5. Conclusions . 



Garnet o genes is begins in February and most clam gonads are in the ripe 

 stage by May or June. Although most remain in this stage throughout the 

 summer, some partial spawning does occur. Complete spawning occurs 

 mainly during September and October in Monterey Bay and Pismo Beach. 

 Clam gonads reach an earlier ripe stage at Manresa Beach and Pismo Beach, 

 and a later ripe stage at the subtidal Moss Landing station. Intertidal 

 Moss Landing clams reach the ripe stage at an intermediate time. 

 Gametogenesis and time of spawning may be correlated with water tempera- 

 ture and phytoplankton abundance in Monterey Bay. The sex ratio for 

 large and small clams did not differ significantly from 1:1. Clams mature 

 when 20 millimeters long in Monterey Bay and San Diego, but because of 

 higher growth rates of juvenile clams at San Diego, the clams at Monterey 

 Bay ripen 1 year later than clams at San Diego. 



VI. AGE AND GROWTH 



1 . Introduction . 



The first studies of the growth of Pismo clams were done at Pismo 

 Beach by Weymouth (1923) , who noted the existence of age classes in 

 size-frequency plots and demonstrated the existence of rings in the shell 

 which he assumed to be annual rings. He also drew the first growth curve 

 for the species. Herrington (1930) confirmed Weymouth's work and added 

 data from other locations, including Monterey Bay. The most recent study 

 of growth in Pismo clams was done by Coe and Fitch (1950) who monitored 

 5 years of growth in several southern California populations and noted 

 fluctuations in the populations. Until this study, the growth in Pismo 

 clams had not been investigated for more than 20 years. 



This study had two major objectives: (a) Establish the growth rates 

 of different-sized clams in the natural environment in Monterey Bay and 

 from that, construct a generalized growth curve, and to provide an 

 independent assessment of the validity of annual growth rings in Pismo 

 clams which were used by three previous investigators to assess age 

 structure and growth; and (b) determine the age structure of natural 

 populations of intertidal clams in Monterey Bay. 



2. Methods and Materials . 



Several methods were used to hold marked clams for growth studies. 

 To determine growth rates of different-sized clams, individual clams 

 were marked with a unique number, the length measured, and returned to the 

 beach. Pismo clamshells are exceedingly thick, so a permanent number was 

 engraved in the outer shell with an engraving tool. Over the 2-year 

 period, a total of 997 clams were marked. 



The first group of 161 marked clams was established on 29 February 

 1972 at Moss Landing at a depth of 10 feet below mean lower low water (MLLW) 



44 



