed as one of several likely mechanisms contributing to the lack of recovery in this population (Trites & Donnelly 2003, Fritz & Hinckley 2005, Sigler et al 2009). …”

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

“…Eastern Bering Sea pollock fisheries would also be a possible candidate for integrating cooperative acoustic surveys and local management to mitigate hypothesized competitive interactions with northern fur seals (Callorhinus ursinus) [43] that could be a factor in its continued population decline [44]. There has been considerable research on nutritional requirements for both Steller sea lions and northern fur seals [30,[45][46][47] and the catch accounting system for bottom fish described for the Aleutian Islands [11] is similarly implemented in this fishery.…”