SILLIMAN: EXPERIMENTAL POPULATIONS OF TILAPIA MOSSAMBICA 



10 20 30 



THICKNESS (mm) 



40 



Figure 5.-Bimonthly thickness frequencies of catches, test 

 (selectively fished) population. Vertical broken lines indicate 

 selection points. Numbers in panels indicate months from start. 



(Table 3). Numbers of fish growing above the 

 selection point rapidly diminished in the test 

 population (Figure 5) so that insufficient numbers 

 were available from which to catch the target 

 percentage. To continue exploitation, it was 

 necessary to lower the selection point to 22 mm as 

 shown. With few exceptions, all fish caught from 

 the test population were above the selection point. 



Catches from the control population were taken 

 representatively over all sizes larger than fry and, 

 therefore, represented the size composition of the 

 stock above the fry size (Figure 6). Significant 

 amounts of recruitment at months 43.2, 45.2, 55.1, 

 63.2, 67.1, and 69.1 (Table 4) appear as modes of 

 small fish in the size frequencies, and the more 

 prominent ones can be traced through succeeding 

 frequencies. 



A summary of catch size frequencies (Figure 7) 

 clearly reveals the differences between catches 

 from test and control populations. It is evident 

 that the selection device employed was almost 

 completely effective. The appearance of roughly 

 equal proportions of males and females in the test 

 catches, after lowering the selection point, is also 

 apparent. It can be seen that a selection curve was 



at work, such that fish were not fully retained until 

 they had reached a thickness of about 2-4 mm 

 above the selection point. 



The relation of yield to exploitation was as- 

 sessed by fitting a Fox (1970) exponential 

 surplus-yield model to data of catches and stock 

 (Figure 8). The method of fitting used requires 

 equilibrium yields. Although absolute equilibrium 

 obviously was not attained, it was considered that 

 the biomass and catch levels (Figure 2) at months 

 29-35 (zero exploitation), 59-61 (10% target rate), 

 and 69-73 (20% target rate) represented 

 sufficiently close approximations to equilibrium 

 for fitting the model. The calculated maximum 

 sustainable yield (1.39 kg per 2 mo) from the 

 selectively fished test population was substantially 

 lower than that for the unselectively fished control 

 (2.36 kg). If we wish to consider a comparable 

 commercial fishery, however, we might assume 

 that only the fish above the selection point are 

 commercially desirable. Catch thickness frequen- 

 cies for the 22-mm selection point (Figure 7) 

 showed that 97% of the fish in test catches were 

 above the selection point as compared with 40% for 

 the control catches. Although these data cannot be 



T. mossombico , control 



10- 

 5- 



in 



g: 5- 



Ll_ 



o 0- 



39.2 



^ /XnA 



41.3 



A >v /^ /U 



43.2 



45.2 



AVsA AA 



47.3 



48.9 



51.2 



53.2 



A^ A'^^^y^ 



55.1 



_cv_ 



Aa^^^^^s^ 



59.0 



61.1 



A^ 



A A 



KL 



^^AA\ aAA 



6 5.4 



JS^ 



67.1 



^A^/^y\/\ y<. ,^_ 



69.1 



A. 



Ar^ A_ 



71.2 . 



Aa.^ /VA ^^ 



73.2 



A^^V/^ /\ 



75.1 



57.2 



A^ ^ .^A/v ^ y\/\ /S. 



—I — " — "-r" — — r— ■-■ 1 — 



10 20 30 40 



THICKNESS (mm) 



10 20 30 40 



THICKNESS(mm) 



Figure 6.— Bimonthly thickness frequencies of catches, control 

 (unselectively fished) population. Numbers in panels indicate 

 months from start. 



501 



