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

Abstract

Spatial and temporal variation in the diet of Steller sea lions Eumetopias jubatus was investigated using scat (fecal) samples collected from 4 regions in the Kodiak Archipelago. Over 2700 scats with identifiable prey were collected from the northern, eastern, southern and western sides of Kodiak Island from 1999 to 2005. Of 76 prey types identified using hard remains, the most important species in terms of frequency of occurrence and numerical abundance were Pacific sand lance Ammodytes hexapterus, walleye pollock Theragra chalcogramma, arrowtooth flounder Atheresthes stomias, Pacific cod Gadus macrocephalus, salmon Oncorhynchus spp., and Pacific herring Clupea pallasi. Significant differences in diet composition were found among regions, seasons, and years, suggesting that the diet of sea lions is strongly influenced by local and temporal distributions and abundances of prey. Herring dominated scat collections from the west coast and accounted for most (14 to 30%) of the differences in regional diet in spring and winter. Annual variation in diet was relatively low in winter, whereas spring and fall diets varied from year to year, with regional-specific shifts in dominant prey. Results from our study generally agree with diet studies conducted in the 1990s, but differ markedly in the relative importance of the major prey species.

A new method to evaluate the nutritional composition of marine mammal diets from scats applied to harbor seals in the Gulf of Alaska

“…), cephalopods (specifically GPO), salmon, and Pacific cod. In contrast to previous studies of harbor seals and SSL diets from the Kodiak archipelago (McKenzie and Wynne, 2008;Pitcher, 1980), walleye pollock did not contribute largely (b 3%) to the average harbor seal diet. As harbor seals are opportunistic predators, less pollock occurring in the overall diet more recently may reflect a decrease in their local availability (Grigg et al, 2009).…”

“…Steller sea lions (SSL; Eumetopias jubatus) are also known generalist predators that frequently have dietary overlap with harbor seals (Pitcher, 1981). Between 1999 and 2005, SSL around the Kodiak archipelago had diets predominantly consisting of sand lance, walleye pollock, arrowtooth flounder (Atheresthes stomias), and Pacific cod (Gadus macrocephalus) (McKenzie and Wynne, 2008). Because the sample collection period in McKenzie and Wynne (2008) had some overlap with the sample collection period for the present study and due to the potential for dietary overlap between harbor seals and SSL, we expected to see similar prey species dominating the diets of both pinnipeds.…”

“…A Shannon diversity index (SDI), a commonly used index for evaluating diet diversity of pinnipeds (Herreman et al, 2009;McKenzie and Wynne, 2008;Sigler et al, 2009;Sinclair and Zeppelin, 2002;Trites et al, 2007;Waite et al, 2012), was estimated for each collection period within each year as follows:…”

Steller sea lion foraging response to seasonal changes in prey availability

“…gadids such as pollock) was proposed to have caused nutritional stress and contributed to the decline of the western stock of Steller sea lions (Alverson 1992, Trites & Donnelly 2003. In contrast, eastern stock abundance has increased by about 3% yr -1 since the 1970s (Pitcher et al 2007), yet pollock occurred at higher frequency in Steller sea lion scats in inside waters of SE Alaska, including Frederick Sound, Stephens Passage and Lynn Canal (Table 2) (Womble & Sigler 2006, Trites et al 2007, than in the coastal rookeries of SE Alaska (~60%, Trites et al 2007), the central (~40 to 60%) and western (~80%) Gulf of Alaska and eastern (~60%), central and western (~10%) Aleutians (Sinclair & Zeppelin 2002, McKenzie & Wynne 2008. Pollock comprised 34% of the energetic contribution and 39% of the biomass contribution in Frederick Sound, approaching the 45% biomass contribution of pollock to Steller sea lions in the Kodiak area after sea lion abundance began to decline there (Atkinson et al 2008a).…”

“…2). Salmon and pollock were important in the eastern Aleutian Islands and western Gulf of Alaska, while salmon, pollock, forage fish and arrowtooth flounder were dominant in the central Gulf of Alaska, and salmon, pollock and forage fish occurred most frequently in SE Alaska (Table 2) (Sinclair & Zeppelin 2002, Trites et al 2007, McKenzie & Wynne 2008.…”