species had a depth refuge from predation by sea 

 otters, which excavated deeper than 50 cm. Tresus 

 nuttallii attains larger size than S. nuttalli (pers. 

 obs.), and if sea otters prefer larger clams, they 

 may have preyed preferentially upon T. nuttallii. 

 However, clams remained abundant in small 

 areas of the harbor in spite of heavy predation by 

 sea otters. Densities under Wharf No. 2 averaged 

 about 17 clams/m^; and in this area they appear to 

 have a partial refuge from sea otters, which may 

 have found it too difficult to dig through the debris 

 of chunks of asphalt and clumps of barnacle tests 

 embedded in the sediment. No such impediment to 

 digging exists in areas A and B, where clams have 

 persisted in somewhat lower densities of about 14 

 and 9/m^, respectively. However, the species com- 

 position of clams was the same under Wharf No. 2 

 and in areas A and B, regardless of predation in- 

 tensity. 



By following tagged animals, Loughlin (1977) 

 showed that certain sea otters made daily foraging 

 trips to Monterey Harbor from rafting locations as 

 far as 2 km away. In the present descriptions of 

 their dive paths, sea otters feeding on items other 

 than clams apparently located prey in a random 

 manner similar to Shimek's (1977) description of 

 an otter patting the surface of rocks and feeling 

 the cracks. Observations of the bubble paths of 

 otters taking clams in areas A and B of the harbor, 

 however, indicated that they usually did not spend 

 time searching for a suitable place to dig, nor did 

 visual selection of a patch of clams appear to occur. 

 If the density of clams in area A averaged 14/m2, 

 and if an average spot dug up by an otter was 0.5 x 

 1.5 m (0.75 m^) as observed in this report, then 

 random digging in area A would produce about 10 

 clams. This is greater than the average number of 

 six clams taken by otters on a series of dives. 

 Perhaps the otters had learned the location of the 

 clam patches, and because sediment clouds nor- 

 mally prevented visual cues as soon as the sub- 

 strate was disturbed, they simply dug haphazard- 

 ly within the patch. Indeed, Gentry and Peterson 

 (1967) compared the underwater visual acuity of 

 sea otters with the sea lion, Zalophus califor- 

 nianus, and harbor seal, Phoca vitulina, and pro- 

 posed that vision in otters may be better adapted 

 for aerial situations of predator detection rather 

 than for underwater prey location. 



The strategy of repeatedly enlarging the hole to 

 capture clams is a good one, because it makes 

 efficient use of the labor to start the hole on initial 

 dives. Anyone who has dug in sand at the seas' --e 



knows that it is relatively easy to enlarge a hole, 

 and it would be advantageous to do this rather 

 than dig straight down for each individual clam. 

 The behavior of digging like a dog has also been 

 reported by Shimek (1977) for a sea otter taking 

 subtidal echiuroid worms and is apparently simi- 

 lar to the behavior of sea otters taking clams in 

 shallow subtidal and intertidal waters in Alaska 

 (Calkins 1978). The holes reported by these au- 

 thors were only half the size of freshly dug holes at 

 Monterey Harbor, however. The first author has 

 observed similar (1.5 m across and 0.5 m deep) 

 holes dug by otters in the sand channels in 12 m of 

 water off kelp forests at Pacific Grove, Calif. In 

 areas such as Prince William Sound and Monterey 

 Harbor, where otters forage heavily on clams, 

 their digging must cause a major disturbance of 

 the infaunal community. 



Sea otters have been termed "keystone pred- 

 ators" (Estes and Palmisano 1974; Estes et al. 

 1978), because they regulate populations of 

 epibenthic invertebrates, perhaps through a pro- 

 cess of switching between prey species. At Mon- 

 terey Harbor there is circumstantial evidence that 

 sea otters have had a major impact on two other 

 prey items. Surveys by the California Department 

 of Fish and Game showed C. antennarius and C 

 productus were taken in abundance by fishermen 

 from the Monterey wharves prior to the return of 

 sea otters, but they were rarely taken at Monterey 

 in 1972-74, while still caught in abundance at 

 piers north of the range of sea otters (California 

 Department of Fish and Game"*). Observations on 

 the scuba dives reported here for 1976-77 confirm 

 that cancer crabs are rare in the harbor. Mytilus 

 edulis and M. californianus formed dense clumps 

 on wharf pilings prior to the return of sea otters 

 (Haderlie^), but mussels are small and uncommon 

 there now (Haderlie and Donat 1978). Curiously, 

 large specimens ofB. nubilus are still abundant on 

 the pilings and were not taken in appreciable 

 numbers by sea otters, even though these barna- 

 cles were taken frequently by otters at other loca- 

 tions in the Monterey area (pers. obs.). The factors 

 which regulate prey selection by sea otters remain 

 poorly understood. 



■"California Department of Fish and Game. 1976. A pro- 

 posal for sea otter protection and research and request for the 

 return of management to the State of California. Calif. Dep. 

 Fish Game, Oper. Res. Branch, Vol. 1: Text and summaries, 

 271 p. 



^E. C. Haderlie, Professor, Naval Postgraduate School, Mon- 

 terey, CA 93940, pers. commun. May 1976. 



162 



