Herrington [1930) also showed that growth slowed in the winter and led to 

 formation of rings in the shells. 



Since the Pismo clam has one major spawning season per year, year 

 classes in the adult population can be identified. Theoretically, the 

 year classes should be apparent as modes in a size-frequency graph. The 

 modes may be resolved by graphical analysis on probability paper (Harding, 

 1949; Cassie, 19S4) to estimate the mean size and standard deviation of 

 the component year classes. 



On 12 October 1973, 71 quadrats (1 square meter) were taken at 

 Monterey Bay Academy and 303 Pismo clams were collected and measured. 

 The clams were grouped by length in 4-millimeter-size classes, a size- 

 frequency diagram was prepared, and the modes were resolved into component 

 age classes on probability paper. The results are given graphically in 

 Figure 21. Five groups were found which were assumed to represent five 

 age classes. 



Herrington (1930) and Coe and Fitch (1950) calculated ages of clams 

 at various lengths; Herrington, by counting the annual rings in the shell, 

 and Coe and Fitch, by actual measurement of marked clams. Only Herrington 

 reported the average sizes for the age classes in Monterey Bay. Comparison 

 of the results of this study with Herrington and Coe and Fitch is given 

 in Table 9. The means of the length (age) do not agree. The large 

 disagreement in year class V is easily explained. Few clams over 110 

 millimeters in length were obtained to validate mean and variance which 

 was believed due to the heavy clamming pressure which cropped off every- 

 thing near the legal limit of 126 millimeters. A single clam of legal 

 size or above was not collected, indicating the clams were probably 

 removed by clammers each year. Therefore, what is designated as class V 

 with a mean of 120 millimeters is probably a combination of all the 

 survivors of older age classes. The 120 millimeters average is simply 

 an average of all the age classes greater than class IV. 



The year class I, with a mean length of 11 millimeters, represents 

 clams spawned in 1972 and their first year. The clam's average size is 

 less than reported by Herrington for Monterey Bay and Coe and Fitch for 

 La Jolla, but well within the range reported for Pismo Beach (Table 9). 

 A similar situation prevails for classes II and III. The mean length 

 of class IV appears to be greater than that at La Jolla or Monterey Bay. 

 However, there is an extremely large spread in Herrington 's data. 



Four age classes are believed to be present at Monterey Bay Academy 

 and a fifth size class comprising the few representatives of all elder 

 classes. The growth rate of clams in Monterey Bay is also slower than in 

 more southern localities. 



4. Conclusions . 



Small Pismo clams in Monterey Bay grow at greater rates than large 

 clams at all seasons of the year. The rates of growth for all clams vary 



