From the Idaho Department of Fish and Game

November 29, 2004
Are Three Better Than Two?
Joe Kozfkay, Fisheries Research Biologist
Idaho Department of Fish and Game

For many anglers, Idaho is a world-class trout fishing destination because of the hundreds of miles of pristine streams and rivers.

Idaho’s abundant, widespread and easily caught state fish, the cutthroat trout, accounts for much of the fame in angling circles. Keeping the beloved cutthroat from disappearing by mixing with another popular trout is one of the major challenges facing Idaho Fish and Game biologists, a problem that new knowledge of genetics is helping to solve.

The term “state fish” is a bit of a misnomer, as Idaho is home to three of 14 recognized species of cutthroat trout: the Yellowstone, the westslope, and the Bonneville cutthroat trout. All may be recognized by their brilliant orange slashes on the undersides of their jaws and unique spotting patterns. Yellowstone and westslope cutthroat trout are found throughout eastern, central, and northern Idaho, whereas the Bonneville cutthroat trout is found in Southeast Idaho. These populations are among the strongest remaining anywhere in the world. Recently however, some concern has arisen about their future.

The rainbow trout is another popular species with Idaho anglers due to its reputation as being a hard fighter, an active jumper, and excellent table fare. Two forms of rainbow trout are native to Idaho, the steelhead and desert redband. Additionally, Fish and Game has had an active stocking program for a non-native coastal form of rainbow trout. This stocking program has supported popular fisheries across the state for decades.

Fish and Game fisheries biologists have learned that non-native rainbow trout have the ability to breed with native cutthroat trout and thus produce hybrids or aquatic “mutts;” fish that are neither full rainbow nor full cutthroat trout. Should hybridization occur often enough over a long period of time, Idaho’s three cutthroat species might be lost forever.

To remedy this problem, Fish and Game simply quit stocking rainbows at locations with self-sustaining cutthroat populations and instead shifted those rainbows to other waters. However, rainbow trout do not always stay where you put them. It seems rainbows like their freedom and often choose to swim long distances, upstream or downstream, to find spawning areas and mates. This problem took biologists back to the drawing board. How do you stop highly mobile hatchery rainbows from spawning with wild, native cutthroat?

It turns out there is a way, but the process is a complex one. When trout spawn, the female’s eggs possess two sets of chromosomes and the male’s sperm possess one set. After the eggs are fertilized, the chromosomes recombine and each egg inherits one set of chromosomes from the female and another set from the male — similar to humans. The third set is then kicked out of the egg. Rarely in the wild, an egg will “forget” to kick out the third set and the fish becomes what is known as a triploid (possessing three sets of chromosomes). Triploid fish look, swim, jump, and taste like normal fish, except for one important difference—they never develop normal eggs or sperm and are unable to reproduce (i.e., they are sterile).

Through experimentation with this natural process, researchers found that they could create triploid trout both by exposing trout eggs to pressure and by placing trout eggs in a warm water bath shortly after fertilization. Both processes inhibit a trout egg’s ability to kick out that third set of chromosomes and voila, a triploid fish is born.

After several detailed experiments, Fish and Game biologists adapted these techniques for use in the state’s rainbow trout egg collection facility at Hayspur Fish Hatchery. And since 2001, approximately nine to 10 million rainbow trout eggs have been sterilized in this manner each year. Raised to catchable size, these fish are now being stocked in waters across the state.