NOTES 



OBSERVATIONS OF SEA OTTERS DIGGING 



FOR CLAMS AT MONTEREY HARBOR, 



CALIFORNIA 



Although the feeding behavior of the sea otter, 

 Enhydra lutris, is frequently observed from the 

 surface, few underwater observations of foraging 

 sea otters have been published. Faro (1969) and 

 Houk and Geibel (1974) described the underwater 

 behavior and tool use of sea otters when they re- 

 moved abalone from rock substrates. Shimek 

 (1977) observed a sea otter foraging for snails and 

 presumably other invertebrates by patting the 

 surface of rocks and feeling into cracks. Shimek 

 also described a sea otter digging up the echiuroid 

 worm, Urechis caupo, from a silt and cobble sub- 

 strate. Further deductions about underwater 

 foraging behavior have been made from collec- 

 tions of abalone shells with the characteristic "ot- 

 ter break" hole in the middle (Wild and Ames^) 

 and from observations of aluminum beverage cans 

 bitten by otters to remove octopus (McCleneghan 

 and Ames 1976). Some sea otters can be enticed 

 underwater to take food offered them (pers. obs). 

 However, these latter observations of underwater 

 food manipulation are of limited value because the 

 otters also take items unpalatable to them (e.g., 

 the holothuroid Stichopus californicus) , and be- 

 cause the otters were clearly interacting with the 

 diver observer. These accounts of underwater 

 foraging indicate that sea otters use primarily tac- 

 tile sensitivity of the forelimbs to locate and cap- 

 ture prey, whereas all other marine mammals 

 (pinnipeds and cetaceans) use their jaws to cap- 

 ture prey. Radinsky (1968) hypothesized that the 

 sea otter evolved forelimb tactile sensitivity sepa- 

 rately from the aonychoid otters. 



The large impact of sea otters on Pismo clam, 

 Tiuela stultorum, populations in California has 

 been documented (Stephenson 1977; Miller et 

 al.2), and in Prince William Sound, Alaska, 81% of 

 the food items taken by sea otters were bivalves, 

 especially Saxidomus gigantea (Calkins 1978). 



•Wild, P. W., and J. A. Ames. 1974. A report on the sea 

 otter, Enhydra lutris L., in California. Calif. Dep. Fish and 

 Game Mar. Resour. Tech. Rep. 20, 93 p. 



=*Miller, D. J., J. E. Hardwick, and W. A. Dahl- 

 strom. 1975. Pismo clams and sea otters. Calif. Dep. Fish 

 Game Mar. Resour. Tech. Rep. 31, 49 p. 



FISHERY BULLETIN: VOL. 78, NO. 1, 1980. 



The Alaskan otters "dug out clams with their 

 forepaws while maintaining a head downward 

 position" in intertidal and shallow subtidal water. 

 However, Shimek's (1977) description is the only 

 detailed underwater observation of sea otters 

 foraging on soft substrate. Detailed observations 

 of sea otters taking prey from soft substrates are 

 more difficult than those on rock, because the ot- 

 ter's disturbance of the bottom often results in 

 clouds of sediment obscuring further vision. In the 

 present account, we describe underwater ob- 

 servations of sea otters digging clams in a silty 

 sand substrate and present information about the 

 impact of this foraging on the distribution and 

 abundance of subtidal clams at Monterey Harbor, 

 Calif. 



Observations 



In 1976-77 we observed sea otters eating large 

 numbers of the Washington c\a,va, Saxidomus nut- 

 talli, primarily in two specific areas of Monterey 

 Harbor (A and B of Figure 1 ). From vantage points 

 along the floating boat slips and elevated wharves, 

 we observed sea otters at the surface feeding on 

 211 prey items: S. nuttalli (88.6%); the crabs 

 Pugettia producta (4.2%) and Cancer sp. (3.3% — 

 probably C. antennarius or C. productus, but not 

 C. magister); the rock jingle hiwalve Pododesmus 

 cepio (1.4%); and unidentified items (2.4%). Dur- 

 ing spring 1976, as many as four sea otters were 

 foraging at one time in the harbor vicinity, but an 

 average of about one sea otter was observed on 38 

 counting trips made to the area. 



The underwater path of foraging sea otters 

 could often be observed from the surface by follow- 

 ing the trail of air bubbles escaping from their 

 compressed fur. The paths of foraging dives made 

 in the inner harbor were often contorted, 50-60 m 

 or more long, and lasted 45-80 s. These dives usu- 

 ally produced no prey, but the prey taken were 

 mostly crabs and rarely clams. On those dives that 

 resulted in the capture of kelp crabs, sea otters 

 usually (eight out of nine dives) finished their 

 search with a swim under 10-20 m of the floating 

 docks in the inner area of the harbor. During scuba 

 dives in this area, we repeatedly observed kelp 

 crabs on the undersides of these floats and rarely 

 elsewhere. It was difficult to observe the paths of 



159 



