FISHERY BULLETIN: VOL. 69. NO. 2 



to 1967. Landings in 1967 totaled approximately 

 5,000 bu and although several vessels fished the 

 area in 1968, the overall catch was light. Mor- 

 tality during the survey period was therefore 

 assumed largely due to natural causes. 



Catch curves using Ricker's (1958) method 

 of plotting loge (average number per drag per 

 month) were constructed (Figure 3). A linear 



Transect A 



H \ 1- 



H — I — I — I- 



A, B Old C CombinM 



Figure 3. — Catch curves for transects A, B, C, and 

 combined. 



regression was applied to the descending curve 

 according to Beverton and Holt (1957), using 

 b as an estimate of F -{- X. Since F was negli- 

 gible, b is an estimate of X or natural mortality. 

 The instantaneous mortality rate (0 is equal to 

 b and the monthly mortality rate (a) was de- 

 rived from Ricker's appendix. 



Data used in the analysis represented the 1967 

 year class except in transect D where catch 



data were too sparse for analysis. December 

 1968 data were not available except for transect 

 C. October 1968 was omitted from transect A 

 because of insufficient data. 



The ascending left limb of the curve reflects 

 recruitment. Mortality during this period is 

 diflicult to determine because small scallops are 

 inaccessible to the dredge. The dome varied 

 among transects but occurred from October to 

 December or February. The descending limb 

 extended from December to October (August 

 in transect A). All limbs are curvilinear but 

 vary in shape. The curve for transect A is vari- 

 able. That for transect B is concave from Feb- 

 ruary to June then becomes convex. Transect 

 C curve is almost linear. 



Monthly mortality rates, computed assuming 

 a linear relationship (as in Beverton and Holt, 

 1957) , are given in Table 3. Two time periods 



Table 3. — Monthly mortality rates (a) and instanta- 

 neous mortality rates (i) for transects A, B, and C, 

 individually and combined, for various time periods. 



were treated: one from October to December 

 (the optimum fishing season) and the other from 

 December to October (the descending limb). 

 The latter period includes the spawning season. 

 Although accurate rates for the first period 

 (the dome) could not be accurately ascertained, 

 a crude mortality estimate was desired for the 

 fishing season. 



Monthly mortality rates ranged from 1% to 

 31% for the dome and 18% to 27% for the 

 descending limb. Averages were 12% and 23% 

 respectively. 



Age classes were difficult to distinguish and 

 all data may not have been from the 1967 year 

 class. This could have caused difl'erences in the 



402 



