not impaired in progeny spawned at salinities as high as 18 ppt, and 

 that brackishwater-spawned progeny possess a higher capacity for 

 survival and growth in seawater than freshwater-spawned progeny when 

 environmental temperatures approach lower tolerance limits. 



Seawater survival and growth of progeny spawned at salinities 

 higher than 18 ppt has not been studied. Available information 

 suggests that advantages of spawning at high salinities will be 

 gained at the cost of lowered seed production at these salinities. 



DEVELOPMENT OF COST-EFFECTIVE FEEDS FOR SEAWATER CULTURE 



An approach to reducing costs associated with use of prepared 

 diets during nursery culture in land-based systems, is the indirect 

 utilization of cheap animal wastes as fertilizer for production of 

 natural planktonic foods. To assess the feasibility of this 

 approach, growth of Florida red tilapia receiving a prepared diet 

 was compared to fish reared in seawater pools enriched with chicken 

 manure. Growth of fish in the fed pools continued at an exponential 

 rate to day 80, while growth in manured pools became asymptotic by 

 about day 30, indicating that food availability in manured pools was 

 growth limiting (Ernst, Ellingson, 011a, Wicklund, Watanabe, and 

 Grover, in press). Significant differences in fish growth among 

 manured pools were evident, suggesting that food availability varied 

 significantly between manured pools. Plankton and fish gut-content 

 analyses supported the conclusion that identical methods of manuring 

 do not necessarily produce identical biological communities and fish 

 growth (Grover, 011a, O'Brien, and Wicklund, in press). Further 

 research is needed to determine fish densities at which a 

 sustainable yield of the food resource and acceptable fish growth 

 rates could be maintained. Supplementary feeding with prepared 

 diets in manured systems may also be practical. 



REARING EXPERIMENTS IN FLOATING SEA CAGES 



In May 1986, a pilot study was conducted to assess survival and 

 growth of Florida red tilapia in floating cages placed in a seawater 

 (37-40 ppt) channel near Barraterre on Great Exuma, Bahamas 

 (Watanabe, Wicklund, 011a, Ernst and Ellingson, in press). The 

 technical feasibility of rearing Florida red tilapia at high 

 densities in floating sea cages using prepared diets was 

 demonstrated in this study. 



A detailed cage production study involving 9,600 sex-reversed 

 fingerlings (10.1 g mean weight) and twenty-eight l-m^ experimental 

 cage units was initiated at Barraterre in August 1987. The effects 

 of feed rate and stocking density on growth from fingerling through 

 market stages, using a commercially available diet (Purina Tilapia 

 Chow, 32% protein), are being assessed. After 43 days of culture, 

 mortalities were negligible, and mean body weights among the 

 experimental groups ranged from 48.6 to 78.2 g, while food 

 conversion ratios (dry weight: wet weight) ranged from 1.4 to 2.6 



342 



