FWP concerned about winterkill in numerous northeast Montana ponds

Region 6 Press release

GLASGOW - The climate in northeast Montana the last few years hasn't been the best for area fishing ponds, and Montana Fish, Wildlife and Parks is asking anglers to help identify ponds that may have winterkill of fish.

Following a very dry winter in 2020-2021, the summer of 2021 was one of the driest on record and was followed by another winter of less than average precipitation. This has left fish populations in many ponds and small reservoirs across northeastern Montana with little room to breathe.

What causes fish to winterkill?

There are many factors that influence a pond to winterkill, but it usually starts with low water and increased vegetation.

"Unfortunately, the ongoing drought left many ponds and small reservoirs in eastern Montana five to 10 feet lower going into the 2021-22 winter," Jared Krebs, Glasgow-area biologist, said. "These lower water levels allowed for greater light penetration through the water column and created conditions for increased aquatic vegetation growth."

While this vegetation growth is not necessarily a bad thing during the summer, snow-covered ice during the winter prevents sunlight from penetrating the water, causing the aquatic vegetation to die. This dead vegetation then begins to decompose, a process which consumes oxygen from the water. Like humans, fish need oxygen to survive.

As more oxygen is consumed from the water, there is less available for fish. If oxygen levels become too low, fish may begin to die. As these dead fish decompose, more oxygen is removed from the water and the entire process begins to escalate.

"All of this points to the stark reality that numerous ponds likely winterkilled in recent months, and anglers should not be surprised if they find dead fish this spring as the ice has melted off," Krebs added.

How do FWP fisheries biologists deal with waters that winterkill?

FWP will work to stock fish into those ponds which have winterkilled. For rainbow trout fisheries, this usually means stocking hatchery trout as soon as conditions are favorable, if biologists determine a pond is still habitable for fish.

Another option is to move fish, such as bluegill or perch, to winterkilled ponds from other healthier ponds. These "donor" ponds are productive and can support the removal of many fish if necessary.

Additionally, biologists actively install and maintain windmill aerators on area ponds. Windmill aerators generate oxygen for fish through tough winter months and work best on waters that have good depth and generally good habitat. They do not work well in shallow ponds with marginal habitat.

"If a pond enters the winter with already low water levels, especially if its max depth is less than 12 feet, even this supplemental aeration may not be enough to guarantee fish survival through a winter," Krebs said.

Lastly, biologists are continually looking for pond restoration projects where they can drain, dry and dredge an existing pond to increase depth and improve fish habitat.

"These projects are extensive and involve several years of planning and coordination to complete," Krebs added. "They also can be quite expensive but can result in long term fisheries benefits."

How can anglers help?

Anglers are asked to notify FWP if they find dead fish in area waters. Notifying FWP of a winterkill allows FWP to adjust stocking rates accordingly and ensure anglers have productive places to fish in the future.

If anglers notice a fish kill, or have other questions or concerns, they can call the local FWP office in Havre at 406-265-6177 or Glasgow at 406-228-3700.