Trites et al Diets of Eumetopias jubatus in Southeast Alaska 



245 



sumed over a number of days (Tollit et al., 2003). In 

 other words, a scat likely does not represent a single 

 meal, but is probably a composite of one or more feeding 

 trips. Frequency of occurrence (Figs. 2-4) represents 

 the probability that a particular prey type was con- 

 sumed and does not represent the number or mass of 

 prey consumed. However, with large sample sizes, the 

 ranked importance of any particular prey type appears 

 to equate with frequency of occurrence or numbers of 

 prey (Sinclair et al., 1994; Antonelis et al., 1997; Sinclair 

 and Zeppelin, 2002). The split-sample frequency of oc- 

 currence technique is another approach that deals with 

 these biases by assuming that all prey species identified 

 in a scat were consumed in equal mass and that each 

 scat contributes an equal amount of information to the 

 overall diet (Fig. 5, Table 3). Split-sample estimates tend 

 to correlate with simple frequencies of occurrence to give 

 a reasonable proportional description of diet, even when 

 the assumption that all prey in a meal were consumed 

 in equal quantities is not always true. 



Prey associations 



The remains found in sea lion scats likely delineate asso- 

 ciations and distributions of prey species by region. In 

 Southeast Alaska, the hard parts recovered from scats 

 indicate that Steller sea lion prey were not randomly 

 distributed, given that some prey species were found 

 together more frequently than expected (if estimated 

 occurrence was based on chance alone). For example, 

 occurrences of gadids tended to be associated with occur- 

 rences of cephalopods (Fig. 8), whereas salmon were 

 found most often with forage fishes (herring and sand 

 lance). An association was also noted between flatfish 

 and "other" species (primarily skates — Figs. 2 and 8). 

 These associations may reflect groups of prey that are 

 commonly associated with each other because of habitat 

 similarities (e.g., depth or substrate similarities). 



Associations of prey in scats may also reflect prey-spe- 

 cific foraging strategies of individual Steller sea lions. 

 In some cases, prey associations may reflect secondary 

 prey, whereby a species was consumed by the prey spe- 

 cies actually targeted by the sea lion. Hard remains of 

 a fish may occur in a sea lion scat not because it was 

 depredated directly but because it had been consumed 

 by a fish that was then eaten by a sea lion. In our 

 case, 10 of over 60 species were found in more than 5% 

 of the scats and were presumably preferred prey that 

 were directly targeted by the sea lions (Fig. 2). The low 

 frequencies of the remaining 50-i- species (Fig. 2) may 

 reflect preferred species that were in low abundance 

 in Southeast Alaska, or they might indicate incidental 

 prey and those that had been consumed by their pre- 

 ferred prey. 



Eastern versus western diets 



Our systematic survey of Steller sea lion diets in South- 

 east Alaska during the 1990s was prompted by a desire 

 to gain insight into a possible dietary basis for the 



population decline that occurred in the Gulf of Alaska 

 and Aleutian Islands. Dietary differences of Steller sea 

 lions between the two regions may help to explain why 

 one population declined while the other increased. 



The most common prey of 61 species identified in 

 Southeast Alaska in frequency of occurrence were wall- 

 eye pollock. Pacific herring, sand lance, salmon, ar- 

 rowtooth flounder, rockfish, skates, squid, and octopus. 

 Looking further afield (west) we found that Steller sea 

 lions targeted a similar suite of species in the Gulf of 

 Alaska in the 1990s as those we noted in Southeast 

 Alaska, although the relative abundances of each dif- 

 fered considerably (Sinclair and Zeppelin, 2002; Trites 

 et al., 2007). The most common prey reported in the 

 Gulf of Alaska in order of importance were pollock, 

 salmon. Pacific cod, arrowtooth flounder, sand lance, 

 herring, and Irish lords (Hemilepidotus sp.). Further 

 west in the Aleutian Islands, however, Atka mackerel, 

 salmon, cephalopods. Pacific cod, Irish lords, and pol- 

 lock dominated the sea lion diet. 



It is unclear what role pollock and Atka mackerel 

 stocks alone have played in the different trajectories of 

 Steller sea lion populations. Pollock were consumed in 

 the Kodiak area — both prior to what was thought to be 

 the beginning of the decline (Pitcher, 1981) and after 

 the decline was under way (Calkins and GoodwinM. 

 Calkins and Goodwin' noted that although sea lions ate 

 more pollock in the Kodiak area after their decline in 

 terms of frequency of occurrence, the pollock they ate 

 were significantly smaller after the decline began. No 

 similar data are available for Southeast Alaska. 



Since the mid 1970s, pollock has been one of the most 

 dominant species in the Bering Sea and Gulf of Alaska 

 ecosystems (Livingston, 1993; Trites et al., 1999; Con- 

 ners et al., 2002). Unfortunately, little is known about 

 the relative importance of pollock in the Southeast 

 Alaska ecosystem. Pollock have been little exploited, 

 and relatively little is known about their distribution in 

 Southeast Alaska. Surveys conducted during 1950-62 

 (Alverson et al., 1964) and again during 1976-77 (Parks 

 and Zenger, 1978) caught 64-129 lbs of pollock per hour 

 in select areas of Southeast Alaska using a 400-mesh 

 eastern otter trawl. The 1950-1962 surveys covered on- 

 ly outside waters from Hazy Islands to Dixon Entrance, 

 whereas the later surveys were distributed throughout 

 both inside and outside waters. Parks and Zenger (1978) 

 estimated pollock biomass in Frederick Sound at 0.94 t/ 

 nmi^. A more recent unpublished estimate indicates 

 that the biomass may have been as much as seven times 

 this level in Frederick Sound in 2001 (Sigler^). Thus, 

 pollock would appear to be an important species in the 

 ecosystem within some areas of Southeast Alaska. 



The most striking difference between the diets of 

 Steller sea lions in the different regions of Alaska is 

 the diversity of prey consumed. Steller sea lions feed- 



3 M. Sigler. 2004. Personal commun. National Oceanic 

 and Atmospheric Administration, National Marine Fisher- 

 ies Service, Auke Bay Lab, 11305 Glacier Highway, Juneau, 

 AK, 99801. 



