131. This technique is used to produce female fish only, avoiding the production of males which perform poorly and which can become sexually mature in less than one year. To start the process the male hormone, methyl testosterone, is introduced into fry food; this turns all these fish into males. Those fish which would have been males anyway are identified by the presence of males in their subsequent offspring and they and their offspring are not used in production. On the other hand the male fish which, without the hormone treatment would have been females, produce only female offspring when mated with females. This technique is now commonly used and each year a small group of female fry is set aside for masculinisation so as to provide males for continuing the line. In effect the Y chromosome (male determining) has been eliminated from the stock.