Fish sex may be manipulated by the direct application of hormones. However, direct hormonal methods are not permitted in many countries, indeed within the European Union only the use of certain androgens (methyltestosterone) is permitted in fish farming and then only under certain derogations. This method is currently used on rainbow trout in order to obtain large female individuals for the market, and to avoid the problem of precocious male puberty. Hormonal treatments have certain limitations and associated problems. One major limitation with steroid treatments is that one protocol is not effective for all species (dosage, timing of treatment, method of administration etc.), even for species that are genetically very close (e.g. with the salmonid family). Alternatives to hormonal treatments, such as methods based on the environmental sex determination sensitivity of some fish have great potential. However, despite significant advances in the understanding of environmental sensitivity, especially for temperature, these methods are not yet practicable by the industry.