In the Diaz increment method, the change in 

 length per unit time, Al/At, is graphed against 

 the average length during the time interval, 1. 

 A straight line, fitted to the plot by least 

 squares, will have a slope of -k and an inter- 

 cept ka. Application of this technique to the 

 modal lengths at our irregular sampling times 

 produced a nonsense result (table 2). We there- 

 fore superimposed on the original data a 

 monthly grid (fig. 9) and applied the Diaz 

 technique to data derived from points of inter- 

 section between the modal progressions and the 

 grid. Growth parameters were determined for 

 1- and 3-month intervals between data points 

 (table 2). 



In the Ford-Walford technique, the length at 

 any age, Ij, is graphed against the length after 

 a specified time, 1, ^ p The time interval may 



be arbitrary but must be the same between all 

 values of Ij and Ij ^ !• A straight line fitted 

 to the points will have a slope of e"*^ and an 

 intercept of a(l-e ). Since the time interval 

 must be standardized for this technique, Ford- 

 Walford plots were made by using the inter- 

 sects between lines of progression and the grid 

 in figure 9, again at both 1- and 3-month 

 intervals (table 2). 



A von Bertalanffy growth curve was con- 

 structed from the mean values for a and k 



Table 2. — Gro\rth parameters for Rang! a cuneata 

 estimated by serval Diaz and Ford-Walford 

 plots'"- 



Means (of four results): 75.62 0.0193 



■"■ All but the first Diaz method used points 

 of intersection between length progressions 

 and monthly grid from figure 9. 



(table 2) by assuming the length at t = to be 

 0.375 mm. which is the approximate length of 

 Rangia at the time of larval settling. Thus b is 



^^'^75 "62'^^^ °^ 0.995. The von Bertalanffy 

 curve derived is shown in figure 10. The 

 original data points were fitted on the same 



X 



»— 

 o 



z 



5 6 7 



AGE (YEARS) 



10 



12 



Figure 10.— Theoretical growth of Rangia cuneata expressed by von Bertalanffy growth curve with 

 mean values of a and k (table 2). Points are modal lengths of size groups A-E from figure 9, 

 fitted to the curve as explained In the text. 



22 



