Lake fisheries affected by long winter

Some extreme winter conditions this year led to abnormally high fish fatality rates in the Blackfoot Valley.

The ice had mostly melted off Brown's Lake by May 1. While this would normally mean the start of fishing season this year had local fishermen worried. In the wake of that melting ice, scores of dead fish were found.

On Upsata Lake, the shoreline was covered with dead yellow perch, largemouth bass and northern pike.

"The ice on Brown's Lake was over 24 inches thick," said Ladd Knotek, a fisheries biologist for Montana Fish, Wildlife and Parks. (FWP) "That's the thickest I've seen in 15 years."

The winter of 2022-2023 had roughly "three more weeks of ice cover than usual," added Knotek.

With that and the deep snowcover, sunlight couldn't reach aquatic plants, and many of them died and decayed in the water. This consumes the oxygen levels and depletes the available oxygen for the fish.

Some plant breakdown occurs every winter, but this winter was excessive.

Small, shallow lakes are the most vulnerable because they hold less water and oxygen and typically have more vegetation that dies in the winter.

Flowing water, such as rivers and streams, typically maintains higher levels of dissolved oxygen than standing water like ponds and lakes, so they rarely experience winterkill events. Creeks and springs that enter a lake can also provide some refuge for fish by adding oxygenated water.

Brown's Lake is home to two strains of rainbow trout, and one, which is a n early season spawner, wasn't able to recover the energy stores they used while still under the ice.

"Different fish species vary in susceptibility to low oxygen," Knotek said. "Salmonids like trout and kokanee are more susceptible while warmer water fish such as bass, suckers and perch are less susceptible."

Brown's Lake has a maximum ice depth of 27 feet and Upsata is up to 40 feet deep, according to the FWP.

Restocking of Brown's Lake with fingerling rainbow trout will be according to schedule in June and September.

Harper's Lake lost 80 large rainbow trout (retired brood stock) over the winter, with limited mortality of other fish in the lake. Restocking of the lake with catchable rainbow trout and westslope cutthroat trout has already begun and will continue this spring.

Upsata Lake's fish kill was severe and biologists are evaluating the status of the fishery before determining management actions.

No unusual winter kill mortality has been observed on other area lakes and ponds, including Beavertail Pond, Frenchtown Pond and larger lakes in the Seeley Lake area.