Trites et a\. Diets oi Eumetopios /ubotus in Southeast Alaska 



237 



species not in the reference skeleton collection at the 

 time of identification or could have been too far digested 

 to be identifiable. 



We grouped the identified species of prey into eight 

 categories for statistical analysis. These included ga- 

 dids, forage fish, salmon (Salmonidae), flatfish, rockfish 

 iSebastes spp.), cephalopods, hexagrammids, and other 

 prey (Fig. 2). Scats that contained more than one spe- 

 cies from a particular group were scored as containing 

 only a single occurrence of that group. For example, a 

 scat containing both Pacific herring (Clupea pallasii) 

 and sand lance was scored as having a single occur- 

 rence of forage fish. Hexagrammids do not inhabit the 

 waters of Southeast Alaska in significant numbers but 

 were included as a prey category so that diets could 

 be compared across regions of the North Pacific where 

 hexagrammids are consumed in greater numbers (Mer- 

 rick et al., 1997; Sinclair and Zeppelin, 2002). 



The diversity of the diet was calculated for the eight 

 prey groups by using the Shannon-Wiener species di- 

 versity index (Rickleffs and Miller, 2000), which yields 

 a value between 1 and 8, where a value of 1 indicates 

 that only one of the eight groups was consumed, and 

 a value of 8 indicates that all eight were equally con- 

 sumed. Merrick et al. (1997) used this index to de- 

 termine the dietary diversity of Steller sea lions that 

 consumed seven prey groups in the Gulf of Alaska and 

 Aleutian Islands. We therefore pooled rockfish with 

 other prey to create the same seven categories used by 

 Merrick et al. (1997) to compare the diversity of diet 

 across all regions of Alaska. We compared our estimate 

 of dietary diversity to those presented by Merrick et al. 

 (1997) for diet data collected between 1990 and 1995. 

 However, we recalculated the diet diversities presented 

 in their paper (from their split-sample frequency of oc- 



currence data) because of a calculation error in their 

 published values. 



Seasonal diets were calculated for rookeries in sum- 

 mer (Forrester Island. Jun-Aug, 1993-99) and haulouts 

 in fall (Sep-Nov, 1993 and 1995-96), winter (Dec-Feb, 

 1996-1997), and spring (Mar-May 1996). Average sum- 

 mer diet (Jun-Aug) was calculated from the three rook- 

 eries — weighted by the average number of pups counted 

 at each site during 1993-1997 (pup counts serving as 

 an index of population size; Trites and Larkin, 1996; 

 Pitcher et al., 2007). The summer data were weighted 

 to indicate what the average Steller sea lion ate in 

 Southeast Alaska, rather than to describe what the 

 average rookery diet was. Fall, winter, and spring diets 

 were given equal weight and averaged to describe the 

 nonsummer diet (haulouts, Sep-May) because animals 

 are more evenly distributed during the nonbreeding 

 season and haulout counts were not available for each 

 of the seasons. 



The relative importance of prey in the diet was quan- 

 tified as "simple" and "split-sample" frequency of occur- 

 rences. The simple frequency of occurrence indicates 

 what proportion of scats contains any particular prey 

 type. They do not sum to 100%. For example, 80% of 

 the scats examined may contain gadids, and 50% may 

 contain forage fish — meaning that some scats contained 

 both prey types, and others contained only gadids or 

 only forage fish. The second method we used, the split- 

 sample frequency of occurrence (Olesiuk et al., 1990; 

 Olesiuk, 1993), yields the proportion of the overall diet 

 made up of any single prey type. These proportions do 

 sum to 100%. With the split-sample method, it is as- 

 sumed that the scat contained remains from all prey 

 consumed in the previous meal and that the prey were 

 consumed in equal volumes. 



