234 



Abstract — The diet of Steller sea lions 

 (Eumetopias jubatus) was determined 

 from 1494 scats (feces) collected at 

 breeding (rookeries) and nonbreeding 

 (haulout) sites in Southeast Alaska 

 from 1993 to 1999. The most common 

 prey of 61 species identified were wall- 

 eye pollock (Theragra chalcogramma) , 

 Pacific herring (Clupea pallasii). 

 Pacific sand lance {Ammodytes hexa- 

 pterus). Pacific salmon (Salmonidae), 

 arrowtooth flounder {Atheresthes sto- 

 mias). rockfish [Sebastes spp.), skates 

 (Rajidae), and cephalopods (squid 

 and octopus). Steller sea lion diets 

 at the three Southeast Alaska rook- 

 eries differed significantly from one 

 another. The sea lions consumed the 

 most diverse range of prey catego- 

 ries during summer, and the least 

 diverse during fall. Diet was more 

 diverse in Southeast Alaska during 

 the 1990s than in any other region of 

 Alaska (Gulf of Alaska and Aleutian 

 Islands). Dietary differences between 

 increasing and declining populations 

 of Steller sea lions in Alaska correlate 

 with rates of population change, and 

 add credence to the view that diet may 

 have played a role in the decline of 

 sea lions in the Gulf of Alaska and 

 Aleutian Islands. 



Diets of Steller sea lions iEumetopias jubatus) 

 in Southeast Alaska^ 1993-1999 



Andrew W. Trites' 

 Donald G. Calkins^ 

 Arliss J. Winship^ 



Email address for A. W. Trites: tritese'zoology.ubc.ca 



' Marine Mammal Research Unit, Fisheries Centre 

 Room 247, AERL - Aquatic Ecosystems Research Laboratory 

 2202 Main Mall, University of British Columbia 

 Vancouver, BC, Canada V6T 1Z4 



2 Alaska Department of Fish and Game 

 333 Raspberry Road 

 Anchorage, Alaska 99518-1599 



Manuscript submitted 10 January 2006 

 to the Scientific Edfitor's Office. 



Manuscript accepted: 6 October 2006 



by the Scientific Editor. 



Fish. Bull. 105:234-248 (2007).234 



Steller sea lion iEumetopias jubatus) 

 populations in the Aleutian Islands 

 and Gulf of Alaska began declining 

 in the mid-1970s and were listed as 

 endangered under the U.S. Endan- 

 gered Species Act in 1997 (NMFSi; 

 Trites and Larkin, 1996; Loughlin, 

 1998). The cause of the population 

 decline is uncertain but may be linked 

 to a decrease in the quantity, quality, 

 or availability of prey, in turn caused 

 either by commercial fisheries or by 

 a natural change in the ecosystem 

 (Alaska Sea Grant, 1993; DeMaster 

 and Atkinson, 2002; Trites et al., 

 2007). Stomach contents and scat 

 analysis indicate that the diets of 

 the declining population may have 

 changed from primarily small, fatty, 

 schooling fishes (such as capelin (Mal- 

 lotus villosus) and sand lance {Ammo- 

 dytes hexapterus)) in the 1950s to one 

 increasingly dominated by walleye pol- 

 lock {Theragra chalcogramma), Atka 

 mackerel {Pleurogrammus monopter- 

 ygius), and flatfish (Pleuronectidae) in 

 the 1970s, 1980s, and 1990s (Mathi- 

 sen et al., 1962; Thorsteinson and 

 Lensink, 1962; Pitcher, 1981; Calkins 

 and Goodwin^; Merrick et al., 1997; 

 Sinclair and Zeppelin, 2002).- 



Merrick et al. (1997) found a posi- 

 tive relationship between the rate of 

 population change and the diversity 

 of summer Steller sea lion diets in the 

 declining population during the early 

 1990s. Regions that had the highest 



rates of decline had the lowest diversi- 

 ties of diet. The greater the diet diver- 

 sity, the slower the rate of population 

 decline. Additional diet data (through 

 to 2001) supported the conclusion 

 that diet diversity had some influence 

 on population success (Sinclair and 

 Zeppelin, 2002; Sinclair et al., 2005). 

 Merrick et al. (1997) hypothesized 

 that animals with less diverse diets 

 may have experienced difficulty ob- 

 taining enough prey. Others have hy- 

 pothesized that consumption of larger 

 proportions of lower energy-dense prey 

 may have exacerbated the effect of 

 diet diversity by increasing the food 

 requirements of sea lions (Alverson, 

 1992; Rosen and Trites, 2000; Trites 

 and Donnelly, 2003). Sea lions with 

 less diverse, low energy-dense diets 

 may also have been more sensitive to 

 changes in overall prey abundance, 

 and could have theoretically incurred 

 higher rates of predation from killer 



1 NMFS (National Marine Fisheries Ser- 

 vice). 1992. Recovery plan for the 

 Steller sea lion {Eumetopias jubatus), 

 92 p. Prepared by the Steller Sea Lion 

 Recovery Team for the National Marine 

 Fisheries Service, 1315 East-West High- 

 way, Silver Spring, MD 20910-3282. 



2 Calkins, D. G., and E. Goodwin. 1988. 

 Investigation of the declining sea lion 

 population in the Gulf of Alaska, 76 p. 

 Unpublished report. Alaska Department 

 of Fish and Game, 333 Raspberry Road, 

 Anchorage, AK 99518-1599. 



