Fish kill documented in the Milk River and Rock Creek

HINSDALE — Fish, Wildlife and Parks fisheries biologists investigated a report of a fish kill on Rock Creek and the Milk River north of Hinsdale in Valley County.

According to the concerned landowner, who reported the kill to FWP July 13, “Lots and lots” of dead fish were observed in the Milk River around the confluence with Rock Creek.

Upon investigation, biologists first observed dead fish downstream of the confluence of the Milk River and Rock Creek. Further searching found dead fish 9.5 miles from the mouth of Rock Creek up to the Rock Creek Dam, although the full upstream extent was not determined. No dead fish were observed in the Milk River upstream of the confluence with Rock Creek.

“Whatever killed these fish was highly toxic as it killed carp, bullheads, catfish and a host of warm water fish species that tolerate poor water quality,” regional fish manager Steve Dalbey said.

Species observed include walleye, northern pike, carp, catfish, goldeye, bullhead, shorthead redhorse, river carpsucker, white sucker, freshwater drum, and buffalo. 

Although the source of the fish kill was not determined, the timing and rate of decomposition of the fish suggest that the kill may have occurred July 9-11. The kill may be related to the high intensity rain events that occurred on July 8 and 9. 

According to the National Weather Service, 0.5 inches of rain fell the evening of July 8 and another 0.25 to 1.0 inches of rain fell the evening of the July 9.  

Live fish were collected in the area at the time of the investigation indicating that the event was acute in nature and no abnormalities were observed. 

“All of this points to a toxicant that may have entered Rock Creek above the Rock Creek dam during the rain events of July 8 and 9 and moved through the river,” Dalbey said. “It’s difficult to pinpoint where this occurred unless more fish are found in and around the affected reach of river.”

FWP will follow up with fish collection from the affected area to look at possible contaminants in the fish tissue. No fish consumption advisories are in place.

If members of the public have any information on what may have led to this fish kill or have other questions or concerns, they are asked to call their local FWP office:

• Glasgow: 406-228-3700

• Havre: 406-265-6177