From the Idaho Department of Fish and Game
November 29, 2004
Three Better Than Two?
Joe Kozfkay, Fisheries Research Biologist
Idaho Department of Fish and Game
anglers, Idaho is a world-class trout fishing destination
because of the hundreds of miles of pristine streams and
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.
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.
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
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.
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?
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).
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.
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.
thanks to some dedicated fish researchers, Idaho anglers can
have their cake and eat it too. Popular rainbow trout
fisheries can be supplemented with non-native sterile hatchery
rainbow trout, and the state’s living gems: the Yellowstone,
westslope, and Bonneville cutthroat trout can be protected
from the threat of hybridization.