Yellowstone cutthroat trout are starting to show signs of recovery after nearly 30 years of efforts to eliminate the invasive lake trout threatening their population.
Lake trout aren't native to Yellowstone Lake but scientists became aware of their presence there in 1994.
Prior to 1994, Yellowstone Lake was home to the largest population of Yellowstone cutthroat in the world. But lake trout have voracious appetites and they like to eat cutthroat. Their introduction decimated the population.
"By 2008 that population of 4 million had dropped down to somewhere in the range of 200 to 300 thousand. So more than a 90% decline. And all of a sudden the refuge for Yellowstone cutthroat was under severe threat of being wiped out. The numbers of lake trout had expanded dramatically," said Yellowstone Lake Special Project Manager Dave Sweet.
It's been an uphill battle to remove the trout since then, but the efforts have started to show some positive results.
The main method to remove lake trout is gill nets. Those are nets strung across the bottom of the lake that catch fish of a certain size, like adult lake trout. The amount of trout caught per net is known as a Catch Per Unit of Effort (CPUE). The overall CPUE of the lake has been dropping.
"The CPUE peaked in 2011/2012. The overall CPUE was about nine lake trout for every hundred meters of net set for one night. That has declined to an overall CPUE right now of about three. So a huge decrease," said Sweet.
Lake trout will likely never be fully eradicated from Yellowstone Lake, but Sweet predicts that the population will be low enough that they will soon be able to reduce their efforts to maintenance netting.
"The news has just been tremendous for the last couple of years and this year, it really is tremendous. Those big adult fish have declined by 90%. They now represent less than 2% of the overall population of lake trout in that system," said Sweet.
The return of cutthroat could likely have cascading effects on the ecosystem.
"Forty different species are dependent upon that fish. The lake trout does not substitute for the cutthroat. It doesn't go up the tributaries to spawn, so it's not available to the grizzly bears. It doesn't spend a lot of time in shallow water, so the Ospreys can't get at it, the bald eagles can't get at it, the river otters can't get at lake trout," said Sweet. "So you can't just say, well, all those other species could eat like trout. No, they can't. They can't get at 'em, because they live deeper in the water column."
Removing the lake trout has been expensive, costing around $2 million each year, but that number should drop with the transition to maintenance netting. It's been a collaborative effort between the park service, many volunteers, and several private entities.
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