from the massive barnacles encrusting the pihngs. 

 There were considerably fewer craters in this area 

 compared with the adjacent area A, and our at- 

 tempts to dig into the substrate under the wharf 

 proved difficult as a result of the debris embedded 

 in the sediment. 



Sea otters were in the process of foraging on 

 clams during several of the scuba dives in area A. 

 Although these otters were not bothered by our 

 presence under water, attempts to observe pre- 

 cisely how they were capturing clams usually 

 failed because they stirred up large clouds of sedi- 

 ment that obscured all of their activity. When the 

 otters stopped foraging and the clouds of sediment 

 dispersed, a large hole up to 1.0-1.5 m across and 

 0.5 m deep had obviously resulted from their dig- 

 ging. The sides of these holes were initially nearly 

 vertical, but collapsed within minutes. 



Details of a sea otter digging for clams were 

 observed by the first author on a single occasion on 

 30 March 1977, when a strong current rapidly 

 dispersed the clouds of sediment. Upon observing 

 a young male otter begin a typical sequence of 

 foraging dives in area A, the observer moved along 

 the bottom and approached the digging site from 

 an upstream direction. The otter was clearly visi- 

 ble at a distance of 5 m and was just leaving the 

 bottom after completing the second longer dive of 

 the series. He returned to the bottom within 20 s 

 but abandoned the initial digging site, leaving a 

 small hole about 0.5 m across and 25 cm deep. 

 Instead, on this third dive, he moved immediately 

 to a new spot about 4 m away and began to dig 

 rapidly with his front paws in a fashion very much 

 like a dog, producing a large conical cloud of sedi- 

 ment extending downstream. Digging lasted 

 about 45 s, followed by a 20 s surface interval. On 

 the fourth dive the otter resumed digging in the 

 same spot, and as during all digging periods, he 

 faced into the current. The observer was able to 

 approach < 1 m from the sea otter by creeping up in 

 a prone position on the bottom while the otter 

 substantially enlarged the hole to a short trench 

 about 1 m long, 0.5 m across, and 25 cm deep by 

 digging rapidly with both front paws. His back 

 flippers were moving at a slower rate, which prob- 

 ably helped maintain his position and also ap- 

 peared to assist in digging. Toward the end of the 

 digging on this dive the otter began to roll re- 

 peatedly from side to side to enlarge the front end 

 of the trench laterally, until he apparently en- 

 countered a clam and suddenly surfaced for 45 s. 

 On the fifth dive this rapid process of rolling and 



lateral digging with the front paws continued 

 again for about 30 s until another clam was caught 

 and the activity suddenly stopped. The hole at this 

 time was over 0.5 m deep and the otter's body was 

 entirely below the level of the substrate surface 

 while digging. The otter used this process of lat- 

 eral digging on three more dives lasting about 30 s 

 each with 40-60 s surface intervals, before the 

 observer ran out of air and surfaced. The trench at 

 that time was over 1.5 m long and remained about 

 0.5 m wide and deep. The otter terminated the 

 series of feeding dives with one additional dive 

 while the observer was at the surface. It paid no 

 apparent attention to the observer's close presence 

 during the entire series. Simultaneous observa- 

 tions by the second author from the surface indi- 

 cated that none of the first three dives (including 

 two dives at the first spot) produced a clam, but 

 that each of the six subsequent dives resulted in a 

 single clam. The otter did not use a rock to open the 

 clams. 



Discussion 



In 1966, prior to the return of sea otters to Mon- 

 terey Harbor, Calif., Department of Fish and 

 Game divers made qualitative surveys of the bot- 

 tom and used a garden hose to remove several 

 clams from the substrate for identification. The 

 bottom topography was smooth, clams were abun- 

 dant, and T. nuttallii was the dominant species 

 removed from as deep as 50 cm in the substrate 

 (Ebert^). Follow-up survey dives soon after the 

 return of sea otters indicated that clams were less 

 abundant and the bottom topography was hum- 

 mocky (Ebert, see footnote 3). Although definitive 

 quantitative data are not available for that period, 

 and although construction and dredging opera- 

 tions in the inner marina portion of the harbor 

 may have had important impact on clam popula- 

 tions, information in the present report indicates 

 that sea otters may have limited the abundance 

 and distribution of S. nuttalli and T. nuttallii and 

 that T. nuttallii is now only a minor species. The 

 cause of this apparent shift in dominance from T. 

 nuttallii toS. nuttalli is unclear. Our limited mea- 

 surements of the depths of these clams in the sub- 

 strate indicated that larger individuals were 

 found deeper (to about 50 cm), but that neither 



^E. E. Ebert, Director, Marine Culture Laboratory, California 

 Department of Fish and Game, Granite Canyon, Coast Route, 

 Monterey, CA 93940, pers. commun. June 1979. 



161 



