MISSOULA — The hot weather has taken its toll on the reservoirs of the lower Clark Fork River, where some fish have died as the result of low oxygen.
On Tuesday, a Montana Fish, Wildlife & Parks announcement said anglers had reported dead or dying fish in Thompson Falls, Noxon and Cabinet Gorge reservoirs.
Biologists investigated and concluded that dissolved oxygen in the reservoirs had declined to levels where fish weren’t getting enough to survive, according to FWP.
As water warms, it carries less dissolved oxygen, which can stress aquatic species. At a certain point, some species can’t get enough oxygen and drown.
July’s high temperatures warmed Montana rivers enough to prompt hoot-owl fishing restrictions to limit stress to trout.
But the Clark Fork River carried that warm water to the downstream reservoirs where the high air temperatures added heat to the stagnated surface water.
Low dissolved oxygen is more prevalent in shallow water in the summer, particularly along shores with decaying vegetation.
Deeper regions of a larger body of water such as a lake often still hold enough oxygen for fish unless decomposing aquatic vegetation further depletes the oxygen at low levels.
However, if a severe storm stirs up the water, the low dissolved oxygen layers can mix with other layers, causing oxygen levels to drop through the lake.
Such an event occurred on Idaho’s American Falls Reservoir in mid-July and the low oxygen levels killed more than 250 rainbow trout.
Trout are more sensitive to low oxygen levels than other lake species such as bass.
Both Noxon and Cabinet Gorge reservoirs contain aquatic invasive plants, including Eurasian watermilfoil, which was first documented in Noxon Reservoir in 2007.
The spaghetti-like milfoil forms dense mats that eventually decay in the waterbodies they inhabit, causing degraded water quality and low-oxygen conditions that are harmful for aquatic life.
Once milfoil becomes established, it is extremely difficult to eliminate. But in 2021, FWP started trying to physically remove the plant along with applying herbicide in select areas.
Drought also contributes to low dissolved oxygen due to less water in rivers and lakes or reservoirs. Smaller amounts of water warm more quickly and contain less oxygen overall.
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.
High nutrient loads — excess nitrogen and phosphorus from fertilizers or human and animal waste — can encourage algal growth in the water that can further deplete oxygen.
FWP requests that anglers report any collections of dead fish they find.
Noxon Reservoir contains 12 species of fish, including yellow perch, largemouth and smallmouth bass, northern pike and walleye, which was illegally introduced in the 1980s and ‘90s.
Brown trout have also been caught in Noxon. FWP did not provide information on the number or species of fish found dead or when reservoir conditions might improve.
