More than 800 whitefish, along with almost 20 trout, have been found dead along the Madison River, most of them below Ennis Dam, according to Montana Fish, Wildlife & Parks.
The dead fish were first reported to FWP biologists on May 18 by boaters floating below the dam in Beartrap Canyon.
By 3 p.m. on May 20, FWP staff had observed about 814 whitefish, 11 brown trout, six rainbow trout and three suckers dead along a 10-mile stretch of the river from just upstream of Ennis Dam to the Warm Springs Boat Launch.
A cause of the mortalities has not been determined. Biologists have collected samples from dead and dying fish for testing.
“We are working hard monitoring the situation and investigating the causes of the mortalities,” said Eileen Ryce, FWP Fisheries Division administrator. “We appreciate the reports from the public on the dead fish they’re seeing, and that’s been an important part of the monitoring we’ve done.”
The lower Madison is known for being susceptible to water temperature spikes in the summer because Ennis Lake is relatively shallow. The high temperature in the nearby community of Ennis ranged from 70 to 81 degrees May 15-18.
According to a U.S. Geological Survey water gauge in the lower river, the temperature on May 17-19 spiked at a high of about 60 degrees after the dam released more water, jumping from about 1,500 cubic feet per second to around 1,700 cfs.
The die-off is reminiscent of the summer of 2016 when thousands of whitefish died in the Yellowstone River, prompting an emergency closure of 183 miles of the stream. The following summer another 175 whitefish expired in the Yellowstone. The culprit in 2016 was later determined to be proliferative kidney disease.
PKD is caused by a naturally occurring microscopic parasite. Periods of high water temperatures and low flows, coupled with increased angling pressure, can stress fish making them more susceptible to dying from the parasite. Whitefish seem especially sensitive.
The report of dead fish comes on the heels of a Wednesday Montana Standard story reporting that FWP fisheries biologists are seeing an unexplained drop in brown trout populations in rivers across southwestern Montana.
"In the Big Hole’s most popular section, from Melrose to Brownes Bridge, brown trout numbers are the lowest they’ve been in 50 years of surveys. Where there were 1,800 fish per mile in 2014, there are now 400," Michael Cast reported.
FWP staff will continue to monitor conditions on the Madison River. If anglers see dead or dying fish, they can call the FWP Bozeman office at 406-577-7900.
Walleye, northern pike and smallmouth bass are keeping anglers busy on that section of Fort Peck.