in tilapias, salinity tolerance varies ontogenetically, suggesting 

 that seawater survival and growth may be affected by the age or size 

 at which seawater acclimation is begun (Watanabe, Kuo and Huang, 

 1985b). Preliminary results of studies on responses of Florida red 

 tilapia transferred to seawater at different times of the sex 

 reversal period suggest that survival was relatively poor among 

 groups acclimated to seawater before 35 days post-hatching 

 (Watanabe, unpublished data). Corresponding effects on growth are 

 under study. Additional studies are required to determine the 

 relationships between salinity tolerance and age and body size to 

 provide a practical basis for determining optimal time for transfer 

 to seawater. 



Production of seedstock in brackish- or seawater 



An alternative approach to reducing low-salinity water 

 requirements for holding broodstock and early rearing is to 

 maintain and spawn broodstock at elevated salinities. There is 

 evidence suggesting that in tilapias, exposure to a saline 

 environment at the early embryonic stages may confer adaptive 

 advantages to these individuals which improve growth and survival 

 in seawater (Watanabe, Kuo and Huang, 1985a). This approach is 

 generally limited by the fact that, in tilapias, normal 

 reproduction is inhibited by increasing salinity (Watanabe and Kuo, 

 1985; Ridha, Al-Ahmad and Al-Ahmad, 1985). 



The effects of salinity on reproductive performance of Florida 

 red tilapia were studied. Adult breeders were maintained in 

 laboratory aquaria at salinities of 1, 9, 18, 27 and 36 ppt under 

 controlled photoperiod (14 L: 10 D) and temperature (28°C). Egg 

 production and spawning was observed at all salinities, although an 

 inhibitory effect of salinity on reproductive performance was 

 evidenced by a marked decline in fertilization and hatching success 

 at salinities above 18 ppt (Watanabe, Burnette, 011a, and Wicklund, 

 in press). Nevertheless, viable yolksac-absorbed fry were produced 

 at all salinities, including full seawater (36 ppt). These results 

 suggest that although seed production at salinities as high as 36 

 ppt is possible, productivity declines at salinities higher than 18 

 ppt. Seed production at high salinity may be practical in areas 

 where low-salinity water is lacking. 



Seawater survival and growth of progeny spawned at different 

 salinities 



A study comparing survival and growth of progeny spawned in 

 brackishwater (18 ppt) with those spawned in freshwater (2 ppt) 

 showed that survival and growth were not significantly different 

 between these groups under a mean water temperature of 27°C. 

 However, when temperatures abruptly declined to below 25°C, 

 survival and growth remained significantly higher in brackishwater- 

 spawned progeny (Watanabe, French, Ernst, 011a, and Wicklund, in 

 press). These results suggest that seawater survival and growth are 



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