The sex ratios of hatchlings from nests incubated in exposed hatcheries were determined by histological analysis in the dry season of 1997. The study was repeated from November 1997 to January 1998 during an El Niño phenomenon that brought drought conditions instead of the anticipated wet season. Green turtle Chelonia mydas eggs were transplanted to 12 nests in an open-air beach hatchery and the incubation temperature was monitored with thermocouples. Sampks of 15 hatchlings from each nest were sexed by histological examination of their gonads. The mean incubation temperature during the critical period for sex determination was 31.3 ± 0.8ºC, and the mean critical period was estimated to last 15 days, from day 18 to day 33. On average, incubation lasted 50.3 days. The hatchery produced 100 %female green turtle hatchlings (n = 169), consistent with the previous study, but a departure from the mixed sex ratio (female: male at 7:3) of hatchlings incubated in situ, even during the El Niño event. Shading of nests by vegetation in the wild and by netting in the open-beach hatchery reduced temperatures and contributed to mixed sex ratios. Open-beach hatcheries are not recommended as a conservation strategy especially during dry conditions when only shading of the existing open-beach hatchery can bring the incubation temperature down to produce mixed ratios of hatchlings. The hatcheries at Gullisan, Selingan and Bakkungan practice shading methods to obtain mix sex ratios.
