Conservation Issue

Invasive and incompatible non-native fish species pose significant threats to aquatic ecosystems. These non-native species can often outcompete native species for food and space, disrupt food webs, and alter habitats. These species can lead to declines in native fish populations through genetic impacts of hybridization and create long-term ecological imbalances. Non-native fish such as common carp and brook trout are able to outcompete native trout or other native species in many places where they have been stocked in the western United States. “Trojan Male” or “YY” Fish is a promising new technology to help managers control and eradicate unwanted fish populations by incrementally eliminating female non-native fish, while leaving the existing native or desirable fish unharmed.

Trojan Male Fish Approach

Like mammals, most fish species have two sex chromosomes: X and Y. A natural genetic female will have two X chromosomes (XX), and a natural genetic male will have one X and one Y chromosome (XY). In the development of YY brood stocks, very young fish are exposed to the female hormone estradiol in their feed at a specific level and for a specific length of time. The treatment makes the male fish become egg-producers, but they are still genetically male. These egg-producing males are then bred with normal XY males, which means 25% of the offspring will have two Y chromosomes, hence referred to as YY males. Some YY males produced in that generation are exposed to estradiol, which then converts them into egg-producing YY males. This process allows a hatchery to have a group of egg-producing YY males and a group of sperm-producing YY males, which are kept for brood stock. After that, hormone treatment is unnecessary because only Y chromosomes are present in the eggs and sperm of the brood stock, and all the offspring will be YY males. These YY offspring are stocked into target areas with hopes that YY males will cross with females. Over time, the population should shift towards a greater number of males since their offspring will all be normal XY males.  Eventually, with no females remaining in the population the reproduction of the fish drops to zero. Preventing further reproduction by the invasive species is one tool to help eradicate the undesirable species. Recent modeling exercises for brook trout indicate this process can be expedited by depressing non-naive wild fish populations through electrofishing, netting, or other mechanical removal methods.

Benefits to Anglers

The YY Fish technique has some advantages over traditional non-native fish control.  It can be a very selective method, impacting only the target fish species allowing desirable species to remain in a location for recreational or conservation purposes while the undesirable species are simultaneously eliminated.  The targeted approach not only protects native species but also enhances the success of chemical fish removal where eradication efforts may not be 100% successful at removing non-native fish.  Stocking YY fish after chemical reclamations increases the chances for successful non-native fish removal and better conservation outcomes.  As this is a new technology, managers are continuing to investigate how this technology could enhance other fish management techniques already in use across the country.

Why WAFWA?

WAFWA is leading YY Fish efforts due to our organization’s ability to coordinate and leverage activities by various member states and agencies using this technology.  While YY Brook Trout are the first established and utilized brood stock, this approach has promise for a wide variety of species.  Aligning the needs of various agencies for YY fish with production and development from participant brood stock sources allows all member agencies to benefit.