SILLIMAN: EXPERIMENTAL POPULATIONS OF TILAPIA MOSSAMBICA 



Table 4.-Recruitment and stock. i? INT = Pn + l-Pn + Mint + Cint, where Pis stock, INT is 

 interval between counts n and n+l,Ris net changei , M is recorded mortality and C is catch, all 

 in numbers. S is mean stock in kilograms for previous interval. 



'Rf^f > indicates recruitment of at least the indicated number of fish; R^^j < indicates 

 unrecorded mortality of at least the indicated number and R^f^-^ = indicates either no recruit- 

 ment and unrecorded mortality or the two exactly balanced. 



^Adjusted to a standard 2-mo interval length. 



^Indicated intervals omitted because of re-equalization of stocks. 



''Exclusive of 46 new fish added for genetic variability. 



mo. Each value of i?iNTwas therefore compared 

 with the mean stock (S) for the preceding 2-mo 

 interval (Table 4). 



The stock-recruitment data were highly varia- 

 ble and were, therefore, treated as group means 

 based on 5-kg intervals of S, considering negative 

 values of Ri^^ to be equal to zero. Although the 

 data indicated no regular relation (Figure 3), they 

 were fitted with parabolas to indicate central ten- 

 dency, even though fits were poor. These, based on 

 30 pairs of observations each, were: 



Selectively fished stock 



R^+i = 6.163 S^ - 0.5209 S/ 

 Unselectively fished stock 



Rn+i = 3.304 S^ - 0.2158 S^f, 

 where N is number of the 2-mo interval, R is in 



-r 

 12 



4 6 8 10 



MEAN STOCK, INTERVAL N (kg) 



14 



16 



Figure 3.-Stock and recruitment. Test popuhition was selec- 

 tively fished; control, unselectively. Parabolas shown were fitted 

 by least squares. 



499 



