FWP confirms natural fish die-offs in regional waterbodies

KALISPELL — Montana Fish, Wildlife & Parks has received reports of dead or dying fish in Thompson Falls, Noxon, and Cabinet Gorge reservoirs in Sanders County and Rogers Lake in Flathead County. FWP biologists determined the small-scale fish kills were the result of a lack of dissolved oxygen, which can be common during the summer months.

What are Summer Fish Kills?

Most summer fish kills are natural events that occur when dissolved oxygen in a waterbody becomes depleted. Similar to winter fish kills, summer die-offs associated with low oxygen levels can be common in shallow productive waterbodies or bays of larger waters. Warm water holds less dissolved oxygen than cold water, which makes it harder fish to get enough oxygen. 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?

FWP thanks the anglers who reported the local die-offs and appreciates any information on possible fish kill events. Please contact your 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.