Female rainbow trout show improved uniformity of growth, more desirable carcass traits and reduced expression of sexual maturation compared to mixed-sex stocks, thus improving efficiency and return for the trout farming industry. As a result, all-female stocks are much more desirable, and currently comprise the vast majority (over 90%) of the rainbow trout sold in the US. Current methods of producing all-female stocks involve the use of small amounts of hormones to effectively convert genetically female trout into functional males, which in turn are used to create all-female offspring. The use of hormones for this purpose is continually being evaluated by the FDA, however the ability to use the hormones long term is not certain. Thus, an alternative method for production of all-female stocks is needed to ensure future efficiency and stability of rainbow trout aquaculture. In our rainbow trout breeding populations we have identified rare individuals who are genetically female yet they produce functional male gonads despite having never receiving hormone treatments. They are thought to contain a genetic variant which, through selective breeding, we have harnessed and intend to utilize for production of all-female rainbow trout. Crossing these rare males with normal females produces 100% female offspring. This alternative method of producing all-female rainbow trout will be evaluated for large scale production, and identification of the genetic mechanisms involved will offer insight into sex differentiation in trout and other species. Should the new method prove viable for production of all-female rainbow trout without the use of hormones, it would ensure continued efficiency and stability of rainbow trout aquaculture.
