TANK CULTURE OF TILAPIA 



41 



Twenty-five fish were measured from each of 

 the lots of 1,000. 2,000, and .'^.OOO fisli, 40 from the 

 lot of -1,000 tish, and 50 from the lot of (i.OOO lisli. 

 Table 14 gives the weekly averaj^es of lenj!:th and 

 weight of fish in the randondy collected sub- 

 samples, wliile figure 14 shows the regression lines 

 fitted to the growth data. 



Table 14. — Weekly average length (mm.) and weight (g.) 

 and average absolute growth rates in random siihsamples of 

 tilapia from various concentrations of fry, 1959 



10 11 12 13 14 15 



TIME (WEEKS) 



Figure 14. — Regression of length on time for five lots 

 of tilapia reared in various concentrations of fisli in 

 tanks. 



Examination of the regression coefficient from 

 each lot indicated that young in the tank stocked 

 with 1,000 fish had the best growth rate (2.9 mm. 

 per week) during the 12-week period, and that 

 growth rates were somewhat slower with an in- 

 creasing lunnber of fisli per lot. Fish in tanks 

 stocked with 2,000 and 3,000 fry had almost 

 identical growth rates (2.4 mm. per week) but 

 less favorable growth than the 1,000-fish lot. The 

 lot of 6,000 fish, for some unknown reason, had a 

 l)etter growth rate than the lot containing 4.000 

 fish (2.2 and 1.8 nnn., respectively, per week). 



Statistically, tlie differences in growth among 

 the li\e lots of fish were significant (/''=41.92, 

 /'<0.01 ). From the results of the experiment, it 

 follows tiial the ideal stocking density for nursery 

 waters ( f ly tanks) would be 9 fry per cubic foot 

 or 15 fry per square foot of surface area (1,000- 



fish lot). Some lesser concentration, as yet unde- 

 termined, might prove even better. However, 

 from a practical standpoint, these very low stock- 

 ing rates might not be the most feasible economi- 

 cally, as the space requii-ements and construction 

 costs for fry-rearing tanks would be enormous in 

 a commercial operation. 



Earlier, we mentioned that newly emerged fry 

 almost always were captured near the surface of 

 the water. Upon transfer to nursery waters, the 

 fry continue to exhibit this behavior. Therefore, 

 we believe that stocking density should be related 

 to surface area rather than to volume of water. 



DIET AND GROWTH 



An experiment to compare the growth rates 

 obtained on an inexpensive, commercially avail- 

 able feed and the more expensive, but highly 

 nutritious, trout feed was initiated in April 1959 

 simultaneously with the experiment on relation 

 of space to growth at tlie Kewalo plant. An as- 



