Abstract
The relationship between production in subtropical earthen ponds of fry suitable for hormonal sex inversion and degree days was quantified for Nile tilapia (Oreochromis niloticus; Egypt strain) and blue tilapia (O. aureus). Degree days were calculated for each trial as the sum of mean daily water column temperature minus the reference temperature, 15°C. No fry from either species were harvested at fewer than 124–126 degree days. Total fry production for each increased linearly as cumulative degree days increased from 129 to 281 degree days. Production of fry suitable for hormonal sex inversion was positively related to cumulative degree days only from 115 to 215 degree days. Tilapia fry too large for hormonal sex inversion were not observed until greater than 200 cumulative degree days and increased with increasing degree days thereafter. Optimal production in earthen ponds in a subtropical climate of Nile tilapia and blue tilapia fry suitable for hormonal sex inversion was obtained at 190–215 degree days.
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