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Protect Our Parks

Yellowstone Lake Cutthroat Trout Threatened by Non-native Lake Trout

Yellowstone National Park fisheries biologists resort to "netting" and help from anglers to save thinning population of native cutthroat trout in Yellowstone Lake.

80,000 non-native lake trout removed from Yellowstone Lake

Fisheries biologist likens removal of non-native lake trout from Yellowstone Lake to “weed control”

Yellowstone Lake is home to the largest remaining population of Yellowstone cutthroat trout in North America. But since non-native lake trout were discovered in Yellowstone Lake by an angler in 1994, the famous cutthroat population has been threatened.

Lake trout aren’t welcome in the lake. Fisheries personnel have removed the fish in large numbers. To date, more than 80,000 lake trout have been removed.

The potential damage a lake trout population can cause cutthroat is enormous.

For starters, a lake trout lives through two or three generations of a cutthroat. A lake trout may live for 25-40 years, a cutthroat, maybe 10 years.

“Lake trout are so long-lived,” says Pat Bigelow, fisheries biologist. “A seven-year-old cutthroat is sort of an old fish. If you have seven years of serious impact on the juveniles coming up, you could lose your whole population.”

A big lake trout taken from Yellowstone Lake weighed 21.5 pounds. A big cutthroat in the lake – maybe even a trophy catch – would weigh about five pounds.

As soon as a lake trout reaches four years old, it begins eating cutthroats, which make up half of their diet. A couple years later, it eats cutthroats almost exclusively. Typically, a mature lake trout will eat 50 cutthroats a year. The non-native lake trout are not only eating several cutthroats a year, but they compete with the native fish for the same food sources. Lake and cutthroat trout both feed on leaches, amphipods, and lake midges.

A Life Cycle for Domination

Lake trout are a reproductive bunch, spawning eight to 10 years in a row, each time yielding 1,000 eggs per kilogram of body weight. This means a 20-pound lake trout could produce 10,000 eggs. If just one percent of those eggs survive, you’re talking 100 new lake trout born to a single fish in one year. On the other hand, a cutthroat may spawn only once or twice during its lifetime.

It doesn’t take a calculator to conclude that lake trout will do serious damage in a short amount of time if left unchecked at Yellowstone Lake.

A panel of fisheries experts that met in 1995 to assess the lake trout’s presence and likely impacts on Yellowstone cutthroat population estimated that, with effective suppression of lake trout numbers, the cutthroat population decline might be held to 10- to 20 percent of present levels. Without some control of lake trout, the experts predicted the cutthroat population would be reduced by 70 percent in 100 years.

Other Impacts

The cutthroat population is valued ecologically, economically and socially. Cutthroat trout live and spawn in shallow streams and waters, providing prey for at least 42 species of birds and mammals. Grizzly bears, otters, eagles, white pelicans and osprey are just a handful of the animals that stand to lose a valuable food source if the Yellowstone Lake population is diminished.

Furthermore, the cutthroat trout in the lake help generate $36 million in revenues resulting from the world-class sport fishing found in Yellowstone and surrounding communities. Anglers come from all over the world to fish for these native wild fish.

Netting to the Rescue

Since 1995, fisheries staff in Yellowstone have been netting lake trout. Some fisheries biologists have called the effort a “forever project.”

In 2001, Yellowstone received a new gill-netting boat, which significantly aided fisheries biologists in their netting efforts. The boat was designed as a scaled-down version of commercial gill netters used on the Great Lakes. The boat, which is 32 feet long and 14 feet wide, has a net lifter designed for pulling nets off the bottom of the lake.

Before the boat’s arrival, fisheries biologists had to pull in the large nets that were full of lake trout varying in size from a half a pound to 20 pounds. It was grueling work.

Last year, 18,000 were netted – five thousand more than the previous year.

“We did really well last year,” says Bigelow. “We increased our effort and as a result removal was up 40% over the year before.”

Trying to ‘hit ’em hard’ During Spawning

Most of the lake trout currently being netted are two or three years old. These immature lake trout are found at depths of 150-250 feet. Netting is also being done in some 50-foot depth areas of the lake, where crews are able to net some large, adult lake trout.

Medium-sized lake trout, however, are difficult to net because they share many of the same waters as cutthroat trout. To net this age group of lake trout, fisheries biologists have located some key spawning areas in the lake and during the lake trout’s spawning season, have been able to net large numbers of medium-sized lake trout.

“When lake trout reach five or six years old, they become sexually active,” says Bigelow. “We’re looking to net them during times when they’re segregated from the cutthroats, which means catching them during the spawning window.”

Bigelow says an additional spawning area near West Thumb Geyser Basin was located last year and resulted in some increased removals of mature lake trout. During recent months, fisheries personnel have been busy trying to determine additional spawning areas via mapping, says Bigelow. The goal is to find additional spawning areas this year during netting efforts so more of the medium-sized lake trout can be removed from the lake.

Spawning for lake trout begins at the end of August and runs through September.

Anglers Help

In recent years, Yellowstone Park has issued a release inviting anglers to help control the lake trout population in Yellowstone Lake by allowing anglers to fish spawning areas of the lake during September and October. There are no creel limits.

As lake trout mature, they become more predacious, Bigelow says, and they intermingle in the shallow water with the cutthroats. They move less, and are harder to catch in the gill nets.

“They have to swim into the nets for us to catch them,” explains Bigelow.

Bigelow says fisheries biologists are having a difficult time catching these older lake trout, but that anglers are doing quite well catching lake trout in this age group.

Anglers are urged to fill out response cards after fishing the lake to report their catches. This helps fisheries biologists better monitor catch rates.

A “Forever Project”

Bigelow says it’s not likely that the last lake trout will ever be caught in Yellowstone Lake. A more realistic goal is to get ahead of the problem, she says.

She compares the lake trout removal effort to weed control. The lake trout problem isn’t going to go away, but with great sustained efforts fisheries biologists might be able to prevent it from taking over the lake completely.

Like weeds in a yard, the unwelcome lake trout that’s populating the famous Yellowstone cutthroat’s home might be reduced in numbers. Its damage should be minimized as a result of continued netting and removal of lake trout, says Bigelow.