The Free Online Aquaculture Dictionary

Hornorum Tilapia (Oreochromis hornorum)

Thanks to Mike Sipe for pictures
Oreochromis hornorum is a tilapia species native to Africa. the main traits of the species are that they are hardy and quite euryhaline. Used occasionally as a pure species in aquaculture, but more commonly now is used as an F1 hybrid where crosses with O. mossambicus result in nearly 100% male offspring. This is a very desirable trait as mixed populations of tilapia breed profusely and perform poorly as a result. the development of coloured strains of O. mossambicus has enabled cross checking of the breeding program. All fish resulting from the crossing of an orange mossambicus and  a grey hornorum, are copper coloured. The appearance of any orange or grey fish in the stock means that the parent fish were incorrectly sexed and the offspring are not necessarily all male. 

The process of all male production involves using a female mossambicus and a male hornorum. The mossambicus posses XX chromosomes and the hornorum male, ZZ chromosomes. As the Z chromosome exerts a dominance over the X, the resulting offspring are all ZX  i.e. male.