What is a Triploid Trout?

Fisheries biologists use all sorts of tools and tactics to manage the waters we fish. Sometimes these can conflict with angler interests. For example, if biologists wipe out an invasive species, it takes the angling opportunity for that species away.

On the other hand, sometimes these tools can be an angler’s friend. One of these, that’s beneficial for both the fishery and the fishermen, is triploid trout.

A cutthroat trout being held in someones hands.
In appearance, triploid trout are pretty much identical to diploids.

What is a triploid trout?

In a nutshell, triploid trout have three sets of chromosomes, while normal trout have two and are called diploids. Having three sets of chromosomes instead of the standard two makes these fish infertile. Triploidy isn’t limited to trout, or even to fish. Bananas, a fruit we eat without seeds, are triploids.

Triploid fish (and other triploid critters) are not genetically-modified organisms (GMOs). In GMOs, genetic material has actually been altered, often by introducing desired traits from other organisms’ DNA. In the case of triploids, no genetic material has been engineered. There is simply an extra set of the existing genetic material, causing infertility.

Triploids vs. diploids

Visually, triploid trout appear to be nearly identical to diploid trout. Male triploids can still produce gonads and exhibit spawning behavior even though they’re sterile. Females don’t produce gonads, although that wouldn’t be apparent to an angler.

One area where they may differ is size, but even that’s up for debate.

Some people claim that triploid and diploid trout are generally comparable in size. The Idaho Department of Fish and Game set out to compare triploid and diploid West Slope cutthroats and found no significant difference between the two.

Others, especially those advocating the stocking of sterile fish, argue that triploids grow larger. This, they claim, is due to the fact that sterile fish spend their energy on growth rather than reproduction. One study found that all-female triploids grow larger than all-female diploids. The comparison of only females is necessary since male triploids can produce gonads, potentially affecting growth.

Another group found that triploids started off with slower growth rates, but eventually caught up to diploids. This could have been due to the fact that the fish were not sexually mature at their first recapture measurement.

Regardless of whether there is a size difference, it seems to be relatively insignificant, at least for the average fisherman. In reality, most people probably wouldn’t notice a difference upon catching a triploid fish.

How triploid trout are made

The concept of creating a triploid trout is simple: cause an egg to retain a chromosome it would normally expel. In normal fertilization, after an egg and sperm combine, one chromosome is kicked out, leaving either XX (female) or XY (male) chromosomes behind. If the extra chromosome is retained, the offspring will either be XXX (triploid female) or XXY (triploid male).

A cluster of trout fry.
Once hatched, diploid and triploid trout can be raised the same way.

There are several ways to cause a chromosome to be retained. Two of the most common ways are pressure treatment and heat shocking.

In pressure treatment, the fertilized egg is given enough time to produce the polar body with the third chromosome, ready to be expelled. Before it can separate out, the egg is subjected to high pressure, causing the polar body to remain. Heat shocking also requires waiting a set amount of time (the time can vary based on the temperature used) and then applying heat to the eggs to cause the third chromosome to remain.

Once triploid trout have hatched, they can be raised like diploid trout and released as stocker fish.

a small trout in a person's hand.

How triploid trout are used

Triploid fish can be a great tool for biologists. Like many hybrid fish, triploids are sterile. There are two very useful purposes for sterile fish: as infertile predators and to maintain the genetic purity of wild stocks.

Infertile predators are great for controlling unwanted, usually invasive, species. Hybrids are also often used for this. If there’s a lake overrun by invasive fish, introducing a predator that can’t reproduce and become its own problem is a great solution. The sterile predatory fish will lower the population of the invasive species while also providing a sport fishery for anglers.

Additionally, triploids can provide a compromise between wild and stocked fisheries. There’s often a debate whether hatchery fish should be introduced into wild-stock fisheries. This is a common issue with steelhead. Anglers may want more fish in the river, but adding hatchery fish can be a slippery slope. They’ll often spawn with wild fish, tainting the existing genetics and lowering the quality of the fishery. By stocking triploids, biologists can add more fish to the system without worrying about mixing the genetics of wild and stocked fish.

Overall, triploid fish can be an asset both for fisheries managers and for fly fishermen. They provide great angling opportunities and protect wild stocks that could otherwise be threatened.



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This is a list I made and use for my own trips, and I think any backcountry angler will find it handy.