THE SPAWNING CYCLE OF SOFT-SHELL CLAM, MYA ARENARIA, IN 



SAN FRANCISCO BAY 



Shelly E. Rosenblum and Thomas M. Niesen' 



ABSTRACT 



Fi'ur populations Myit urcuaria in San Francisco Bay were studied for 1 year to determine the spawning 

 cycle. The spawning- cycle was well synchronized among; the four populations. Gametogenesis had com- 

 menced by late February and spawning occurred uninterrupted from April through summer. Cessation of 

 spawning occurred from September to October. The protracted spawning period of M. arenaria populations 

 in San Francisco Bay is f)robably related to the long period of moderate water temperatures (March- 

 October) which occur there. Size at first reproduction was placed at a shell length of 25 mm. Sex ratios of Af. 

 arenaria > 25 mm in shell length did not differ significantly from 1 : 1 . No evidence of hermaphroditism was 

 observed. 



The soft-shell clam, Mya arenaria, was once popular 

 with clam diggers in San Francisco Bay. During the 

 early 20th century, owners of bay front property 

 fenced off portions of the mud flats in order to ex- 

 clude clam predators, thus insuring bountiful 

 harvests of M. arenaria (Bonnot 1932). Today as the 

 "trend toward the improvement of San Francisco 

 Bay water continues, "^ the potential for a recreation- 

 al shellfishery exists again. Agencies for communi- 

 ties on the bay have begun to look at this potential. 

 Recently (1982), the digging of clams in San Fran- 

 cisco Bay received official clearance for the first time 

 in 30 yr.3 



The spawning cycle of the soft-shell clam has been 

 studied extensively on the east coast. Ropes and 

 Stickney (1965) examined populations from the Cape 

 Cod-New England region. They did not encounter 

 clams in the ripe stage of gametogenesis until May, 

 and by September spawning was over. Brousseau 

 (1978) reported a biannual cycle of spawning forM. 

 arevjiria from Cape Ann, MA. The first spawning 

 occurred between March and April and was of short 

 duration. A separate, second spawning took place 

 from June through July. Porter (1974) studied M. 

 arenaria from populations at Skagit Bay, WA. He 

 noted a single yearly spawning from late May to 

 early September. 



'Department of Biological Sciences, San Francisco State Univer- 

 sity', 1600 Holloway Ave., San Francisco, CA 94132. 



2Jones and Stokes Associates, Inc. 1977. San Francisco Bay 

 shellfish: an assessment of the potential for commercial and recrea- 

 tional harvesting. Prepared for the Association of Bay Area 

 Governments, 171 p. 



^Champion, D. 1982. Clam digging OK'd on part of San Fran- 

 cisco Bay. San Francisco, Chronicle, 27 August 1982, p. 1. 



The climate of the San Francisco Bay area, and 

 hence the seasonal water temperature fluctuations of 

 San Francisco Bay, are much less extreme than that 

 of the New England or Washington areas and most 

 of the other locations from which M. arenaria has 

 been examined. The question investigated in this 

 study is whether the spawning cycle of M. arenaria 

 from San Francisco Bay would differ from that of 

 other areas reported in the literature. 



MATERIALS AND METHODS 



Description of Study Sites 



Specimens of M. arenaria were collected from 

 September 1979 through December 1980 from four 

 sites around central San Francisco Bay (Fig. 1): 1) 

 Candlestick Point- adjacent to the causeway leading 

 to the Candlestick Park Stadium (lat. 37°42'32"N, 

 long. 122°23'28"W); 2) Burlingame Lagoon -just 

 south of San Francisco International Airport (lat. 

 37°35'12"N, long. 122°20'10"W); 3) Foster City- 

 immediately north of the San Mateo Bridge, off 

 Third Street (lat. 37°34'20"N, long. 122°23'28"W); 4) 

 Point Isabel -north of the Golden Gate Fields race 

 track on the eastern shore of San Francisco Bay (lat. 

 37°53'59"N, long. 122°23'28"W). These areas were 

 selected because they experience annual variations 

 in temperature and salinity (Conomos 1979), factors 

 which are known to affect bivalve spawning cycles 

 (Loosanoff and Davis 1951; Swan 1952; Matthiessen 

 1960, Pfitzenmeyer 1962; Stickney 1964). Substra- 

 tum conditions were classified according to field 

 observations; no particle size analyses were con- 

 ducted. 



Manuscript accepted October 1984. 



FISHERY BULLETIN: VOL. 83. NO. 3, 1985. 



403 



