Doroff and DeGange: Prey composition and foraging success of Enhydra lutris 



709 



sea otter predation. There were likely preexisting 

 differences in the community structure among our 

 study areas that were not assessed, such as the dis- 

 tribution and abundance of bivalve species prior to 

 sea otters re-occupying the study areas. However, we 

 believe that comparisons of study areas are valid 

 given the similarities in habitat and infaunal inver- 

 tebrate assemblages among study areas documented 

 by Kvitek et al. (1992). 



Saxidomus may appear resilient to sea otter pre- 

 dation pressure over the short term because it is 

 present in high densities in our study areas (Kvitek et 

 al., 1992). Saxidomus was found in higher densities 

 than was any other forage species and it was selected 

 preferentially (based on differences between in situ 

 population of clams and the shells discarded by forag- 

 ing otters) in intermediate and frontal areas (Kvitek et 

 al., 1992). Saxidomus was also the most abundant clam 

 (in situ) in the established area; however, Protothaca 

 was selected preferentially (Kvitek et al., 1992). 

 Protothaca was not identified visually as sea otter prey 

 in the established area; however, only 239c of the clams 

 could be identified to species. 



We believe the classification of our study areas and 

 those used by Kvitek et al. ( 1992) were correct; how- 

 ever, our methods lacked the refinement needed to 

 distinguish between intermediate and frontal areas. 

 Kvitek et al. ( 1992) was also unable to detect differ- 

 ences between the intermediate and frontal areas by 

 measuring prey size directly from the shells of clams 

 consumed by sea otters. However, there were differ- 

 ences in the size of the in situ population of clams 

 between areas (Kvitek et al., 1992). Newly exploited 

 habitat in our study was represented by an area es- 

 timated to have been occupied 1-4 years by sea ot- 

 ters. Rapid changes may occur within the first year 

 that sea otters occupy unexploited habitat. Garshelis 

 et al. (1986) observed an approximate twofold de- 

 crease in kcal/dive in areas occupied by sea otters 

 <1 year compared with areas occupied 1-2 years. Co- 

 incident with the change in kcal/dive was a shift in 

 prey from crabs to clams between areas studied by 

 Garshelis et al. (1986). In the Kodiak Archipelago, 

 we did not observe differences in mean kcal/dive or 

 changes in prey composition between intermediate 

 and frontal areas. Changes in prey composition, such 

 as the potential removal of green sea urchins from 

 the study area, may have occurred in the frontal area 

 during the first year and were undetected. 



Adult sea otters in the established area appear to 

 have compensated for reduced prey size by retriev- 

 ing more prey items per dive. However, they still 

 obtained less clam biomass (and subsequently less 

 caloric intake) per dive than otters in the intermedi- 

 ate and frontal areas, suggesting that they may need 



to forage longer to meet minimum daily caloric needs. 

 Interestingly, juveniles in established areas did not 

 appear to compensate for reduced bivalve prey size by 

 increasing the number of prey captured per dive. Ju- 

 veniles may be less efficient foragers and may compen- 

 sate by increasing their consumption of Mytilus spp., 

 which are an easily obtainable intertidal prey (Estes, 

 1981; VanBlaricom, 1988; Doroff and Bodkin, in press). 



Acknowledgments 



We thank Larry Barnes, John Baird, Elizabeth Belan- 

 toni, Walt Cunningham, Dave Douglas, Dan Monson, 

 Jay Nelson, Doug Sheperd, Annetta Smith, Susan 

 Stanford, and Shelli Vacca for assistance with data 

 collection. Walt Cunningham and Susan Stanford 

 graciously hosted us at their home on Bare Island. 

 Logistic support was provided by Jay Bellinger and 

 staff of the Kodiak National Wildlife Refuge and Al 

 Bayer and the crew of the MV Tiglax. Shelli Vacca 

 and especially Bob Stehn provided computer support. 

 For reviewing drafts of this manuscript, we thank 

 Brenda Ballachey, Edward Bowlby, James Bodkin, 

 Dan Esler, Dave Garshelis, Rikk Kvitek, and three 

 anonymous reviewers. 



Literature cited 



Alt matin. J. 



1974. Observational study of behavior: sampling 

 methods. Behavior 49:227-267. 

 Calkins, D. G. 



1978. Feeding behavior and major prey species of 

 the sea otter, Enhydra lutris, in Montague Strait, 

 Prince William Sound, Alaska. Fish. Bull. 76:125- 

 131. 

 Costa, D. P. 



1978. The ecological energetics, water, and electro- 

 lyte balance of the California sea otter, Enhydra lu- 

 tris. Ph.D. diss., Univ. California, Santa Cruz, 75 p. 

 Doroff, A. M., and J. L. Bodkin. 



In press. Sea otter foraging behavior and hydro- 

 carbon levels in prey. In T. R. Loughlin (ed.), Im- 

 pacts of the Exxon Valdez oil spill on marine 

 mammals. Academic Press. 

 Estes, J. A., and G. R. VanBlaricom. 



1985. Sea otters and shellfisheries. In R. Beverton, 

 J. Beddington, and D. Lavigne (eds.), Conflicts be- 

 tween marine mammals and fisheries, p. 187- 

 235. Allen and Unwin, London, England. 

 Estes, J. A., N. S. Smith, and J. F. Palmisano. 



1978. Sea otter predation and community organi- 

 zation in the western Aleutian Islands, Alaska. 

 Ecology 59:822-833. 



