FISHERY BULLETIN: VOL. 73. NO. 3 



X 



15- 



10- 



5- 



T. mossambicQ 



Control, month 39.2-48.9 



20- 



15- 



10- 



5- 



Test 



^ 0-f 



o 



q: 15-1 



UJ 



m 10- 



3 5- 



z 



0- 



Control, month 51.2-75.1 



30- 

 25- 

 20- 

 15- 

 10- 

 5- 



0-- 



Test 



immature- 

 mole- 

 female. 



20 30 40 



THICKNESS (mm) 



— I 

 50 



FiGUKB T.-Sammary of catch thickness frequencies. Test 

 population was fished selectively; control, unselectively. Vertical 

 lines indicate selection points. 



converted to weights, it is certain that the com- 

 parison for fish above the selection point would be 

 more favorable to the test catches than was true 

 for all sizes of fish. 



It is possible to calculate the efficiency of con- 

 version of food to fish flesh under both types of 

 fishing. The amount fed per 2-mo period was 3.75 

 kg (433 g per week from Table 1, for 8% wk). 

 Maximum sustainable yields (MSY's) of 1.39 kg 

 and 2.36 kg indicate 37 and 63% conversion 

 efficiencies for selective and unselective fishing, 

 respectively. Since the theoretical MSY's 

 represent a considerable extrapolation (Figure 8), 

 it is of interest to calculate from equilibrium yields 

 actually attained during the experiment. The larg- 

 est yields were under the 20% per 2-mo target rate 



control 



-| 1 



•8 1.2 1.6 2.0 2.4 



EFFECTIVE EFFORT 



control 



12 



BIOMASS (kg) 



FiGUBE 8.-Fitting of Fox (1970) model. Catch-per-unit-effort 

 (CPUE) is considered proportional to biomass; effort is in arbi- 

 trary units. Regression lines shown are least-squares fits. Target 

 exploitation rates were 0, 10%, and 20% per 2 mo, left-to-right in 

 upper panel, reversed in lower panel. 



for both populations. Effective exploitation rates 

 and corresponding yields were: selectively fished, 

 17.1% and 1.09 kg; unselectively fished, 17.9% and 

 1.35 kg. These yields indicate 29 and 36% efficiency, 

 respectively. The values are in fair agreement 

 with the 33% calculated by Silliman (1970) for the 

 initial growth of the populations. 



Genetic Response 



Knowledge of the number of generations in- 

 volved is essential to any genetic experiment. 

 Progression of the two most prominent thickness 

 frequency modes (months 63.2 and 69.1) in the un- 

 selectively fished population (Figure 6) gave an 

 indication of the growth rate of the fish. Frequen- 



502 



