Trout in Montana's stretch of the Kootenai River have fared much better than fish farther downstream in Idaho since Libby Dam went online in 1972.
Trout populations in the river near the town of Libby are six times greater than in the Idaho stretches involved in a nutrient enhancement project.
But the Montana fish are small.
Fish, Wildlife and Parks Department biologists are trying to solve a downhill slide of issues, including:
* Small average fish size near Libby.
* Declines in trophy fish below the dam.
* Didymo, an invasive species, also known as "rock snot," that outcompetes native aquatic life.
"Montana's Kootenai fish are struggling, we know that much," said Jim Vashro, FWP regional fisheries manager in Kalispell.
"We're not sure if it's nutrients, complications from the didymo, or a combination."
Montana's record rainbow trout was caught in the Kootenai in 1997. The fish — 33.1 pounds and 38.6 inches long — joined other catches in drawing attention to the trophy rainbow that ambushed prey below Libby Dam.
"The nutrient levels are pretty good in the tailrace section, and the trout feed on kokanee that come through the dam — a combination that grows very big fish," said Jim Dunnigan, FWP fisheries biologist in Libby.
"But we're seeing a decline in the trophy fish that may be related to the didymo that showed up in the Kootenai below the dam in the late 1990s."
Didymosphenia geminata, or "didymo," is a single-celled invasive alga threatening streams across the United States. It attaches to underwater surfaces forming large, dense, slimy mats that slough off and float downriver.
Rock snot is an appropriate moniker.
Montana is launching experimental treatments as scientists look for a way to control the spread of didymo.
"Maybe that's the cause of the decline we see in the big trout, maybe not," Dunnigan said.
The fishery may be malnourished, one of the problems found to be plaguing fish populations downstream in Idaho.
Dunnigan said Montana is closely watching the river fertilization project being conducted by the Kootenai Tribe and Idaho Fish and Game.
"If Idaho has a favorable result, Montana hasn't discounted doing something like that in the interest of putting more size on our fish," he said.
Researchers have begun a study to understand why the trout are surviving in abundance but not growing large.
"Maybe they're just not living long enough to get large," he said. "Are they not growing or not surviving?"
Meanwhile, the state also is working on habitat related issues, including putting screens on irrigation intakes to prevent fish from straying into waters that go dry.
"The river is a much different place for fish than it was before the 1970s," Dunnigan said.