Figure l. — Monterey Harbor, Calif. High densities of clams 

 were foraged by sea otters in areas A and B. 1 = Fisherman's 

 Wharf; 2 = Wharf No. 2; 3 = north and east sea wails of the inner 

 harbor; 4 = breakwater for the outer harbor. (Traced from an 

 aerial photograph in Haderlie and Donat 1978.) 



dives made in the middle of the outer harbor, but 

 sea otters usually surfaced without prey 50 m or 

 more from the start of the dive. However, the paths 

 of feeding dives in the two locations where clams 

 were taken in abundance were usually quite short, 

 only 10 m or less. 



The usual sequence of dives in the harbor region 

 began with otters making one to three 50-90 s 

 dives that produced no prey. After about 10-20 s on 

 the surface and a little grooming, the otters usu- 

 ally dove again to the same spot. A series of short 

 (25-40 s) dives followed the initial dives, and each 

 of these invariably resulted in a single S. nuttalli 

 about 10 cm long. The otters took 30-90 s to open 

 and eat the clams before diving to the same spot. 

 Sometimes they pounded the clams on a rock anvil 

 on their chest; other times they simply twisted or 

 pried the clams open with their teeth. An average 

 of 6 and as many as 19 clams were taken in a single 

 series of these dives. Following such a series, ot- 

 ters usually spent up to 30 min grooming before 

 they swam away, sometimes to forage in a new 

 location. 



In spring 1976, we conducted an underwater 

 survey of most of the bottom of the inner and outer 

 harbor, noting variations in the substrate and 

 counting protruding clam siphons in haphazardly 

 tossed y^ m^ quadrats. Depths in the harbor ranged 

 from 2 to 8 m, with area A being 4.5 m and area B 

 2-3 m (Figure 1). The substrate in much of the 

 enclosed inner harbor was black mud and silt, and 

 most of the rest of the harbor (including areas A 

 and B) was silty sand. The two areas where otters 

 fed extensively on clams had high densities of 

 clam siphons: for area A,x = 13.5/m2, SD = 8.9, n 

 = 19; for area B,x = 9.3/m2, SD = 7.2, n = 16. The 

 area under Wharf No. 2 adjacent to area A had 

 even higher densities of siphons: x = ITA/m.^, SD 

 = 11.9, n= 18. However, other areas of the harbor 

 had siphon densities < l.O/m^, and the black mud 

 of the inner harbor had densities <0.04/m2. ^g 

 inserted a slender rod down siphon holes in the 

 substrate until the rod contacted the clam shell, 

 and, with considerable difficulty, we used a stream 

 of freshwater from a garden hose to obtain a few 

 8-14 cm long clams from area A. We used these 

 specimens to distinguish the two species present 

 by the morphology of the protruding siphons. The 

 species composition in areas A, B, and under 

 Wharf No. 2 were the same: 95% wereS. nuttalli, 

 5% were the gaper clam, Tresus nuttallii. In this 

 way we also determined that the clams were lo- 

 cated 10-50 cm into the substrate and that larger 

 individuals of both species tended to occur at the 

 deeper end of this range in the sediment. We re- 

 corded the densities of clam siphons in area A and 

 also in the adjacent area of highest density under 

 Wharf No. 2 at approximately bimonthly intervals 

 from February 1976 to March 1977. The densities 

 and proportions of the two species of clams did not 

 change (ANOVA,P>0.05). 



The bottom in the two areas where sea otters 

 took large numbers of clams was littered with 

 hundreds of shells, both on the surface and mixed 

 into the sediments. About 58% of the shells did not 

 have pairs of connected valves, and about one- 

 third of the valves were broken. Of 89 shells sam- 

 pled, 99% wereS. nuttalli and 1% wereT. nuttallii. 

 The bottom topography was hummocky in these 

 areas, and there were many craters 0.5-1.0 m 

 across and 10-15 cm deep. The bottom under 

 Wharf No. 2, where the density of clam siphons 

 was highest, was mixed with debris consisting of 

 chunks of asphalt apparently from resurfacing of 

 the road on the wharf and of clumps of large bar- 

 nacle, Balanus nubiluS, tests which had fallen 



160 



