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Fishery Bulletin 105(2) 



or hunting by native peoples was reduced. Another is 

 that these sea lions may have begun consuming more 

 abundant prey or they had a higher quality diet that 

 enhanced birth and survival rates. Unfortunately there 

 is little or no information to shed light on this impor- 

 tant question. 



There are few data available on fish stocks preyed 

 upon by sea lions or on the diet of Steller sea lions 

 in Southeast Alaska prior to the 1990s. Pollock does 

 not appear to have been abundant in fishery surveys 

 and was not thought to be present in commercially 

 available quantities. However, Imler and Sarber (1947) 

 reported that pollock were found in five of the seven 

 stomachs taken in the vicinity of the Brothers Islands 

 and White Sisters Islands in 1946 — and accounted for 

 68% of the stomach contents by volume. The only other 

 dietary information comes from the stomachs of five 

 Steller sea lions shot at Forrester Island in May 1986 

 (Calkins and Goodwin^); for these sea lions Pacific cod 

 (Gadus macrocephalus) accounted for the largest single 



60 



30 



a 



E 



60 



30 



Rookeries 



Jun-Aug 

 dd=5,3 



Haulouts 



Sep-May 

 dd=4,1 



GSFRFIOCH 



Figure 5 



Split-sample frequency of occurrence of prey 

 types in Steller sea lion iEumetopias jubatus) 

 scats from Southeast Alaskan rookeries (For- 

 rester, Hazy, and White Sisters) in summer 

 (Jun-Aug) and from haulouts the rest of the 

 year (Table 1). Diet at rookeries is the weighted 

 average (by the average 1993-97 pup counts) of 

 the three mean rookery diets (averaged across 

 years). The haulout diet is the average of the 

 three mean seasonal diets (averaged across 

 years). Diet diversity (dd) was calculated by 

 using a Shannon-Wiener index. Types of prey 

 consumed were F = forage fish, S = salmon, 

 G = gadids, R = rockfish, Fl = flatfish, O = other, 

 C = cephalopods, and H = hexagrammids. 



prey occurrence at 58% of the total volume, and pollock 

 accounted for 32% of the contents by volume and were 

 present in three stomachs. 



The scats we collected during the 1990s revealed 

 that gadids were an important part of the diet and 

 that pollock was the predominant gadid. Relatively few 

 Pacific cod were noted (Fig. 2). Pollock were the most 

 frequently occurring food item in all scats examined 

 from haulouts during the nonbreeding season and were 

 second only to salmon by frequency of occurrence in all 

 scats taken from rookeries during the breeding season. 

 Pollock may have been important in the diet of Steller 

 sea lions in the 1940s and 1980s, but the sample sizes 

 taken at that time are inadequate to draw a firm con- 

 clusion or to determine whether diet changed over time 

 (Trites and Joy, 2005). 



Attempts to reconstruct the size of the pollock con- 

 sumed during the 1990s in Southeast Alaska from the 

 lengths of bones recovered in the scats revealed that 

 adult fish (>45 cm fork length) were present more fre- 

 quently in the diets of Steller sea lions on the outer 

 coast sites than they were present in the diets of Steller 

 sea lions from the inside waters (Tollit et al., 2004a). 

 The largest proportions of fish consumed were of adult 

 and subadult sizes. Juvenile pollock (^20 cm) contrib- 

 uted insignificantly to the overall sea lion diet (Tollit 

 et al., 2004a). 



Potential biases 



The use of scats, like all measures to quantify diet, 

 entails caveats. Fortunately, the assumptions that under- 

 lie our analyses in Southeast Alaska also underlie the 

 interpretation of scats we sought to compare them with 

 from the Gulf of Alaska and Aleutian Islands. Some of 

 the limitations that can restrict the interpretation of 

 the dietary data from scats collected during the 1990s 

 from all parts of Alaska are also greatly reduced by the 

 large sample sizes (>1000 scats) collected in both regions 

 (Trites and Joy, 2005). However, greater caution must 

 be exerted when comparing stomach contents of Steller 

 sea lions shot from the 1950s-1980s with scats collected 

 during the 1990s, particularly for species such as squid 

 and octopus whose beaks can be caught on the stomach 

 lining and are often regurgitated rather than passed 

 through the intestinal tract. 



Our seasonal diet estimates largely compared what 

 lactating females were eating during summer with what 

 lactating females and all other age and sex classes were 

 eating during fall, winter, and spring. Whether or not 

 the different age groups forage in different areas or have 

 different prey preferences is not known. It should also be 

 noted that the seasonal descriptions of diet came from 

 samples that were primarily collected in tv/o time peri- 

 ods — February to July (1996-99) and June to December 

 (1993-95). Our conclusions about seasonal changes in 

 diet could therefore be biased if significant changes 

 occurred in the prey field over our decade of sampling. 

 We assumed that feeding conditions remained relatively 

 stable during the 1990s; our conclusions were based on 



