708 



Fishery Bulletin 92(4), 1994 



Table 2 



Frequency and estimated biomass of Saxidomus giganteus retrieved per dive by adult sea otters, Enhydra lutris, in 

 established (occupied for >25 years), intermediate (occupied 5-15 years), and frontal (occupied <5 years) study 

 areas along the Kodiak Archipelago, Alaska, 1987-1988. 



Study area 



Size class 

 (mm) 



Proportion 

 in sample 



Mean number 

 of prey/dive 



Estimated 

 wet-tissue weight 

 obtained/dive (g) ' 



Estimated 



caloric content 



(kcal) 



Established 

 Intermediate 



Frontal 



<50 

 50-90 



<50 



50-90 



>90 



<50 



50-90 



>90 



0.83 

 0.17 



0.18 

 0.71 

 0.11 



0.28 

 0.62 

 0.10 



I 6 

 1.1 



1.2 



16 

 33 



33 



10 

 21 



21 



Wet-tissue weight=2.14 (10 -4 ((shell length I" 7H :r-=0. 86 for Saxidomus giganteus where shell lengths equal 40, 70, and 100 mm representing 

 small, medium, and large size classes, respectively (Kvitek et al.. 1992). 



irrespective of the number of years the habitat had 

 been occupied by sea otters. Clams, particularly 

 Saxidomus, were the predominant prey identified in 

 all study areas, although 35-77% of the clams were 

 not identified to species. Green sea urchins were 

 absent in the diets of sea otters in established areas 

 but were found, infrequently, in the prey composi- 

 tion in intermediate and frontal areas. Sea urchins 

 were apparently locally abundant in intermediate 

 and frontal areas prior to the initiation of our study 

 (Kvitek et al., 1992; Stanford and Cunningham 7 ). Sea 

 urchin abundance had been reduced to low levels by 

 sea otter predation in other regions of Alaska and in 

 California (Lowry and Pearse, 1973; Estes et al., 

 1978; Laur et al., 1988; Kvitek et al., 1989) and it is 

 likely that sea otter predation affected urchin popu- 

 lations in the Kodiak Archipelago. 



Juvenile sea otter diets contained a higher propor- 

 tion of mussels than that of adults. A higher occurrence 

 of mussels in the diet of juveniles than of adult sea 

 otters has also been demonstrated by other studies con- 

 ducted in Alaska (VanBlaricom, 1988; Doroff and Bod- 

 kin, in press; Johnson 6 ). Mussels are an easily obtain- 

 able intertidal prey, and young sea otters may rely on 

 mussels as a food source until they become more profi- 

 cient foragers (Estes et al., 1981; VanBlaricom, 1988). 



Sea otters at Kodiak were highly successful in se- 

 curing prey, even where prey had been reduced by 

 years of otter predation (Kvitek et al., 1992). There- 

 fore, forage success was not a useful criterion for dis- 

 criminating among study areas that varied in the 



Stanford. S., and W. Cunningham. Bare Island. Port Bailey, AK 

 99615. Personal commun., June 1987. 



duration of sea otter occupancy. For sea otters, for- 

 age success may vary with prey type, hunting tac- 

 tics, or locality (Ostfeld, 1991) and may not be re- 

 lated to prey abundance or biomass (Estes et al., 

 1981). Ostfeld (1991) suggested, however, that for- 

 age success is a useful means of comparing forage 

 strategies and habitat characteristics for sea otters. 

 The lack of variation in forage success among our 

 study areas may have resulted, in part, from simi- 

 larities in habitat (Kvitek et al., 1992). Kruuk et al. 

 (1990) recommended caution in defining and using 

 the concept of forage success on a per dive basis and 

 suggested that a more meaningful approach would be 

 to examine the biomass captured per unit of effort. 



We estimated the average biomass and subsequent 

 caloric value captured on a per dive basis for sea ot- 

 ters. Sea otters foraging in habitat occupied an esti- 

 mated 1-15 years obtained approximately twice the 

 biomass of otters foraging in habitat occupied >25 

 years. This suggests that sea otters foraging in long- 

 occupied habitat may need to compensate for reduced 

 prey size and abundance through increased alloca- 

 tion of time for foraging to meet minimum daily ca- 

 loric requirements (Costa, 1978; Estes et al., 1982; 

 Estes et al., 1986; Garshelis et al., 1986). Biomass 

 and caloric values were similar for intermediate and 

 frontal areas. Possible explanations for the lack of 

 disparity between intermediate and frontal areas are 

 1) preexisting habitat differences among study ar- 

 eas, 2) resilience of Saxidomus to sea otter preda- 

 tion over the short term (see Kvitek et al., 1988), or 

 3) an error in the classification of study areas. 



We made the assumption that observed differences 

 in foraging characteristics resulted primarily from 



