Initial lab tests conducted to pinpoint the cause of a fish die-off on the Madison River last week have revealed no conclusive cause.
“The fact that it was larger whitefish that died leads you down the disease path,” said Mike Duncan, a Region 3 fisheries biologist for the Montana Department of Fish, Wildlife & Parks.
The virus that causes proliferative kidney disease, which resulted in the death of thousands of whitefish in the Yellowstone River in 2016, was identified in some dead Madison fish but not others, so “It doesn’t seem to be the cause,” he said.
More than 800 whitefish and almost 20 trout were found dead downstream of Ennis Dam last week, mainly in Beartrap Canyon down to Beartrap Creek.
“The number of dead whitefish declined substantially below Beartrap Creek,” Duncan said, although dead fish were found all the way to the Warm Springs Boat Launch.
Some of the dead and dying fish collected for testing did have gill deformation, which is indicative of stress, Duncan said. What could have caused that stress is uncertain.
The fish die-off occurred as temperatures in the region soared into the 80s for two days. Because Ennis Lake, above the dam, is shallow its waters can heat up. A U.S. Geological Survey gauge lower down the river showed the water temperature spike to about 60 degrees after the dam increased its water output from about 1,500 cubic feet per second to 1,700 cfs.
Duncan said those flows and temperatures aren’t anything out of the ordinary for that stretch of water, which can spike as high as 80 degrees in the summer.
“It was unseasonably warm, but not like we see in August,” he said. “So it’s an odd time of year for this to show up.”
Water samples taken in the wake of the die-off showed dissolved oxygen levels were good. So if a contaminant was in the water it had dissipated quickly. What’s more, if it were a water quality issue small fish should have died in greater numbers, Duncan said. The water’s pH seemed fine as well as its conductivity, a measure of dissolved solids. The dam powerhouse was not operational at the time of the fish kill.
Whatever the cause, it seems to have temporarily run its course. Duncan said no more dead fish have shown up despite repeated searches along the river.
Next week, FWP crews will capture live fish above Ennis Lake and below the dam for analysis and baseline data on the presence of the virus that causes PKD.
“That may rule some things out,” he said.
How badly the whitefish population may have been decimated by the die-off event is uncertain, since FWP has no baseline data on whitefish numbers in that stretch of water.
Next week the temperatures in the Ennis area is predicted to spike into the 80s for three days, which may replicate the conditions of last week’s fish kill.