Influence of sexual genotype on the behaviour of females (genotype WZ) and pseudofemales (genotype ZZ) in the tilapia Oreochromis aureus

α-ethynylestradiol sex-reversed males of Oreochromis aureus(pseudofemales, ∆F, genotype ZZ) are used in aquaculture to produce a male monosex population by crossing with ZZ homogametic normal males. When placed with males (M) and females (F) in the same spawning tank, the spawning rate of F is higher than for ∆F. In order to understand this phenomenon, comparisons were made between the behaviour of 18 F (446 ± 96 mm) and 18 ∆F (401 ± 59 mm). ∆F showed a more aggressive behaviour and were significantly more dominant than normal F in fighting pair experiments (F × ∆F) or in fighting group experiments in four different stocking densities (8, 12, 16 and 83 fish 10 -3 l with F/∆F ratio = 1). ∆F were also more aggressive towards males than F were. The results support the idea that behavioural differences exist between F and ∆F. These differences are probably due to the effect of the sexual genotype on behaviour. © 2002