Zeppelin et al.: Sizes of walleye pollock and Atka mackerel consumed by Eumetopias jubatus 



517 



10 



20 



30 



40 



- 25 



20 



B Atka mackerel 



50 60 70 



Overlap 53°o 



80 



15 



10 



10 20 30 



Estimated fork length (cm) 



40 



50 



Figure 5 



Relative frequency histograms of the estimated fork length of wall- 

 eye pollock and Atka mackerel consumed by Steller sea lions (SSL) 

 compared with relative frequency histograms offish caught by the 

 walleye pollock and Atka mackerel commercial trawl fishery. 



mackerel fishery. In the summer the overlap in size of 

 fish consumed by sea lions and the size of fish caught 

 in the pollock fishery is 67% and there is a 51% overlap 

 in the size of fish caught in the Atka mackerel fishery. 

 When seasonal data were pooled, overlap between the 

 size of fish caught in the commercial fishery and the 

 size of fish consumed by sea lions was 68% for walleye 

 pollock (Fig. 5A) and 53% for Atka mackerel (Fig. 5B). 



Discussion 



Regression formulae 



Regressions of cranial structure measurement on fish 

 fork length with the use of multiple structures was 

 an effective tool for estimating size of fish consumed 

 by Steller sea lions. Sample sizes of measurable prey 



remains from scats were enhanced by using a number 

 of cranial structures in addition to otoliths. Body size 

 estimates of only 13.2% of the pollock and 6.9% of the 

 Atka mackerel prey in this study were based on otoliths 

 alone. Fork-length estimates can be considered accurate 

 regardless of which structure was used in the estimate 

 because all r 2 values were high (range: 0.78-0.99). Like- 

 wise errors associated with the application of DCFs 

 were consistent across structures (Tollit et al., 2004b, 

 this issue). Confidence intervals around size estimates 

 generally overlapped across structures; however, it was 

 not surprising that different structures yielded slightly 

 different mean sizes because different bones can origi- 

 nate from different scats. 



The use of multiple cranial structures may also re- 

 duce bias resulting from variability in recovery and 

 passage rates of structures from different species or 

 sizes of fish (Pierce and Boyle, 1991; Browne et al., 



