Environmental sex determination:: the effect of temperature and salinity on sex ratio in Oreochromis niloticus L.

This paper reports the effects of environmental conditions during the period of sex differentiation on the sex ratio of the Nile tilapia (Oreochromis niloticus). Different sex genotypes were exposed to varying temperatures (putative all-female, all-male and all-YY males) and salinities (putative all-female progeny only) for a minimum period of 21 days after first feeding and were on grown prior to sexing by gonad squash. The majority of the putative all-female progeny exposed to high temperature (36.54 +/- 0.39 degrees C) produced significantly higher percentages of males compared to controls reared at ambient temperature (27.87 +/- 1.40 degrees C). Similarly, at high temperature, some of the all-male and YY male progenies had significantly lower percentage of males compared to controls. Sex differentiation in YY males appears to be more labile than in normal XY males although this could possibly be attributable to different levels of inbreeding. Low temperature (25.78 +/- 0.24 degrees C) and varying levels of salinity (11.30 to 26.65 ppt) did not significantly affect sex ratios. The apparent sensitivity of sex differentiation to some environmental factors is considered in the context of a predominantly monofactorial genetic sex determining mechanism. Implications for sex control technologies are discussed. (C) 1999 Elsevier Science B.V. All rights reserved.