3. Results . 



The size-frequency distribution of Pismo clams at Moss Landing is 

 shown in Figure 2 and the densities in Table 1. The mean densities of 

 the clams smaller than 47 millimeters long obtained on 11 July 1972, 

 7 July 1973, and 11 October 1973, were tested for deviation with a 

 Student-Newman-Keuls multiple range test (Woolf , 1968) . The mean densi- 

 ties did not deviate significantly. However, when clams larger than 

 47 millimeters were compared, there were substantial differences in 

 densities in the three time periods. There were 13 clams larger than 

 47 millimeters long recorded on 11 July 1972, 9 on 1 July 1973, and none 

 on 11 October 1973. Forty additional square-meter quadrats were sampled 

 at Moss Landing on 10 December 1973, and no Pismo clams larger than 

 47 millimeters long were found. 



Further evidence of the decline of Pismo clams coincidental with the 

 appearance of feeding sea otters became available from the catch-per-unit 

 effort observations in collecting clams for gonad studies. Most collec- 

 tions before April 1973 were made within 15 minutes at Moss Landing with 

 two divers collecting 20 subtidal clams within 45 minutes. Clams became 

 progressively more difficult to obtain by the summer of 1973. By 

 September, divers with 7 hours bottom time had obtained only one clam. 

 Also, the sand bottom contained a large number of broken shells; before 

 April 1973 broken shells were rarely observed. 



Clammers were interviewed during November and December 1973 and 

 January 1974 to determine their success in obtaining legal-sized clams 

 (larger than 126 millimeters long). Eleven clammers at Moss Landing dug 

 13 workdays after the season opened in October and caught only one 

 legal-size clam. The same 11 clammers took 137 legal-size clams in 12 

 workdays the previous season. All the clammers interviewed stated that 

 the clamming was the worst experienced at Moss Landing and some of the 

 clammers had been clamming there for 20 years. 



Several direct observations were made of sea otters feedings on Pismo 

 clams. On one occasion a single otter ate 24 Pismo clams and two mole 

 crabs [Blepharipoda oaoidentatis) . The average estimated size of the 

 clams consumed was 101 millimeters and the otter fed for 2 hours, 15 

 minutes. On another occasion an otter fed on 14 Pismo clams and 37 mole 

 crabs. The average size of the clams was estimated at 127 millimeters 

 and the feeding period at 4 hours. The California Department of Fish and 

 Game observed one otter consuming 22 Pismo clams in 3 hours, 45 minutes 

 at Atascadero State Beach (Paul Wilde, 1973, personal communication). 



The density of otters varied at Moss Landing. At one time 10 otters 

 were observed, but the number declined to 1 after 24 September 1973. 



4. Discussion . 



The interviews and estimated clam densities obtained by the catch-per- 

 unit effort indicate a recent decline in the density of large clams at 



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