Those small blotchy-sided rainbow trout that enthusiastically greet fly fishers’ dry flies on Idaho’s famous cutthroat streams are a bit of a mystery even to biologists.
They seemed to be more common than ever last season, leaving curious anglers to wonder if releasing numerous 8-inch trout had something to do with a big fish hatch last year.
How did rainbows become so common in native cutthroat streams?
And if there are so many young rainbows with those distinctive fingerprints (parr marks) along their sides, why aren’t anglers catching many large rainbow-sided adults?
Turns out that fisheries biologists are curious, too, although getting to the bottom of all the questions is not a research priority.
“We do know that small rainbows aren’t as numerous as they might seem to the angler,” said Joe DuPont, Idaho Fish and Game’s Clearwater Region fishery manager, who has led trout studies in North Idaho streams.
“Snorkel surveys indicate they aren’t the most abundant fish, but when water temperatures warm up, they tend to be more active than cutthroat.”
Drought last year caused lower- and warmer-than-normal flows throughout the region’s waters.
The Coeur d’Alene drainage rainbows were introduced in defunct stocking programs, DuPont said. Rainbows in the Clearwater system’s Lochsa River are steelhead smolts growing up one to five years in the river before making a run to the ocean or remnant steelhead that will spend their lives in the streams.
In Kelly Creek, North Fork Clearwater and Little North Fork, the tiny rainbows are remnants of steelhead runs cut off by Dworshak Dam.
“If they lived as long as cutthroat, you’d probably see those Kelly Creek rainbows get to the 16- to 18-inch range,” he said.
“I can only speculate because we simply haven’t evaluated a lot of these things, but we see small rainbows throughout the North Fork Clearwater and Little North Fork. They just don’t live long.
“Their shorter life history may be related to their genetic background from steelhead. Maybe it’s that they spend a little more time in warmer water than cutthroat.
“A cutthroat might max out in that system at 8 to 10 years – 10 being really old. But the rainbows are living to maybe 4 years old.”
River differences can be a factor.
“Compared with the Coeur d’Alene River, we don’t see bigger rainbows as far upstream on the St. Joe, although we’ve seen some bigger ones downstream from Avery,” DuPont said. “That could be because the Coeur d’Alene is more productive and lower gradient.
“Coeur d’Alene River rainbows will get big, over 20 inches in that system, because of the hatchery stock they come from,” he said, emphasizing the the Coeur d’Alene is one of the most productive rivers in North Idaho.
Rainbows adapted to productive rivers like the Henry’s Fork grow fast and likely lose their parr markings in their first year. Slow-growing rainbows in the Clearwater drainage may keep their parr marks for years.
“Some of that may be related to their habitat in smaller streams and tributaries,” where parr marks provide more camouflage, he said.
At the mouth of the North Fork Clearwater, anglers can catch good-sized, longer-lived “cutbows” created by hybridization of native cutthroat and hatchery rainbows formerly stocked in Dworshak Reservoir.
But hybridization is less likely upstream where cutthroats thrive and rainbows are like fish out of water.
“Nowadays we stock only sterile rainbows in Dworshak and no rainbows in our rivers,” DuPont said.
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