State confirms natural fish kill in Noxon Reservoir

Recent reports of dead or dying fish in Noxon Reservoir in Sanders County are reminders that small-scale summer fish kills can be common when the weather gets hot.

What are summer fish kills?

Montana Fish, Wildlife & Parks recently received reports of dead and dying fish in Noxon Reservoir and confirmed the cause was lack of dissolved oxygen in the water.

Most summer fish kills are natural events that occur when dissolved oxygen in a waterbody becomes depleted. Similar to winter fish kills, summer fish kills associated with low oxygen levels can be common in many small bodies of water during summer. Warm water holds less dissolved oxygen than cold water, which makes it harder on fish. Declining oxygen levels can lead to fish stress and, ultimately, fish mortality.

Most of the time, these oxygen “sags” do not occur in the whole waterbody and do not harm the entire fish populations. These incidents may go completely unnoticed.

Shallow productive waterbodies or bays of larger waters with abundant vegetation are at higher risk of die-offs due to lower volumes of oxygen-rich water caused when plants begin to die and decay.

What can anglers do?

Anglers who see dead or dying fish can report them via FWP’s online portal: sickfish.mt.gov. These reports are helpful for biologists to understand what anglers are seeing on the water and to be able to respond if necessary. Anglers can also contact a local FWP office with specific information including location, date, number and species of fish, and if possible, photos and any other relevant information. This information will help biologists determine if the event was natural or the result of accidental or unauthorized human actions.

Fish kills can occur year-round due to a variety of factors, including dissolved oxygen depletion, extreme water temperatures, fish diseases or the introduction of pollutants.