509 



Abstract— Prey-size selectivity by 

 Steller sea lions lEumetopias juba- 

 tus) is relevant for understanding 

 the foraging behavior of this declin- 

 ing predator, but studies have been 

 problematic because of the absence 

 and erosion of otoliths usually used 

 to estimate fish length. Therefore, 

 we developed regression formulae to 

 estimate fish length from seven diag- 

 nostic cranial structures of walleye 

 pollock (Theragra chalcogramma) 

 and Atka mackerel (Pleurogrammus 

 monopterygius). For both species, 

 all structure measurements were 

 related with fork length of prey (r 2 

 range: 0.78-0.99). Fork length (FL) 

 of walleye pollock and Atka mackerel 

 consumed by Steller sea lions was 

 estimated by applying these regres- 

 sion models to cranial structures 

 recovered from scats (feces) collected 

 between 1998 and 2000 across the 

 range of the Alaskan western stock 

 of Steller sea lions. Experimentally 

 derived digestion correction factors 

 were applied to take into account loss 

 of size due to digestion. Fork lengths 

 of walleye pollock consumed by Steller 

 sea lions ranged from 3.7 to 70.8 cm 

 (mean=39.3 cm, SD = 14.3 cm, n = 666l 

 and Atka mackerel ranged from 15.3 

 to 49.6 cm (mean = 32.3 cm, SD = 

 5.9 cm, rc = 1685). Although sample 

 sizes were limited, a greater propor- 

 tion of juvenile (<20 cm) walleye pol- 

 lock were found in samples collected 

 during the summer (June-September) 

 on haul-out sites (64^ juveniles, ;;=11 

 scats) than on summer rookeries (9% 

 juveniles, n = 132 scats) or winter 

 l February-March) haul-out sites 

 (3% juveniles, n = 69 scats). Annual 

 changes in the size of Atka mackerel 

 consumed by Steller sea lions cor- 

 responded to changes in the length 

 distribution of Atka mackerel result- 

 ing from exceptionally strong year 

 classes. Considerable overlap (>51%) 

 in the size of walleye pollock and Atka 

 mackerel taken by Steller sea lions 

 and the sizes of these species caught 

 by the commercial trawl fishery were 

 demonstrated. 



Sizes of walleye pollock 



(Theragra chalcogramma) and Atka mackerel 

 (Pleurogrammus monopterygius) consumed by 

 the western stock of Steller sea lions 

 (Eumetopias jubatus) in Alaska from 1998 to 2000 



Tonya K. Zeppelin' 

 Dominic J. Tollit 2 

 Katherine A. Call 1 

 Trevor J. Orchard 3 

 Carolyn J. Gudmundson' 



E-mail address: Tonya Zeppelin ifflnoaa gov 



1 National Marine Mammal Laboratory 

 Alaska Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE 



Seattle, Washington 98115 



2 Marine Mammal Research Unit 

 Fisheries Center, Room 18, Hut B-3 

 University ot British Columbia 

 6248 Biological Sciences Road 

 Vancouver, British Columbia, Canada V6T 1Z4 



3 Department of Anthropology 

 University of Toronto 



100 St. George Street 



Toronto, Ontario, Canada M5S 3G3 



Manuscript submitted 28 April 2003 

 to Scientific Editor's Office. 



Manuscript approved for publication 

 25 March 2004 by the Scientific Editor. 



Fish. Bull. 102:509-521 (2004). 



The western stock of Steller sea lions 

 (Eumetopias jubatus) in the Gulf of 

 Alaska and the Bering Sea has experi- 

 enced dramatic and continued declines 

 since the mid-1970s (Loughlin et al„ 

 1992; Loughlin and York, 2000). It is 

 likely that changes in prey availabil- 

 ity linked to commercial fisheries and 

 large-scale oceanographic changes are 

 among the reasons for the continued 

 decline (Loughlin and Merrick, 1989; 

 NRC, 1996). The diet of the western 

 stock of Steller sea lions has been 

 recently assessed (Sinclair and Zep- 

 pelin, 2002), but discrete selection of 

 prey by size has not been described. 

 The size of prey is relevant for under- 

 standing the foraging behavior of the 

 predator as well as the ecological role 

 of the prey (e.g., mortality at a given 

 life history stage). In the case of the 

 Steller sea lion, prey-size selectivity is 

 particularly important for understand- 

 ing spatial and temporal changes in 



diet and is needed for making fishery 

 management decisions. 



Size of fish prey consumed by ma- 

 rine mammals has been estimated 

 by using sagittal otoliths recovered 

 from stomach and more recently scat 

 samples (Pitcher, 1981; Frost and 

 Lowry, 1986; Browne et al., 2002). 

 Significant relationships have been 

 demonstrated between fish fork length 

 (FL) and otolith length (Templeman 

 and Squires, 1956; Frost and Low- 

 ry, 1981; Harvey et al., 2000). The 

 use of otoliths to describe the size of 

 prey taken by Steller sea lions has 

 proved useful in data collected from 

 stomach samples (e.g., Pitcher, 1981; 

 Calkins and Goodwin 1 ). However, few 



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 Road, 

 Anchorage, Alaska, 99518-1599. 



