Over the next week, Washington Department of Fish and Wildlife staff will purge West Medical Lake of fish using rotenone, a naturally occurring pesticide.
The process, known as rehabilitation in department parlance, comes after goldfish were illegally introduced to West Medical Lake – likely as live bait. The fish quickly overtook the lake’s trout population.
Running north and south along the western edge of the town of Medical Lake, West Medical Lake is one of the more popular spots in the region for anglers on the hunt for rainbow trout, which WDFW introduces seasonally.
Once a species like goldfish becomes established, it can quickly outcompete the native trout.
“If other species get in there and they start competing for food, then the fry that you planted will get outcompeted,” said Randy Osborne, a Washington Fish and Wildlife Department fisheries biologist.
Trout fishing is a popular pastime and a big business in Washington. In 2017, there were about 352,000 trout anglers in the state. Those anglers went on 2.13 million trips and spent $66.2 million on trout-fishing related expenditures.
“(But) the cost-effective trout fishery consists of having a monoculture fishery, meaning trout are the only thing in them,” Osborne said.
Stocking the lake with trout fry is a cheap and relatively easy way to create a productive fishery. Although it would be possible to introduce larger, mature trout into the lake – trout that could compete with the goldfish – this is not a cost-effective strategy, Osborne said.
“You can put a ton of these fish (fry) in the water, and once they’re in the water our job is done,” he said.
Spring fry rainbows cost 4 cents each. WDFW usually stocks lakes in late May or early June. That means by the following opening day, roughly a year later, the fish will be between 11 and 12 inches, Osborne said.
Treating West Medical Lake with rotenone will cost $150,000. Planting fry in the lake for a decade will bring that total cost up to $294,200.
In comparison, planting a mature and catchable fish in the lake costs $1.51 per fish. Osborne estimates that treating the lake and then planting mature fish would cost closer to $2 million over a decade.
There have been goldfish in Medical Lake for at least four years, Osborne said.
After treating the lake, WDFW will plant some mature catchable trout alongside the fry. That means anglers will still be able to fish for trout in 2019.
Prior to ordering the treatment of the lake, WDFW held several public meetings, Osborne said. Those meetings were poorly attended. Rotenone treatment can be a contentious management decision, but Osborne said it’s a safe substance and dissipates in a matter of days. Rotenone is found naturally in some roots and trees.
The chemical has been used by biologists since the 1950s.
“You can expect the lake to be toxic to fish anywhere from four to six weeks,” he said.
There are a number of regulations surrounding the use of rotenone. Fish killed by rotenone are not allowed to be eaten or used as feed. WDFW staff wear protective clothing – including respirators – while treating the lake.
The chemical can sicken humans if ingested. According to a Cornell University examination of the pesticide, it has no long-term ecological impacts.
The chemical breaks down “readily by exposure to sunlight. Nearly all of the toxicity of the compound is lost in five to six days of spring sunlight or two to three days of summer sunlight,” Cornell said.
Because of how quickly rotenone breaks down, it’s not considered a groundwater pollutant. WDFW will put about 30,000 pounds of rotenone in the lake.
As for how the goldfish got there, Osborne said, “it’s pretty evident that the goldfish in West Medical Lake got there illegally. There is no inlet or outlet, and they do not fly or walk.”
The goldfish may have been used as live bait, been stocked or perhaps someone “didn’t have the heart to flush them down the toilet.”
Stocking fish in any body of water, without receiving a permit from WDFW, is illegal.
“Please leave the lake management and fish stocking to us,” Osborne said. “If you don’t want your goldfish, bring them to me.”
Treatment of the lake will start Monday and end Friday.
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