Hundreds of dead fish discovered in American Falls Reservoir

Idaho Fish and Game Fisheries biologists and Idaho Department of Environmental Quality(DEQ) staff responded to calls on Friday, July 26 about a possible “fish kill” in the American Falls Reservoir.

According to a news release from Idaho Fish and Game, there were about 250 dead rainbow trout found on the shoreline. It is unclear how many total there are in the reservoir.

The DEQ conducted water tests when they arrived on the scene. The likely cause of death for all the fish is low dissolved oxygen levels. Earlier that week, dissolved oxygen levels in the reservoir were at 9 mg/L. On Friday, they had dropped to 4 mg/L. The DEQ considers 6 mg/L to be the state standard.

The news release said one possible cause for the lower oxygen levels is the high winds that swept the region on Thursday evening may have caused the water from the reservoir to “turn over,” meaning that the water at the bottom moved to the top.

Bodies of water like the American Falls Reservoir are stratified, which means they have different layers of water that vary by temperature and oxygen level. Sudden changes in these levels of water can be dangerous to fish.

Fish and Game Regional Fisheries Biologist Ryan Hillyard said in the news release that the winds may have caused these different levels of water to mix.

“The winds acted like a giant spoon,” Hillyard said in the news release. “That mixing action brought stressfully-low levels of dissolved oxygen to some parts of the reservoir occupied by fish.”

The majority of the fish that were affected by this were rainbow trouts. Biologists say that this is not surprising.

“Trout are more sensitive to low dissolved oxygen levels than are other fish species like bass or perch,” Hillyard said in the news release. “Though we did see a dead bass while at the reservoir, the majority of dead fish were trout.”

The news release said the American Falls Reservoir is Idaho’s largest reservoir when it is full. Events like this have happened before at large bodies of water. They can be alarming, but they usually do not have a large impact. It is possible that many of the fish were able to find refuge in other parts of the reservoir and this event will not have a significant impact on the fishery.