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FWP Concerned About Fish Winterkill

Montana Fish, Wildlife & Parks is asking anglers to help identify ponds this spring that may have winterkilled. 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 ten feet lower, or more, going into the 2022-23 winter,” says Jared Krebs, Glasgow-area biologist. “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. FWP fisheries staff have been monitoring regional ponds this winter for dissolved oxygen and have found dire readings in some water bodies.

“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 melts off,” adds Krebs.

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 suitable for fish.

Another option is to move fish, such as bluegill or perch, to winterkilled ponds from other healthier ponds. These “donor” ponds are usually 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.

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,” adds Krebs. “They also can be quite expensive but generally result in long-term fisheries benefits.” How can anglers help? Anglers are asked to notify FWP if they find dead fish in area waters as the ice comes off this spring. Notifying FWP of a winterkill allows us 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, call your local FWP office at 406-2283700.

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