High water temperatures and low flows in the Thompson River threaten the health and survival of the Thompson River's native bull trout and westslope cutthroat trout and the popular rainbow trout fishery. In order to protect the fishery, Montana Fish, Wildlife and Parks (FWP) is advising Thompson River anglers to voluntarily limit all fishing to mornings only until conditions improve. The angler advisory takes effect, Friday, July 27, on the Thompson River and its tributaries.
The action was recommended by Fisheries Biologist Laura Katzman in Thompson Falls and taken by FWP Region 1 Supervisor Dan Vincent and Fisheries Manager Jim Vashro in Kalispell, who have authority over voluntary drought-related restrictions under FWP's Drought Contingency Plan. The Drought Contingency Plan sets out three options for streams severely impacted by drought: voluntary partial closure advising anglers not to fish between the hours of noon and midnight; partial mandatory closure prohibiting fishing between the hours of noon and midnight; and full closure prohibiting all angling. The FWP Commission approved the plan in June.
"We are advising anglers to fish only in the morning to reduce stress on the trout in an effort to protect the Thompson River fishery during this drought year. Reducing the pressures on the Thompson River fishery during a tough year like this will help maintain the fishery for years to come," Katzman said. "Over the next few weeks we will be evaluating conditions to determine if mandatory closures are needed."
The FWP drought plan calls for an evaluation of the effects of angling on fish populations and potentially imposing voluntary restrictions on bull trout streams when maximum daily water temperatures equal or exceed 60 degrees for three consecutive days and on other trout streams when maximum daily water temperatures exceed 73 degrees for three consecutive days.
A combination of low flows and high air temperatures is causing high water temperatures in the Thompson River this month. The flow on July 20 was 170 cubic feet per second (cfs), less than half of the mean daily flow from 46 years of record (360 cfs for July 20); flows do not usually drop to the level they are now until September of most years. The current flow is less than the 80% exceedance level (266 cfs) and not far above the minimum recorded flow (138 cfs) for July 20. Water temperatures have exceeded 64 degrees 10 days in row. In addition, the water temperature reached 75 degrees on July 4. FWP closes streams to fishing when low water conditions and high temperatures combined with fishing pressure would lead to an unacceptable level of stress on fish.
The Thompson River drainage is important for bull trout spawning. The bull trout spawning migration begins during spring run-off and continues through September when spawning occurs. Bull trout migrate from the Clark Fork River or Thompson River mainstem to Thompson River tributaries during this time period. Although it is not legal to fish for bull trout because they are listed as a threatened species under the Endangered Species Act, anglers do unintentionally capture bull trout. Water temperatures exceeding 60 degrees are stressful for the bull trout without the added angling stress. Conditions in the Thompson River are also approaching levels where angling stress could begin affecting other trout populations. It is critical that sufficient numbers of trout are conserved to repopulate the fishery when conditions improve.
The voluntary restrictions will likely remain in effect until August 30. Conditions will continue to be evaluated, Katzman said.