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MISSOULA (AP) – Anglers are being asked to restrict themselves to mornings only on the Thompson and St. Regis Rivers and tributaries, where flows are near record low levels and water is warm enough to kill fish.

The voluntary “hoot-owl” restrictions – no fishing from noon to midnight as of Friday – are the first drought-related fishing restrictions in Western Montana.

The low, warm water threatens the health and survival of native bull trout and westslope cutthroat trout and the rivers’ popular rainbow trout fisheries, the department of Fish, Wildlife and Parks said.

“The water level in those two drainages is the lowest we’ve ever seen this time of year,” said fisheries biologist Ladd Knotek. “It’s what you might expect in late August or September in a low-water year. It’s bad.”

River gauges on the Thompson River showed a flow of 170 cubic feet per second on July 20, less than half the mean daily flow recorded during 46 years of record-keeping. The river’s record low flow for July 20 is 138 cfs.

The St. Regis River has no gauge station, but flows are extremely low, Knotek said.

On the Thompson, the water temperature reached 75 degrees on July 4 between miles 16 and 19. Water temperatures of 70 degrees or more can be lethal to bull trout, and temperatures of 77 degrees or more can kill rainbow trout.

Last year, the water temperature never reached 75 degrees on the Thompson River, said fisheries biologist Laura Katzman. And the high reading – 73 degrees – didn’t come until July 29 and 30, when the river’s flow was 230 cfs.

Fishing only in the morning, when water temperatures are cooler, reduces the stress on trout and protects fisheries for future years, Katzman said. Low flows and high water temperatures stress fish by reducing the amount of oxygen in the water and concentrating fish in small stretches of a river.

“Trout, particularly native trout, are really sensitive to warm temperatures,” Knotek said.

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The Thompson and St. Regis rivers are important spawning runs for bull trout, a native species listed as threatened under the federal Endangered Species Act. The bull trout spawning migration begins during spring runoff and continues through September, when spawning occurs.

Although it is illegal to fish for bull trout because they are a threatened species, anglers do sometimes catch them, and a fish is more likely to survive being caught and released in cooler morning water.

The St. Regis River supports populations of fluvial and resident bull trout and westslope cutthroat trout, as well as rainbow trout and brown trout in the main stem. The system is a major spawning and rearing area for the lower Clark Fork River, Knotek said.

The mainstem rivers are lower and warmer than average for this time of year, Knotek said, but do not yet warrant any restrictions on fishing.

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