FISHERY BULLETIN: VOL. 77. NO 4 



corresponded to the temperatures found in Little 

 Skookum Inlet during the winter months (Table 

 1 ). The growth for the summer settling clams was 

 much higher, and other studies with Manila 

 clams of the same settling size have reported 

 similar or higher initial growth. Yoshida (1953) 

 found clams settling in early June reached 0.9 

 mm by the end of July. Clams raised in 22°-29° C 

 water under hatchery conditions, with an optimal 

 food supply, were 5 mm 90 days after settling 

 (Rodde et al. 1976). The small size at settlement 

 for the Manila clam and the slow initial growth of 

 clams that settled in the fall underscores the 

 necessity to use a small mesh size when sampling 

 for spat so as not to possibly mask a large part of 

 their early life. 



In addition to a difference in the growth rates 

 between summer and fall settling clams, the 

 lengths of clams that I recovered in June were 

 significantly greater (P<0.001) in treatments 

 without adult clams than in treatments with 

 adult clams. A similar decreased growth with in- 

 creased densities has also been shown for larger 

 Manila clams in other studies. Sagara (1952) 

 found this for clams >30 mm, but indicated that 

 clams <20 mm were not affected by density. 

 Ohba (1956) found decreased growth in 10-12 mm 

 clams and related it to competition for food. In 

 other studies, Hancock (1973) found reduced 

 growth with Cardium edule in areas of overpopu- 

 lation and a marked reduction in size in locally 

 overcrowded areas. Finally, in a hatchery-rearing 

 experiment with 14 mm Manila clams, Langton 

 et al. (1977) found that growth increased with 

 ration size in crowded conditions. A decrease in 

 available food to juveniles was implicated as the 

 controlling factor that caused the decrease in 



Table l. — The daily average and tlie extreme substrate tem- 

 peratures 1 1 cm below surface) between sampling periods for the 

 Manila clam in Little Skookum Inlet, Wash., at the +0.6 m tide 

 level. 



clam growth observed in treatments with adult 

 clams, as compared with those in treatments 

 without adults. There were not sufficient study 

 plots available to determine whether this differ- 

 ential growth continued during the summer. The 

 results of these experiments indicates that the 

 harvest of adult clams from a beach will allow for 

 a better growth of undersized clams. This result 

 coincides with the general view of commercial 

 Manila clam harvesters in Puget Sound 

 (Taylor"). 



Survival 



Approximately 1.2% of the clams that settled 

 in September 1976 survived until June 1977. 

 Japanese studies on Manila clams have reported 

 similar low levels of survivorship 4-9 mo after 

 initial settlements. Ikematsu (1957) found that 

 spat densities of 5,000/m2 in March were only 

 1.0% of the 500,000/m2 he found the previous 

 November. Ohba (1959) estimated settling den- 

 sities of 25,000/m^ in October, but found only 

 8.0% of that (2,000/m2) by the following June. In 

 studies on a number of clam species other than 

 Manila clams, Muus (1973) reported that regard- 

 less of clam densities at settling, the number of 

 clams recovered per unit area decreased rapidly 

 until a density of several hundred per square 

 meter was approached. The level of survivorship 

 from the fall settlement was similar to these 

 studies but although it was low, the number of 

 spat that it represented (250-450/m^) was more 

 than 2.5 times greater than the density of adults 

 (approximately 100/m^) considered as an 

 adequate level at which a beach can be dug com- 

 mercially (Taylor see footnote 4). 



Not only was the survivorship from the fall set- 

 tlement low, but the majority of the clam spat 

 mortality occurred during the first 2 mo after 

 settling (57%) and only about 10% of the clams 

 survived to 0.7 mm long (6 mo). One or more of a 

 number of factors are usually identified as causes 

 of high mortality in biological populations. In the 

 case of benthic marine invertebrates, Hancock 

 (1970) stated that survival after settlement will 

 depend upon: a) environmental conditions, nota- 

 bly temperature; b) food supply, which may be 

 affected by intra- and interspecific competition; c) 

 space competition; d) parasites and disease; e) ac- 



Mustin Tavlor, Totten Seafood, Route 1. Box 372A, Olympia. 

 WA 98502, pers. commun. June 1977. 



896 



