TANK CULTURE OF TILAPIA 



37 



SEX RATIO AND BROOD-STOCK DENSITY 



A ratio of '2 9 : 1 d was used iiiidally at the 

 pilot plant and judged to yield satisfactory re- 

 sults. The best production resulting from this 

 sex ratio was realized during July I!);")? at the 

 pilot plant, wlien production in tank 1 amounted 

 to •2i),'.no fry or 8:27 fry per female (table G). A 

 second grouping of 5 V : I i (total stock, 72) was 

 also tried in tank 2 from August to November, 

 19o7. The production per female per month is 

 given in table 9. The best production for tiiat 

 period and grouping occurred in August, when 

 fry production amounted to 2,532, or 42 fry per 

 female. 



T.^BLE 9. — Production of fry per female per month in tank 2 

 at the pilot plant, August 1957- July 1958 



Table 10. — Sex ratios and concentrations of tilapia used to 

 stock the 12 assault-boat tanks, September- Xovember 

 1958 



' Because of mortalities and transfer of fish, the number of females used 

 to calculate the production per female is based on the number of females that 

 were present in the tank for more than 2 weeks. 



A more detailed experiment designed to deter- 

 mine the ideal sex ratio for maximal yield of 

 fry -was conducted at the Kewalo plant from Sep- 

 tember to November 1958. The effect of brood- 

 stock density on spawning was simultaneously 

 investigated in the same experiment. 



The 12 assault boats mentioned earlier were 

 stocked with various sex ratios and concentrations 

 of fish. The design of this partially confounded 

 factorial experiment is shown in table 10. The 

 brood stock was fed drj' trout feed at the rate of 

 2 percent of its weight daily. It was assumed 

 that any differences in temperature or oxygen c(m- 

 centrations which might occur among the tanks 

 would not bias the results. 



The experiment was terminated after 13 weeks. 

 An analysis of variance of the data (table 11) 

 indicated significant differences (7^=5.73, P 

 < 0.01) between fry production with respect to the 

 sex ratios used in the experiment. (A probability 

 level of 5 percent is considered the maximum value 

 for a conclusion of significance in this report.) 

 The most productive ratio was 3 $ : 1 5 (tanks 

 4 and 5) . Tanks 1 and 2 with a ratio of 2 9 : 1 5 

 were almost equally high in production for the 

 first 2 months of the experiment (fig. 10) . 



30 



SO 70 



CONCENTRATION 



FioURE 10. — Prodtiction i)er fi'iiialc in iclation to sex 

 ratios and foiieentration.s of brood .stock. 



