Table 2.-Population and catch, Tilapia mossambica, in two sizes of tanks. Target 



exploitation rate was 10% per 2 mo. 



10 = 1 July 1970. 



^Initial stocks were: S-6 Immatures, 2 males, 3 females; L-4 immatures, 4 males, 3 

 females. 



3To each stock, 5 Immatures were added. 



■•Stocks were adjusted to 3 immatures, 2 males, and 4 females each. 



sstocks readjusted to 1 male and 3 females eacti. Temperature was increased from 

 24° to 30°C at month 5.7. 



I 2n 



Figure l.-Course of biomass and catch. 



recruitment occurred throughout the experiment 

 after month 6.9 in L but was limited to 2-mo period 

 6.9-9.2 in S. 



Exploitation began at month 19.1 for both 

 populations, at a target rate of 10% per 2 mo. 

 Because of the small numbers of fish in the 

 populations, actual percentages removed (Table 2) 

 varied considerably from 10%. Populations dif- 

 fered in their response, S declining while L 

 remained almost constant (Figure 1). Mean values 

 of catch were S, 111 g; L, 124 g. Although the 

 exploitation data were too few for firm 



conclusions, they suggest a greater yield from the 

 larger tank, under the same catch rate and food 

 amount. Here the response for T. mossambica was 

 reversed from that found by Silliman (1972) for T. 

 macrocephala. If significant, this difference may 

 be due to the fact that T. mossanbica reaches 

 larger ultimate size than T. macrocephala. The 

 presence of a few large individuals in a population 

 of small numbers (Table 2) could lead to a different 

 response of the population to space available. 



Literature Cited 



Kelly, H. D. 



1957. Preliminary studies on Tilapia mossambica Peters 

 relative to experimental pond culture. Proc. 10th Annu. 

 Conf. Southeast Assoc. Game Fish Comm., p. 139-149. 

 Silliman, R. P. 



1972. Effect of crowding on relation between exploitation 

 and yield in Tilapia macrocephala. Fish. Bull., U.S. 

 70:693-698. 

 Swingle, H.S. 



1960. Comparative evaluation of two tilapias as pondfishes 

 in Alabama. Trans. Am. Fish. Soc. 89:142-148. 

 UcHiDA, R. N., AND J. E. King. 



1962. Tank culture of Tilapia. U.S. Fish Wildl. Serv., Fish. 

 Bull. 62:21-52. 



Ralph P. Silliman 



Northwest Fisheries Center 



National Marine Fisheries Service. NOAA 



Seattle, WA 98112 



Present address: 



il35 Baker NW 



Seattle, WA 98107 



686 



