Why YY Males? Using Hatchery Brook Trout to Eliminate an Invasive Species


Kennedy, P.A., K.A. Meyer, D.J. Schill, M.R. Campbell, N.V. Vu. 2018. Survival and Reproductive Success of Hatchery YY Male Brook Trout Stocked in Idaho Streams. Transactions of the American Fisheries Society.


Background: What’s wrong with Brook Trout?

Brook Trout were first introduced to the American West in the early 1900s. Since then, they have hurt native fish populations through competition and predation. Sometimes, they can successfully mate with native species of trout, creating new fish hybrids that also compete with native fish. Over time, the invasion of Brook Trout has spread to more and more streams, weakening native fish populations. Today, scientists are trying to figure out creative ways to remove these invasive trout. One solution may include using genetic technology to manipulate the sex ratio of male to female Brook Trout in the wild.

Like humans, Brook Trout have two sex chromosomes that can be X or Y – males are XY, while females are XX. Researchers are able to create “feminized” trout by exposing XY males to estrogen. These XY males can then produce eggs, which are crossed with other XY males. Approximately a quarter of the offspring of this crossing will be YY males, which normally do not occur in nature.

If a YY male successfully mates with a female, all of their offspring will be male. If the YY hatchery fish can survive and successfully mate after being released in the wild, eventually all of the Brook Trout in that area will be XY male, and therefore unable to reproduce, effectively sterilizing the population.


Drawing of a Brook Trout, courtesy of the Washington Department of Fish and Wildlife website (https://wdfw.wa.gov/fishing/washington/Species/1261/)


Study: How successful are YY hatchery fish in the wild?

A recent study by Kennedy et al. examined the “fitness” (chance of survival and successful reproduction) of YY brook trout hatchery fish when they were released into streams with either resident fish that had been “suppressed” (some fish removed by researchers) or not suppressed. Hatchery fish frequently face low success rates of survival and reproduction when they are released into wild streams, due to the time it takes them to adjust to stream flows, as well as competition with resident fish.

Researchers released approximately 2000 YY hatchery fish in the Big Lost River basin of Idaho in June 2014. They recaptured adult fish in October 2015 to estimate how many of the hatchery trout survived. They also took approximately 100 tissue samples from young trout to estimate the reproductive success of the released fish.

YY Brook Trout from the hatchery had a slightly reduced fitness than their wild counterparts, but were still able to survive and reproduce in the wild. In streams where researchers first removed a portion of the wild Brook Trout, the hatchery trout were more successful.


Why is this important?

Brook Trout have had a negative impact on many native species throughout the Western U.S for the last century. This includes several species of salmon, some of which have threatened or endangered populations that are at increased risk of extinction.

This study represents the first time scientists have tried releasing YY fish of any species into the wild. Harnessing genetic technology in this way is a potential strategy to eradicate invasive species like Brook Trout over time without employing poisonous chemicals or other higher impact methods in wild streams.

There are several questions that need to be addressed before this technology could be considered for use, including cost-effectiveness, the effect of different stocking rates, and the possible impact hatchery fish could have on all wild species present in a specific area.

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Maddie Halloran

Maddie Halloran

I am a second year master's student at Humboldt State University in the Fisheries Biology Department. I'm interested in human impacts on the environment and conservation. When I'm not counting fish you can probably find me outside on an adventure or eating ice cream on my porch.

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