The rapid death of an estimated 4,000 whitefish in the Yellowstone River over the past week has triggered a swift response from Montana Fish, Wildlife and Parks. FWP public information officer Vivica Crowser says we now know what is killing the fish.

"The catalysts for the fish kill is called Proliferate Kidney Disease," Crowser said. "It's one of the most serious diseases that can impact white fish and trout. It's caused by a microscopic parasite. The parasite has been documented in Canada, United States, Europe and in Montana a couple of times but not at any scale close to what we're seeing here."

The unprecedented kill of is being met by an unprecedented response by FWP.

"The closure of the Yellowstone River is all access so not just fishing, but floating swimming anything," Crowser said. "It's from Gardiner, all the way down to Laurel so it's a big stretch of river so not something you see happen really ever but we're doing what we can to remove any additional stress from the fish and trying to keep the parasite contained if we can."

The full closure of the river began on Friday, August 19. Crowser says that there will soon be penalties in place for those that access the Yellowstone River. Many are comparing this parasite to the outbreak of whirling disease back in the 90s.

Photo courtesy of FWP