▶️ The Great Outdoors: Using plant-based chemicals to kill invasive catfish


In September 2022, we told you about efforts to control an invasion of brown bullhead in Crane Prairie Reservoir. There, the invasive catfish are being netted to keep the population in check and reduce their impacts on native species.

There is another way being applied to control catfish — kill them all.

Heart Lake sits 5,700 feet above sea level in the Fremont National Forest between Klamath Falls and Lakeview. It’s a perennially good trout fishery that has gone downhill in recent years due to illegal introductions of brown bullhead catfish and fathead minnows. 

“They end up working against each other, the brown bullhead and the rainbow trout,” said Justin Miles, Klamath watershed assistant fish biologist with the Oregon Department of Fish & Wildlife. “People really come here to catch rainbow trout. We don’t want the brown bullhead in here either and another source population to move to other water bodies. The main reason is making sure people are happy with what they are catching.”

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A small body of water like Heart Lake is an ideal location for applying rotenone because there’s nowhere for the fish to hide. The natural, plant-based chemical kills fish by preventing them from extracting oxygen from the water through their gills. 

At the concentrations used to kill fish, rotenone is not toxic to humans, other mammals or birds. But precautions must be taken during application. 

The chemical breaks down completely in the environment and is not detectable a few weeks after treatment.

“I’ve been working on this for six years. It’s mainly making sure we are doing everything right. There are other projects like this around the state. This is not the only district that has problems with illegal introductions,” Miles said.

Wearing Tyvek suits and breathing through air filters, crews spray liquid rotenone into the shallows through a pump that resembles a fire nozzle. In deeper sections of the lake, powdered rotenone is injected 10 feet below the surface to reach suspended fish and those holding near the bottom.

“We’ve actually done this before. We treated North Twin Lake in the Deschutes District” in 2016, said Ben Ramirez, Klamath watershed district fish biologist. “It snowed on us the whole time. It kept people busy because the second you stopped it got a little cold. We really don’t have issues. The biggest issue is overheating but this Tyvek isn’t bad.” 

Rotenone works quickly. Within minutes of application, we saw fish rising to the surface trying to escape the chemical.

The day after the treatment, biologist Justin Miles counted more than 60,000 dead catfish and minnows. Many will be pulled from the water in nets, but most will sink to the bottom and fertilize the lake for future plantings of rainbow trout.

“We are paid by angler dollars. We work for the angling public,” Ramirez said. “People who want a better fishery here are noticing that the trout sizes are decreasing. We are more than happy to go through the grant process and everything. This is a last-case scenario. We don’t want to be treating water bodies but if we get to that point and we work with the public so that they don’t just put these unwanted fish back in, it’s a fantastic win.” 

It is illegal to introduce any fish into a waterbody or transport live fish for any reason. 

If you happen to see someone transplanting fish or suspect the activity is occurring, you can report it by calling 1-800-452-7888. It’s even easier from a cell phone, just dial *OSP or *677


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