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Fishery Bulletin 102(3) 





otoliths are recovered from Steller sea lion scat, and 

 measurements of otoliths recovered from scats likely 

 underestimate prey size because of partial erosion 

 from digestion (Prime and Hammond, 1987; Del- 

 linger and Trillmich, 1988; Harvey, 1989). Because 

 of the impracticality of collecting stomachs and the 

 low number and poor quality of otoliths found in 

 scats, alternative methods are needed to accurately 

 describe the size of prey consumed by Steller sea 

 lions. 



Archaeological studies routinely use skeletal struc- 

 tures other than otoliths to estimate either fish 

 length or mass (Keys, 1928; Casteel, 1976; Owen 

 and Merrick, 1994; Desse and Desse-Berset, 1996). 

 Wise 11980) used a regression offish length on ver- 

 tebrae length to estimate prey size from scat samples 

 of otters (Lutra lutra) and mink tMustela vison). 

 The regression approach relies on the assumption 

 that the overall size of a given fish and the size of 

 skeletal structures are highly correlated. This as- 

 sumption has been substantiated for cranial and 

 skeletal structures other than otoliths in various 

 North Pacific fish species (Orchard, 2001). Thus, the 

 use of cranial structures appear to be a viable alter- 

 native to the use of otoliths for studying prey size of 

 Steller sea lions. 



Walleye pollock (Theragra chalcogramma) and At- 

 ka mackerel (Pleurogrammus monopterygius) rank 

 among the top prey items of Steller sea lions (Sin- 

 clair and Zeppelin, 2002) as well as being valuable 

 in the U.S. commercial fishery (NMFS, 2003). We 

 estimated fork length for these two primary prey 

 species from scats collected between 1998 and 2000 

 across the range of the Alaskan western stock of sea 

 lions. Fish length was estimated by using regres- 

 sion formulae relating bone or otolith measurement 

 to fork length for seven cranial structures found in 

 sufficient quantities and in good and fair condition in 

 scat samples. Experimentally derived digestion cor- 

 rection factors (Tollit et al., 2004b, this issue) were 

 applied to bone and otolith measurements to account 

 for loss of size due to erosion. The methods developed 

 here proved to be an effective tool to estimate size of 

 prey selected by Steller sea lions and are applicable 

 for other marine mammal diet studies particularly 

 where otoliths are highly eroded. 



Materials and methods 



Development of regression formulae 



Fork-length to bone and otolith-length regression 

 equations were developed for seven cranial struc- 

 tures from walleye pollock and Atka mackerel. Bones 

 and otoliths were selected according to species-specific 

 features, predictability in condition, and prevalence 

 in scats. Bones included the angular (ANG), quadrate 

 (QUAD), interhyal (INTE), dentary (DENT), pharyn- 

 gobranchial 2 (PHAR), and hypobranchial 3 (HYPO) 



A Walleye pollock 



Pharyngobranchial #2 



Quadrate 



Interhyal 



B Atka mackerel 



Otolith 



I 



Hypobranchial #3 



Pharyngobranchial #2 



Interhyal 



Otolith 



Figure 1 



Illustrations of the various planes for bone and otolith 

 measurements used to solve the bone-length to fish-length 

 regression equations for (A) walleye pollock and (Bl Atka 

 mackerel. The structures from the right side of the body 

 are shown for all structures except for quadrates. 



(Fig. 1). Fork length regressions were developed for sagit- 

 tal otolith length (OTOL), as well as for width (OTOW) 

 measurements. All selected cranial structures were 

 paired (having a left and right side) which allowed for 

 enumeration of prey species. Only right-sided bones and 

 otoliths were used to develop the regression equations. 



