FISHERY BULLETIN: VOL. 76, NO. 1 



and 1974 approximated 8-yr monthly means so the 

 bimodal pattern was not an atypical response to 

 above average temperatures. Nevertheless, the 

 temperature patterns of this locale are probably 

 influenced substantially by local topography. 

 More than 60% of the total area of the Annisquam 

 River system is <6 m deep, 30% being intertidal 

 (Jerome et al. 1968). Early spring warming and 

 fall cooling trends would be expected here due to 

 these nearshore influences. Lastly, it appears that 

 the bimodal spawning pattern emerging here may 

 be typical of some populations of M. arenaria 

 found as far north as Plum Island Sound. A spring 

 set of juvenile clams occurs annually on intertidal 

 flats in Ipswich, Mass., (Richard Sheppard pers. 

 commun.) and large numbers of 2- to 4-mm clams 

 appeared in the May and June samples of Smith et 

 al. (1955). Such evidence indicates that a semian- 

 nual pattern may be more prevalent in northern 

 Massachusetts than once believed. 



Orton (1920) first noted that some animals in 

 temperate regions spawn when the temperature 

 exceeds a critical level characteristic of the 

 species, while for others the rate of change is im- 

 portant. Nelson (1928) reported 10°-12°C as the 

 critical spawning temperature for M. arenaria; 

 Belding (1930) reported the exceptionally high 

 figure of 22°C. The data for Gloucester indicate 

 that spawning can occur with equal likelihood at 

 either of the supposedly critical temperatures pro- 

 vided that the gonad is ripe. The significant tem- 

 perature appears to be that at which maturation of 

 the gonad occurs. Similar significance of matura- 

 tion temperature had been reported for the oyster, 

 Crassostrea virginica, by Loosanoff and Davis 

 (1950). 



Gonadal oocyte counts provide an accurate mea- 

 sure of fecundity in M. arenaria since all oocytes 

 are stored in the gonad prior to spawning and 

 nearly total evacuation takes place at spawning. 

 The fecundity values for M. arenaria indicate that 

 the largest females produce the largest number of 

 oocytes. This increase is undoubtedly due to in- 

 creased gonad size made possible by increased 

 shell volume. Average oocyte production by a 

 60-mm clam during a single breeding season (two 

 spawning periods) is about 120,000; lifetime pro- 

 duction would be in the order of 1.5 x 10^ oocytes. 

 Although fecundity of M. arenaria is large, as is 

 typical of species with planktonic larvae (Thorson 

 1950), these estimates are considerably lower 

 than early unsubstantiated ones for this species 

 (Belding 1930), as well as those reported for other 



164 



marine bivalves such as Crassostrea virginica and 

 the hard-shell clam, Mercenaria mercenaria 

 (Galtsoff 1930; Davis and Chanley 1956). 



High fecundity, however, is offset by high mor- 

 tality during pelagic life, metamorphosis, and 

 early settlement. It appears that sources of mor- 

 tality such as predation, disease, and bottom 

 character are more critical factors in explaining 

 fluctuations in recruitment than variability in 

 fecundity rates or spawning frequency. The 

 spawning cycles in which the greatest number of 

 oocytes were released did not correlate with 

 periods of highest recruitment. In terms of spat 

 densities, spring recruitment in both years 

 studied was higher than summer recruitment. 

 Success of some year classes and failure of others 

 indicate that fluctuations in clam populations are 

 largely natural occurrences and may result from 

 things other than fluctuations in the number of 

 oocytes or the number of juveniles or byssus-stage 

 young. 



Spawning times and fecundities of individual 

 females are critical factors in determining first, 

 what constitutes a satisfactory breeding stock and 

 secondly, how to protect it. Numerous studies have 

 been conducted on methods of improving soft-shell 

 clam fisheries (Belding 1930; Turner 1949, 1950; 

 Smith et al. 1955; Smith^). Regulatory efforts have 

 ranged from predator control to establishment of 

 legal size limits for clams, closed seasons, and re- 

 stocking of barren flats. All this work has pro- 

 ceeded in the near absence of basic information of 

 the reproduction and population dynamics of the 

 clam. The dwindling yields of clams on the New 

 England coasts indicate the ineffectiveness of 

 present regulatory procedures and the need for 

 revised management practices. 



In Massachusetts, any clam over 2 in long (51 

 mm) may be harvested. In effect this practice 

 maximizes the removal of the reproductively most 

 valuable individuals in the population. Murphy 

 ( 1968), using genetic models, has shown that adult 

 longevity and iteroparity ( = repeated reproduc- 

 tion) are important adaptations for population 

 stability in species like M. arenaria which exist 

 under conditions of uncertain preadult survival 

 and relatively stable adult survival (Brousseau 



^Smith, O. R. 1952. The results of experimental soft clam 

 farming in Plum Island Sound, Massachusetts. Third annual 

 conference on clam research, U.S. Fish and Wildl. Service, clam 

 investigations, Boothbay Harbor, Maine, p. 46-48. Unpubl. rep. 



