Utah Division of Wildlife Resources

Stocking sterile fish

WHEN WE FIRST shared news that we planned to stock sterile walleye into Red Fleet Reservoir, we got some confused looks. Most were wondering a couple things:

1) Why would you want to make walleye sterile?

2) How do you make walleye sterile?

I’m hoping to help answer these questions in this post.

Walleye are predatory fish that are fun to catch and taste delicious.

Why sterilize fish?

Stocking sterile fish allows biologists to better control their numbers. In the case of sterile walleye in Red Fleet, we can control the population and protect endangered and native fish that are in the Colorado River system downstream from the reservoir. We don’t have to worry about sterile fish establishing reproducing populations and can control the number of walleye in the wild by simply changing the numbers of fish that we stock.

How do you sterilize fish?

It’s a fairly simple — yet strategic — process! The gist can be reduced to five words: collect, spawn, pressurize, test, release. I’ll explain the details.
Collecting walleye

During the last few weeks, we’ve been using nets and electrofishing equipment to collect walleye from Willard Bay.  The nets are a specific size and material so that walleye mortality is minimized and fewer fish of other species are caught.

Expressing milt (sperm) from a male walleye onto the eggs.

Note: We have had some anglers express concern about the impacts that our operation could have on Willard Bay’s walleye population.  I would like to emphasize that we are producing some non-sterile fish that will be stocked into Willard Bay to offset any impacts our spawning operation could have on Willard Bay’s walleye population.

Spawning the fish

Once we’ve collected the fish, we bring them to shore and sort them by sex. We strip the eggs from five females into a plastic dish. Then we express the sperm from three males onto the eggs. Then we add water and stir.

We add a chemical called tannic acid to the eggs exactly one-and-a-half minutes after adding the water.  Walleye eggs are very sticky, and that causes problems when they’re incubated in the hatchery.  The tannic acid reduces the stickiness.

After stirring for three minutes, we rinse the eggs to remove the tannic acid.
Pressure treatment
We then place the eggs into a specially designed sieve that goes into a hydraulic pressure chamber on site.  This chamber subjects the eggs to 9,500 PSI of pressure for 10 minutes.

Removing the sieve from the pressure chamber.

All of the previous steps are timed as it is critical that the eggs are subjected to this pressure beginning exactly seven-and-a-half minutes after they were first fertilized.
Sterile fish

This pressurization step is what makes the fish sterile.

Normally, fish inherit two sets of chromosomes: one from each parent.  In reality, the female provides two sets of chromosomes and the male provides one set.

The extra set from the female is normally “kicked out” of the egg.  The pressurization of the egg interferes with the “kicking out” of the extra chromosome set, and the egg ends up with all three sets of chromosomes. We call these fish “triploid.” Fish that are triploid cannot produce normal sperm and eggs, and thus cannot reproduce.

Finishing up

After the eggs are held at 9,500 PSI for 10 minutes, the hydraulic pressure chamber is de-pressurized and the sieve holding the eggs is removed. We move the eggs to a cooler of water, then they’re transported to the Springville Hatchery where they are hatched.

A look at some of the 26 million walleye eggs being raised at the Springville hatchery

Once the walleye fry hatch and rise up from their eggs (after about 14 days), they’re ready for testing.

DNA testing

We send a sample of about 200 fry via overnight mail to Virginia Commonwealth University. A professor there uses a machine called a flow cytometer to measures the quantity of DNA in the cells of the fry. This determines if the fish are triploid (sterile).

Sterile walleye going into Red Feet on April 6, 2016.


Now the fish are ready for stocking! And that’s how it’s done.