Production of Oreochromis niloticus fry for hormonal sex reversal in relation to water temperature

Summary Recently hatched tilapia fry 9 to 11 mm total length (TL) are preferred for hormonal sex reversal because they are most likely to be sexually undifferentiated. Thirty‐three trials were conducted in Honduras between September 1988 and March 1990 to quantify the effect of water temperature on Oreochromis niloticus fry production in earthen ponds for hormonal sex reversal. Two 0.05‐ha ponds were simultaneously stocked with brood fish in each trial; generally, one pond was harvested after 17 days, the other after 20 days (range 16 to 21 days). Fry production was evaluated in relation to degree‐days from the threshold temperature of 15°C. Harvests averaged 86,000 fry/0.05 ha. A total of 4,897,000 fry were produced, of which 4,363,000 fry were of appropriate size for hormone treatment. No fry production occurred at less than 140 degree‐days; fry production increased significantly with increased degree‐days above this level. Above 195 degree‐days percent of the population retained by a 3.2‐mm vexar‐mesh grader (too large for androgen treatment) increased significantly with increased degree‐days. Fry retained by the grader averaged 14.2 mm TL, while fry not retained averaged 9.5 mm TL. No significant linear relationship between degree‐days and number of fry not retained b the grader was observed between 140 to 280 degree‐days. However, production appeared to peak at agout 210 degree‐days.