If “Shark Week” ever comes to Idaho, it’s star might be the bull trout.
Like a shark, it’s an elusive, predatory fish that’s seldom seen and often misunderstood, but there’s a healthy population of them in the Boise River system that appears to be growing.
Bull trout are native to Idaho and most commonly found in cold, clear mountain streams. In 1998, they were listed as “threatened” under the federal Endangered Species Act, but Idaho’s rivers and streams remain a stronghold for the fish.
Biologists with Boise National Forest, the Bureau of Reclamation and other agencies are learning more about bull trout, and they’re finding some surprises.
They’ve found that rather than living year-round in streams where food is limited, the fish are using large reservoirs as a place to feed and grow, then return to small tributary streams to spawn.
“It’s like a salmon going to the ocean to get big,” said Mary Faurot, fish biologist for the Boise National Forest.
How big? Researchers last fall trapped a 34-inch-long bull trout that weighed about 8 pounds and the estimated age was 7 years old. It was trapped in a tributary of the Boise River along with several others in the 30-inch range.
A bull trout living year-round in the headwaters of the Boise River system will only reach a fraction of that size.
“Fish that choose this life history will not get very large and grow very slow because the conditions are challenging,” said fish biologist Dmitri Vidergar with the Bureau of Reclamation in Boise.
Biologists believe large bull trout are spending most of their lives in large reservoirs, such as Arrowrock, and feeding on abundant suckers, pikeminnows and other fish that thrive there.
“It’s absolutely amazing,” Boise Forest fish biologist Mary Faurot said. “They’ve adapted in a relatively short amount of time to take advantage of the reservoirs.”
In the fall, bull trout in Arrowrock may travel up to 92 miles to reach their spawning grounds.
Part of the reason bull trout are able to make such long migrations is because the Forest Service has removed culverts and other things that block the fishes’ migration routes.
Faurot said about 150 miles of river sections on the Boise National Forest have been reconnected in the last decade.
During fall trapping, biologists captured various age groups of bull trout in good health, which indicates the fish are finding suitable habitat.
Whether that becomes a trend remains to be seen.
“The trick for long-term health of the population is for the fish to consistently find favorable conditions year after year,” Vidergar said.
The Boise River bull trout appear to he headed in the right direction, he said. Data for the Boise River basin suggests increased populations in recent years but still not to the levels they were prior to 2000.
Attitudes toward bull trout have also been gradually shifting. They were once seen as a threat to trout preferred by anglers, such as rainbows and cutthroat.
Some fish and game agencies went so far as putting a bounty on bull trout decades ago.
But now they are protected, and anglers who catch them must immediately release them unharmed.
“The esteem of bull trout has changed over the last 100 years,” Vidergar said. “They are recognized as a valuable part of the Boise River ecosystem and it is exciting to see their numbers improving.”
Roger Phillips: 377-6215
Boise River bull trout adapting to reservoirs
