Aisling Irwin reports from the annual meeting of the American Association for the Advancement of Science in Atlanta. A biologist has mated two male fish, producing some "extra" male offspring with two Y chromosomes instead of the usual XY.

He has developed the technique so that fish farmers will be able to control the sex of their fish. The larger yield that this will bring could make the fish - known as tilapia - such a successful product that it may soon replace cod in the traditional British fish and chip supper.

It will also help to increase the amount of protein that can be produced in developing countries.

Graham Mair, research fellow at the University of Wales, Swansea, has been trying to improve the yield of tilapia (orechromis nilotica), because it is such a versatile tropical fish, growing in fresh or sea water and resisting most diseases. The problem is that tilapia start producing offspring long before they are big enough to be marketable. The resulting fry compete with the adults for food and space and end up in the catch although they are too small to sell. And the males and females both use up energy in the reproduction which would otherwise be used for growth.

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Dr Mair wanted to try producing all-male populations. He and colleagues found that the genes that determine sex in tilapia are as simple as those in humans - ie two X chromosomes make a female and XY makes a male.

He added oestrogen to the fish feed of a generation of tilapia, which led to the males developing female organs although they carried the XY chromosomes. By a sequence of crosses, which included producing YY females, he produced a generation all of which werel YY males.

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These males form an all-male producing broodstock "that could be used by hatcheries to mass produce large numbers of genetically male tilapia," said Dr Mair.

The fish grows best in the tropics and Dr Mair has been working at Central Luzon State University in the Philippines to put the research into practice. He has pushed yield of tilapia up by a third and doubled the financial return to farmers in trials.