Study Of Lake Trout Early Life Stages Aims To Help Rid Yellowstone Lake Of Invasive

By on July 10, 2015
Emergent fry traps are placed on known spawning reefs, such as around Carrington Island in Yellowstone Lake, and capture lake trout fry as they swim out of the substrate. These traps provide a catch-per-unit-effort (CPUE) which can be used to compare relative densities of lake trout fry between spawning sites. (Credit: Lee Simard)

Once a stable home for cutthroat trout, Yellowstone Lake is now threatened by invasive stocks of lake trout. Scientists say that an over-eager angler is likely to blame.

“They were introduced from Lewis Lake, which is on the opposite side of the continental divide,” said Lee Simard, a graduate student in natural resources at the University of Vermont who is helping with studies of lake trout in Yellowstone. “They had to have been transported by someone and then planted, and then the population just took off.”

The lake trout are harmful to the native cutthroats for a lot of reasons. For one, cutthroats have shorter life cycles than lake trout, so it is easier for lake trout to outgrow them in size. This allows the lakers to prey more readily on cutthroats. By age four, cutthroats become the main meal for lake trout, comprising about half of their diet.

Many initiatives are underway to fight the invasive lake trout problem at Yellowstone Lake. The National Park Service is supporting fishing efforts to pull out as many lakers as possible. Work with gill nets at different sites where the fish live has also been employed. But the issue is more complicated than those steps can control, and Simard is working to attack the lake trout before they even grow up.

“We’re doing more on the research side to look at lake trout early life stages,” said Simard, discussing his work that’s focusing on spawning sites. “It could help managers suppress and control populations faster.”

A redside shiner is captured in the custom-built sled after it was washed out of the substrate by jets of water from the front of the sled. Note how the curved bars on either side of the sled prevent it from tipping as it slides over rocks. (Credit: Lee Simard)

A redside shiner is captured in the custom-built sled after it was washed out of the substrate by jets of water from the front of the sled. Note how the curved bars on either side of the sled prevent it from tipping as it slides over rocks. (Credit: Lee Simard)

He and others at the University of Vermont, including his advisor Ellen Marsden, professor of fisheries, and Steven Cluett, captain of the research vessel Melosira, use a lot of different equipment to study spawning sites in Yellowstone Lake. These include things like gill nets and passive traps that capture lake trout and their fry. These have helped study lake trout numbers around spawning areas, but scientists say they aren’t proactive enough. To get around that issue, Cluett designed and built a custom benthic sled.

“We wanted to develop a more active method, so we decided to create a benthic sled,” said Simard. It works by using a pressure washer to send water through its manifold and shoot water into the substrate to dislodge lake trout fry. “The eggs trickle into crevices, but any eggs we tow it over are thrown into the water column and into the net. It washes them out of the substrate and makes them accessible.”

The new sled was used this past spring at 15 different sites in the lake on about 75 different tows. Not many fry were captured as a results of those efforts, says Simard, but there were some found around Carrington Island, which sits on the western shore of Yellowstone Lake.

The sled, as well as much of the other sampling equipment used in Yellowstone Lake, is also being used in a complementary study looking at populations of lake trout in Lake Champlain. But the issue is different there — native stocks of lake trout in Champlain are not successfully recruiting into the population and scientists want to know why.

“In the Great Lakes, there are healthy populations of lake trout, but they’re all stocked,” said Simard. “We’re trying to restore them in the Great Lakes, where they’re native, using the same science that we’re using to remove them from Yellowstone Lake, where they’re invasive.”

The project is being funded by Wyoming Trout Unlimited.

Top image: Emergent fry traps are placed on known spawning reefs, such as around Carrington Island in Yellowstone Lake, and capture lake trout fry as they swim out of the substrate. These traps provide a catch-per-unit-effort (CPUE) which can be used to compare relative densities of lake trout fry between spawning sites. (Credit: Lee Simard)

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