Upsata, Harpers and Browns Lakes experience different levels of winterkill
MISSOULA — Several Blackfoot Valley lakes experienced a winterkill event this year, losing a portion of their fish populations. Fish kills occur when prolonged ice cover and snow reduces oxygen levels in the water. If oxygen falls low enough for a long enough period of time, fish can die.
Montana Fish, Wildlife & Parks fisheries biologists have been surveying the extent of the impact of this year’s long winter on fish populations. So far, the lakes they have found to be affected include:
Lake Upsata: Experienced a severe winterkill event this year, losing a large portion of its fish including largemouth bass, northern pike and yellow perch. Biologists will evaluate the status of fishery in upcoming weeks before determining management actions.
Browns Lake: Biologists observed several hundred dead rainbow trout of various sizes. However, many live fish (spawners) were observed along shoreline, and anglers are already catching non-spawners in deeper water. Browns Lake will be re-stocked with fingerling rainbow trout on its normal schedule in June and September.
Harpers Lake: Approximately 80 large rainbow trout (retired brood stock) died over the winter, with limited mortality of other fish in the lake. Re-stocking of lake with catchable rainbow trout and westslope cutthroat trout has already begun and will continue this spring.
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.
Some lakes and ponds experience higher fish mortality over the winter months when covered in ice and snow because they have decreased levels of dissolved oxygen. Aquatic plants are not able to photosynthesize as well due to a lack of sunlight, atmospheric oxygen is not able to mix through the frozen water, and break-down of dead plants consumes oxygen. When winters are prolonged like this season, reduction of oxygen over an even longer period of time adds extra stress to fish.
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.