SILLIMAN: EFFECT OF CROWDING 



20 22 21 



MONTH (0= 12-1-67 



Figure 1. — Bimonthly stocks and catches, numbers, 

 Tilapia macrocephala, 77.6-liter (S) and 155.2-liter (L) 

 tanks. Percentages indicate target bimonthly exploita- 

 tion rates. 



ining net changes in number (Rint) during the 

 intervals between counts (Table 3), 



Recruitment is defined here as survival of 

 young fish to the size that could not pass through 

 the 3-mm openings of the refuge fence. On 

 this basis, no recruitment was apparent during 

 the period of 10% exploitation, months 0-14 

 (Table 3) . However, it is possible that some did 

 occur, balanced by unrecorded mortality. Deaths 

 not recorded are indicated by negative values of 



R,NT (Table 3) . Also, it is almost certain that 

 spawning occurred in which the resulting young 

 were victims of cannibalism before reaching re- 

 cruit size. Positive values of Rint after month 

 14 show that recruitment occurred in both S and 

 L under the 25% exploitation rate, with recruit- 

 ment in L about double that in S. 



Growth of individual fish can be detected by 

 observing concurrent changes in numbers and 

 biomass. Growth was demonstrated from month 

 to month 12.3, when biomass increased and 

 numbers decreased in both S and L (Table 2, 

 Figures 1 and 2). 



10 12 



MONTH (O- 12-1 -67) 



Figure 2. — Bimonthly stocks and catches, weights, 

 Tilapia inacrocephala, 77.6-liter (S) and 155.2-liter (L) 

 tanks. Percentages indicate target bimonthly exploita- 

 tion rates. 



Table 2. — Population^ and catch, Tilapia maorocephala, in two sizes of tanks during bi- 

 monthly exploitation. 



1 In the 77.6 liter tank the population was replaced after accidental mortality at month 19. Also, sex ratio was 

 changed from ) male;5 females to 3 moles:3 females at month 21.2. 



^ Because of the relatively small numbers of fish, the effective exploitation rates varied considerably from these. 

 In fitting the popLflation model, the effective rates in terms of weight were used. 



3 0= December 1, 1967. 



695 



