Sterilisation may be achieved by one of two methods, depending upon the species:

• Triploidisation
• Hybridisation

Triploidisation

All female triploid offspring may be produced by the indirect use of externally administered hormones and by pressure / temperature treatment of eggs for a few minutes, post fertilisation. Within the European Union, it is forbidden to market hormone-treated fish for human consumption (see "legislation" page for further details). Methyltestosterone may be used to sex-reverse broodstock females (XX) to neo-males (only sperm containing the X chromosome). These neo-males (XX) are then crossed with normal females (XX). Induction of triploidy in the resulting zygotes will produce all-female triploid offspring (XXX). This is currently practiced on rainbow trout and perch. However, current research focuses on the development of alternative, more environmentally sensitive methods that are also species specific.

(Note: female triploids are always sterile, they do not undergo gonad development; however triploid males are only functionally sterile, they develop gonads and undergo puberty, but their sperm does not give rise to viable progeny).

Limitations
Some species of triploids may grow more slowly than diploids. Triploidy is also not 100% successful (98% success rate), and as it is not feasible on a commercial scale to test individual fish to check whether or not they are triploid, there will always be a certain number of individuals within the stock that are not triploid and are thus capable of reproducing.

Hybridisation

A hybrid is the progeny of parents of two different species and is usually sterile. Hybrid fish may be produced for restocking purposes, or because of their resistance to certain diseases. Some hybrid fish species are also produced for human consumption. In certain cases fish hybrids may be fertile, such as those of different sturgeon species.

(Information in this section was prepared with the expert assistance of scientists Alicia Felip, Pierrick Haffray, Olivier Kah and Silvia Zanuy).