Used in hatcheries to manipulate the phenotype sex of fish in the very early stages of development. The sex of fish is genetically determined at some point during its embryonic development and it is possible to influence the sex up to this point - thereafter only radical measures work e.g surgery! With salmonids phenotype is determined after hatching - by feeding hormone- laced food for the first 40 to 60 days of first feeding the phenotype can be pre-selected, i.e. you can decide whether you want all male or all female. The genotype remains fixed within the genes, thus a genetic female (XX) can be forced to become a phenotypic male (grow testes which produce functional milt with XX genotype) which when crossed with normal eggs (XX) produce all female progeny.
The levels of hormone or steroid added to the diet are low (2-20 mg/kg of food) and once the phenotype is set they can be discontinued so that they are not present in the harvest stock. Use of low levels of steroids also improve growth and feed efficiency in salmonids but there are marketing problems with their use.
