TANK CULTURE OF TILAPIA 



43 



Table 16. — Differences in average length (mm.), weight 

 (g.) and average absolute growth rates of 5 lots of iOO fish, 

 in relation to types of water and feed 



' These figures are based on length-weight measurements at end of the 

 fourth week. 



experimental tanks, the experiment was termi- 

 nated at tlie end of 5 weeks. 



A random subsample of 10 fish was collected 

 from each aquarium and the fish measured once 

 each week during the 5-week period (table 16). 

 The results were not suitable for statistical analy- 

 sis. A general summary (table 16) shows that 

 mortality was lowest in fresh water and highest 

 in sea water. Differences in growth rate were 

 slight and most likely of no biological significance. 



A second experiment, which dealt with the 

 effects of salinity on gi'owth of the young, was 

 conducted simultaneously with the experiment on 



6 7 8 9 

 TIME (WEEK-SI 



10 II 12 13 14 IS 



Figure 16. — Regression of length on time for young 

 tilapia reared in brackish and in fresh water. 



the effect of salinity on the rate of reproduction. 



Four assault-boat tanks, two containing brack- 

 ish water with a salinity of about 10 Voo and two 

 with fresh water serving as controls, were stocked 

 simultaneously with 4,000 fry. The fish were fed 

 trout feed twice a day. Salinity determinations 

 (by hydrometer) of the brackish- water tanks were 

 made daily (table 18 lists average and range of 

 salinities for the 2 months of the experiment). 

 Length and weight measurements were made 

 weekly for 8 weeks on a random subsample of 

 40 fish from each tank. The results of the experi- 

 ment are recorded in table 17, while the regres- 

 sion lines describing the growth in each tank are 

 shown in figure 16. 



It is evident from the regression coefficients that 

 the brackish-water environment had a pronounced 

 influence on the growth rate of the young fish. 

 Here again, we are not certain of the effect of the 

 saline environment on young tilapia. but presum- 

 ably it altei-s metabolic processes enough to affect 

 growth considerably. An analysis of variance in- 

 dicated a significant difference in growth between 

 treatments (F= 39.94, P< 0.01), and no significant 

 dift'ei-ence in growth between replicates. 



Another interesting ixspect of this experiment 

 was that the growth rates of the fish in the fresh- 



Table 17. — Average length (mm.) and weight (g.) and 

 average absolute growth rates of random subsamples of 

 tilapia from 4 lots of 4,000 fry reared in brackish and 

 fresh water 



Item 



Initial length.. 

 Initial weight. 



1st week: 

 Length - 

 Weight- 



2d week : 

 Length - 

 Weight. 



3d week: 

 Length- 

 Weight: 



4th week: 

 Length. 

 Weight - 



5th week: 

 Length. 

 Weight. 



6th week: 

 Length . 

 Weight. 



7th week: 

 Length - 

 Welght- 



8th week: 

 Length - 

 Weight. 



Average absolute growth rate 3.8 mm 



Brackish water 



Tank 2 Tank 4 



12.4 

 0.028 



14.1 

 0.042 



15.8 

 0.070 



21.6 



0.178 



26.2 

 0.295 



31.2 

 0.548 



33.4 



0.772 



39.2 

 1.200 



42.8 

 1.412 



12.6 

 0.030 



13.5 

 0.040 



16.5 

 0.072 



19.8 

 0.145 



25.3 

 0.280 



29.9 

 0.500 



34.5 

 0.892 



38.1 

 1.100 



41.8 

 1.378 



Fresh water 



Tank 6 Tank 11 



12.0 

 0.025 



15.2 

 0.055 



17.6 

 0.080 



20.2 

 0.265 



23.8 

 0.242 



25.4 

 0.300 



28.8 

 0.490 



32.0 

 0.632 



34.1 

 0.6S2 



2.8 mm. 



12.1 

 0.025 



13.5 

 0.042 



15.2 

 0.065 



17.5 

 0.225 



2!.0 

 II. 145 



26.5 

 0.380 



29.3 

 0.512 



33.4 

 0.712 



36.0 

 0.870 



