522 



Abstract— Lengths of walleye pollock 

 i Theragra chalcogramma I consumed 

 by Steller sea lions (Eumetopias 

 jubatus) were estimated by using 

 allometric regressions applied to 

 seven diagnostic cranial structures 

 recovered from 531 scats collected 

 in Southeast Alaska between 1994 

 and 1999. Only elements in good and 

 fair condition were selected. Selected 

 structural measurements were cor- 

 rected for loss of size due to erosion 

 by using experimentally derived 

 condition-specific digestion correc- 

 tion factors. Correcting for digestion 

 increased the estimated length of 

 fish consumed by 23% , and the aver- 

 age mass offish consumed by 88%. 

 Mean corrected fork length (FLl of 

 pollock consumed was 42.4 ±11.6 cm 

 (range = 10. 0-78.1 cm, n=909). Adult 

 pollock (FL>45.0 cm) occurred more 

 frequently in scats collected from 

 rookeries along the open ocean coast- 

 line of Southeast Alaska during June 

 and July (74% adults, mean FL = 48.4 

 cm l than they did in scats from haul- 

 outs located in inside waters between 

 October and May (51% adults, mean 

 FL = 38.4 cm). Overall, the contribu- 

 tion of juvenile pollock (<20 cm) to 

 the sea lion diet was insignificant; 

 whereas adults contributed 44% to 

 the diet by number and 74% by mass. 

 On average, larger pollock were eaten 

 in summer at rookeries throughout 

 Southeast Alaska than at rookeries 

 in the Gulf of Alaska and the Bering 

 Sea. Overall it appears that Steller 

 sea lions are capable of consuming 

 a wide size range of pollock, and the 

 bulk offish fall between 20 and 60 cm. 

 The use of cranial hard parts other 

 than otoliths and the application of 

 digestion correction factors are fun- 

 damental to correctly estimating the 

 sizes of prey consumed by sea lions 

 and determining the extent that these 

 sizes overlap with the sizes of pollock 

 caught by commercial fisheries. 



Sizes of walleye pollock (Theragra chalcogramma) 

 consumed by the eastern stock of Steller sea lions 

 (Eumetopias jubatus) in Southeast Alaska from 

 1994 to 1999 



Dominic J. Tollit 



Susan G. Heaslip 



Andrew W. Trites 



Marine Mammal Research Unit, Fisheries Centre 

 University ol British Columbia 

 Room 18, Hut B-3, 6248 Biological Sciences Road 

 Vancouver, British Columbia, Canada, V6T 1Z4 

 E-mail (for D J Tollit). tollitia'zoology ubc ca 



Manuscript submitted 28 April 2003 

 to Scientific Editor's Office. 



Manuscript approved for publication 

 25 March 2004 by the Scientific Editor. 



Fish. Bull. 102:522-532(2004). 



The dramatic decline of the western 

 population of Steller sea lions (Eume- 

 topias jubatus) in the 1980s (Loughlin 

 et al., 1992; Trites and Larkin, 1996) 

 prompted a number of studies to deter- 

 mine what they eat and the extent of 

 overlap of the fish consumed by Steller 

 sea lions and fish caught by commer- 

 cial fisheries. The eastern population 

 of sea lions (east of longitude 144°) 

 located mainly in Southeast Alaska 

 and British Columbia gradually 

 increased as the western population 

 declined (e.g.. Calkins et al., 1999), 

 permitting insightful comparative 

 studies to be undertaken (e.g., Mer- 

 rick et al., 1995; Milette and Trites, 

 2003). Possible explanations for the 

 different population trends include 

 ocean climate, competition with fish- 

 eries, predation, and the amount or 

 the sizes of pollock in the diets of sea 

 lions in the two regions (Loughlin 

 and York, 2000; Benson and Trites, 

 2002; NRC, 2003; Trites and Don- 

 nelly, 2003; Calkins and Goodwin 1 ; 

 Loughlin and Merrick-). 



Reliable estimates of prey size are 

 important not only to investigate prey 

 selectivity and the extent of overlap 

 in size of prey with size of the same 

 species caught by commercial fisher- 

 ies and by other marine piscivores 

 but are also vital for accurately as- 

 sessing prey numbers, biomass, and 

 total consumption (Beverton, 1985; 

 Ringrose, 1993; Laake et al., 2002). 

 One means of estimating prey size is 

 to measure hard parts recovered from 

 fecal remains and to apply allometric 



regressions relating fork length to 

 the size of otoliths (Frost and Lowry, 

 1981) and other bones (Zeppelin et al., 

 2004, this issue). However, the extent 

 of digestion incurred by both otoliths 

 and bones as they pass through the 

 digestive tract must be accounted for 

 to ensure that prey size is not un- 

 derestimated (Tollit et al., 2004, this 

 issue). Application of these two steps 

 is integral to correctly estimate the 

 size of prey consumed by Steller sea 

 lions and other pinnipeds. 



The goal of our study was to esti- 

 mate the size of walleye pollock (Ther- 

 agra chalcogramma) consumed by 

 Steller sea lions in Southeast Alaska 

 between 1994 and 1999 by using 

 new methods outlined by Tollit et al. 

 (2004, this issue) and Zeppelin et al. 

 (2004, this issue). Previous size esti- 

 mates for this region of Alaska are 

 based on the analysis of only eight 

 stomachs collected in 1986 i Calkins 



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

 Unpubl. report. Investigation of the 

 declining sea lion population in the Gulf 

 of Alaska, 76 p. Alaska Department 

 of Fish and Game, 333 Raspberry Rd, 

 Anchorage, AK 99518. 



2 Loughlin, T. R., and R.L. Merrick. 

 1989. Comparison of commercial har- 

 vest of walleye pollock and northern sea 

 lion abundance in the Bering Sea and 

 Gulf of Alaska. In Proceedings of the 

 international symposium on the biology 

 and management of w-alleye pollock. 

 Nov. 14-16, 1988, Anchorage, AK, p. 

 679-700. Alaska Sua Grant KVp Sil- 

 01. LIniv. Alaska Fairbanks, Fairbanks. 

 AK 



