Water temperatures of up to 82 degrees killed hundreds of rainbow and brown trout on the Firehole River this past week as warm, dry weather continues to pummel the Greater Yellowstone ecosystem.

Officials noticed the kill after they issued a fishing advisory Friday for a number of waterways in the park, asking anglers to avoid fishing low elevation waters between noon and 6:00 p.m. The partial fishing closure is voluntary, but could become mandatory if warm conditions persist. The voluntary closure starts Saturday.

According to Yellowstone spokesman Al Nash, water temperatures in a number of waterways have exceeded 73 degrees, which can be stressful or even fatal for trout.

The fish kill in the Firehole ranged from Midway Geyser Basin downstream to Firehole Cascades. The Firehole River maintains higher temperatures than other rivers in the park because of geothermal features that heat the water.  

According to Nash, the highest temperature reading in the Firehole Friday occurred along Fountain Flat Drive where the water temperature reached 82 degrees. The hundreds of dead brown and rainbow trout that park biologists found in the river died no earlier than three days ago.

In years past, Yellowstone officials would normally only close rivers such as the Firehole, the Madison, and the Gibbon if temperatures got too high. This year, however, officials decided to have a voluntary advisory in order to maintain fishing opportunities for Yellowstone anglers.

“We have a strong, involved, responsive user group here,” said Nash. “We know that people come here because of the world class fishing opportunities. We know how important it is to them and we know they’ll join us in protecting this resource.”

Nash said the current advisory is a “little earlier” and more widespread than years past. “I am not aware that we’ve involved this many rivers and creeks before,” he said.

In northern Yellowstone, the advisory applies to; Gardner River below Osprey Falls, Lava Creek below Undine Falls, Lamar River below Cache Creek, all of Slough Creek, Soda Creek below Amphitheater Creek, and Yellowstone River below Seven Mile Hole.

On the west side of the park, the advisory applies to; Madison River, Firehole River, and Gibbon River below Gibbon Falls.

In the southern part of Yellowstone, the advisory applies to Bechler River below Ouzel Creek, Fall River below Rainbow Falls, Mountain Ash Creek below Union Falls, Proposition Creek, Boundary Creek below Dunanda Falls, Robinson Creek, and Snake River below Six Mile Ford.