NorthWestern Energy told the Associated Press a gate that lets water out of the dam malfunctioned Tuesday morning, leaving some channels of the Madison River with little to no water. This left fish lying out on rock beds that were supposed to be underwater.

Morgan Jacobsen, a spokesperson for Fish, Wildlife and Parks, told the AP that over 100 volunteers joined the effort to save the fish, using nets to scoop them up and transfer them to parts of the river that were unaffected.

The Madison River is considered a “blue ribbon” trout fishery. Jacobsen said some trout were saved, though the majority of the fish saved were sculpin.

She added that biologists won’t know the full scope of the damage until next year when they can see how the incident affected “trout spawning areas.”

Kelly Galloup, a fly-fishing shop owner, told the AP there was also concern for the safety of any trout eggs that might have been exposed. He was part of the group of volunteers that helped rescue the fish.

“We scooped yesterday all day on the upper river,” Galloup said. “We’re just concentrating on the side channels, trying to rescue whatever we can find.”

For more reporting from The Associated Press, see below.

Galloup said bigger trout likely moved to deeper pools as water levels dropped. But he expected an impact on fish born last year that were too small to escape the sudden drop in flows.

The Madison is a highly regarded “blue-ribbon” trout fishery that flows out of Yellowstone National Park in Wyoming and eventually feeds into the Missouri River. Hebgen dam is used to control water flows for downstream hydroelectric plants.

Volunteers on Tuesday began scooping fish out of the channels and returning them to the main stem of the river. That work continued Wednesday, and Montana Fish, Wildlife and Parks officials closed down fishing along the upper river until full flows can be restored.

A component needed to fix the failed gate was en route to the site, with repair work anticipated to start Wednesday evening, said Jeremy Clotfelter, director of hydro operations for Northwestern Energy.

“We know what failed but we don’t know how it failed or why it failed,” Clotfelter said. He add that it would take “a couple of hours” to install the replacement component.

Workers had earlier started releasing water over the dam’s spillway, which provided a small increase in flows, said NorthWestern Energy spokesperson Jo Dee Black.