and above. Growth under 36 ppt at a low density (10 fish/tank) 

 was lower than that at a high density. At an intermediate density 

 (15 fish/tank), however, there was a clear trend toward increased 

 growth with salinity due to increased food consumption and 

 declining conversion ratios with salinity (Watanabe, Ellingson, 

 Wicklund and 011a, 1988). These results support previous reports 

 of faster growth in brackish- and seawater than in freshwater in 

 certain tilapias including 0^ mossambicus ( Canagaratnam, 1966; 

 Jurss, Bittorf, Vokler and Wacke, 1984) and Taiwanese red tilapia 

 hybrids (0^ mossambicus x 0^ niloticus ) (Liao and Chang, 1983). 



The influence of behavior on growth at different salinities 



The apparent density-dependent differences in growth response 

 to salinity observed in these studies suggested that behavioral 

 factors influenced these results. Further investigations revealed 

 that agonistic encounters among fish as well as percentages of 

 fish with damaged fins (due to agonistic encounters) declined with 

 salinity, suggesting that growth response to salinity was 

 influenced by inhibitory effects of territorial aggression, which 

 was mitigated by increasing salinity (Watanabe, French, Ellingson, 

 Wicklund and 011a, 1988). This suggested that aggression impairs 

 growth by lowering food consumption (appetite) and increasing 

 conversion ratios. Hence, as aggression was mitigated by 

 increasing salinity, growth was improved. That behavioral 

 interactions may exert inhibitory effects on growth which vary 

 with salinity, was previously suggested for Taiwanese red tilapia 

 (Liao and Chang, 1983). 



Growth in seawater pools under maximum feeding 



Survival and growth of monosex male Florida red tilapia 

 (1.3 g mean weight) in seawater pools (23-m3) under conditions of 

 maximum feeding with a prepared diet (30% protein) were studied. 

 A mean body weight of 467 g was attained after 170 days at a 

 survival rate of 90% (Ernst, Ellingson, 011a, Wicklund, Watanabe, 

 and Grover, in press). Mean specific growth rates decreased as a 

 function of fish size from 9.2%/day to 0.8%/day during the 170 day 

 period, and exceeded reported values for Taiwanese red tilapia 

 hybrids in freshwater. A high growth capacity in seawater from 

 fingerling through market stages was demonstrated. 



PILOT HATCHERY FOR SALTWATER TILAPIA CULTURE 



The methods that have been developed at CMRC for culture of 

 Florida red tilapia in seawater are relatively simple: spawning 

 occurs naturally in brood tanks maintained at low salinity (3-6 

 ppt groundwater) and yolksac-absorbed fry are collected following 

 release by mouthbrooding females. The fry are then sex-reversed 

 by hormone (17 alpha-ethynyltestosterone ) treatment to transform 

 genotypic females to phenotypic males (Guerrero, 1975). Monosex 

 culture prevents unwanted reproduction at an early age which 



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