Environmental factors and sexual differentiation in Poecilia sphenops Valenciennes (Pisces: Poeciliidae)

Because environmental factors play an important role in the sexual proportions in fish and given that the studies that evaluate the interactions of such factors are scarce, the purpose of this work was to determine the combined effect of pH/temperature (23, 26 and 29 °C; pH 6, 7 and 8) on sex determination of Poecilia sphenops Valenciennes. The trial started with newborn organisms (n = 180) that were maintained in aquaria for 6 months until they could be separated by sex. Afterwards, nine groups of four females (0.81–1.48 g) and two males (0.36–0.63 g) were distributed in 20‐L aquaria until fertilization, gestation and offspring birth took part. The offspring were grown under the same conditions until they reached sexual maturity. The results indicated the strong interaction of the factors (P < 0.05) with a lower male/female ratio at pH 7 both at 23 °C (M/F = 0.12) as well as at 30 °C (M/F = 0.10) and higher ratios at pH 8 at similar temperatures (M/F = 0.7) to 23 °C and (M/F = 0.6) to 26 °C. The females dominated in all the combinations; the least favourable combinations were 23 and 26 °C at pH 8 with 0.7 and 0.6 M/F ratios respectively. The results obtained in the laboratory explain very closely the behaviour of the population of P. sphenops from Oaxaca in the natural environment that can be used in aquaculture for baitfish production and also for aquaculture purposes.