A welcome shower on Tuesday broke Kalispell’s 33-day rainless streak, but for Northwest Montana’s trout streams, it may not be enough to stave off fishing restrictions.

A host of records have been broken already this spring and summer for heat and lack of rain, and fish biologists at Montana Fish, Wildlife and Parks are watching water temperatures, which are closely linked to the survival of cold water-dependent fish.

Especially at risk are rainbow, cutthroat, brown and bull trout, according to Ken Staigmiller, the fish health coordinator for the state agency.

“Every fish has an optimal temperature range, and that varies depending on the species,” Staigmiller said. “When they’re living at that upper limit it creates stress.”

Because temperature and dissolved oxygen have an inverse relationship, the stress on fish is doubled the warmer that water gets. Fish need to breathe more as temperatures rise, and getting hooked on a line doesn’t help their chances.

“You’re playing that fish to exhaustion, and you’re turning him loose in an environment that doesn’t have much oxygen,” Staigmiller said. “The longer-term effect is their suppressed immune system also makes them more susceptible to infection.”

Rainbow and brown trout prefer water temperatures around 55 to 57 degrees, and can start going belly-up once the mercury hits 77.

Mark Deleray, the agency’s regional fisheries manager, said his office uses the Thompson River as a proxy for regional stream temperatures, since it historically is the first to reach the temperature threshold that could lead to restrictions.

Once the water hits 73 degrees, initial restrictions could include Fish, Wildlife and Parks imposing “hoot owl” hours for fishing — meaning anglers could only fish between midnight and 2 p.m.

During the recent string of nine straight days when the air temperature exceeded 90 degrees, one of the four parts of the Thompson River routinely tested by fisheries technicians exceeded 73 degrees, while another reached the threshold without going over.

Deleray said his office is currently discussing the possibility of putting restrictions on portions of the river if water temperatures surpass 73 degrees again.

Water temperatures have declined, thanks to a corresponding drop in air temperatures beginning Sunday, but at least two more days of above 90-degree weather are forecast today and Friday.

“We’ll watch the flows, we’ll watch temperatures and see if we end up with the need for further restrictions,” Deleray said. “We’re really fortunate in Northwest Montana: We have cold water, and this is not a regular thing for us like it is in Southwest Montana.”

The last time fishing restrictions were put in place in Northwest Montana was in 2007 on the Thompson River.

On July 3, the agency’s regional office in Bozeman put hoot owl restrictions on the entire length of the Jefferson River and most of the Big Hole River. Officials there are asking anglers to seek out fishing spots less prone to fish kills such as reservoirs and high mountain lakes.

The day before, the state issued hoot owl restrictions on the Blackfoot and Bitterroot rivers, along with the Clark Fork River up to its confluence with the Flathead River.

To get updates on fishing restrictions in Northwest Montana, contact Fish, Wildlife and Parks’ Kalispell office at 406-752-0349 or visit fwp.mt.gov/regions/r1.

Reporter Samuel Wilson can be reached at 758-4407 or by email at swilson@dailyinterlake.com.