A cutbow is what you get when a rainbow trout and a cutthroat trout spawn together. Both belong to the genus Oncorhynchus, both spawn in spring on the same kind of gravel at roughly the same time, and their offspring are fertile, which is the detail that makes the cutbow more than a curiosity. A fertile hybrid can spawn back into either parent population, so where rainbows have been introduced into cutthroat water the two species do not stay separate. They blend, generation by generation, until a river that once held pure natives holds a swarm of fish carrying some mix of both.
Why it happens
Cutthroat evolved in the interior West in isolation from rainbows, which are native to the Pacific slope. The two never met until rainbows were stocked across the Rockies starting in the late 1800s. Where that happened in cutthroat rivers, hybridization followed, and the cutbow is the result. It is an accident of fish culture, not a naturally occurring fish, and in most of its range the presence of cutbows is a measure of how much rainbow influence has crept into a native population.
There is a second, deliberate source. Some state agencies raise and stock cutbows on purpose for put-and-take fishing, since the hybrid grows quickly, takes a fly readily and is not a conservation concern where no native cutthroat remain. Colorado has done this in a number of waters. So a cutbow can be either a sign of a native population being diluted or a hatchery product with no wild history at all, depending on the river.
How to identify one
There is no reliable way to tell a cutbow from either parent by eye, and the honest answer is that many fish anglers call cutbows are simply rainbows or cutthroat that look a bit unusual. That said, a fish that shows both of these traits at once is likely a hybrid:
- A rainbow's pink or red lateral stripe and its dense, small spots spread across the whole body including the head, plus
- a cutthroat's orange or red slash under the jaw, often fainter or shorter than on a pure cutthroat.
Pure cutthroat generally lack the pink lateral band, carry fewer and larger spots concentrated toward the tail, and often have a white tip on the pelvic and anal fins that rainbows lack. Pure rainbows have no jaw slash, though a faint orange smudge can appear on some strains. A cutbow usually lands in between: a stripe plus a slash, or cutthroat spotting on a rainbow-shaped fish. Biologists confirm hybridization with genetic sampling, not appearance, and that is why agencies managing native cutthroat treat visual identification as a rough guide only.
Where cutbows are common on this site
Three river pages here describe the hybrid directly, and all three are places where rainbows overlap native cutthroat.
- Yellowstone River, Montana: the Paradise Valley rainbows hybridize with Yellowstone cutthroat in some sections, so a fish with a slash and a stripe there is not unusual.
- Snake River, South Fork, Idaho: rainbows are present throughout and hybridize with the native Snake River fine-spotted cutthroat in some reaches. Idaho's fish and game agency has run programs encouraging anglers to harvest rainbows on the South Fork specifically to protect the cutthroat from this; check the agency for what is current.
- Teton River, Idaho: rainbows in the valley section with some hybridization with the fine-spotted cutthroat.
The pattern generalizes. Any river on the cutthroat guide that also lists rainbow trout is a candidate, which includes the upper Gallatin, the Cache la Poudre, the Logan, the Bear and the Uinta. The waters where the cutthroat are still pure tend to be the ones with a barrier below them, a waterfall or a dam, that rainbows never got above: the Lamar in Yellowstone is the clearest example here.
Fishing for them
There is no cutbow tactic. The fish behaves like whichever parent it leans toward, and on most rivers that means it holds in faster water than a cutthroat, takes a dry fly a little quicker, and fights more like a rainbow. Fish the river for the species it is known for and you will catch cutbows along the way. The one thing worth doing differently is handling: where the hybrid exists because natives are being diluted, agencies often want rainbows and hybrids removed and cutthroat released, and where the hybrid was stocked, the rules are usually the same as for rainbows. Those rules are specific to the drainage and change. Check the state agency before deciding what to do with the fish in your net.
What a cutbow tells you
Treat the fish as information. A slash-and-stripe fish in a river that is supposed to hold pure cutthroat means rainbows are above whatever barrier was meant to keep them out. A photograph with the location, sent to the state agency's fisheries office, is a small, useful thing. And a cutbow in a stocked reservoir is just a strong, willing trout that will take a Woolly Bugger, and there is nothing wrong with enjoying it for that.