Invasive Lake Trout a Threat to Many Yellowstone Species

(CN) – Lake trout illegally introduced into Yellowstone Lake in the 1980s have had a major impact on not just native trout, but also on other species that feed on the fish.

Scientists from the University of Wyoming and the National Park Service looked at data spanning back to 1972 to analyze the impact that lake trout, first identified in the lake in 1994, has had on the Yellowstone ecosystem. They found in the past 30 years, there has been a significant drop in native cutthroat trout in Yellowstone Lake.

According to the research, which was published in the journal Scientific Advances on Wednesday, the drop in cutthroat trout is partly because the lake trout preys on the cutthroats and their spawn at depths that can’t be accessed by bears, birds, and other trout-eating animals.

The declining population of cutthroat trout has had a ripple effect. Because there are fewer of the fish, the size of zooplankton has increased, which has likely caused a rise in the water’s surface temperature.

And other species have been affected too. Ospreys and bald eagles have had to find new food sources, and some populations have moved away from Yellowstone Lake. The eagles have been observed eating other birds like loons and pelicans, which might have an impact in those populations’ decline.

"Our study illustrates the potential impact of a single, invasive predatory species on otherwise pristine ecosystems," the scientists wrote.

Cutthroat trout are also an important source of food for bears. The study showed how the annual consumption of trout by black and grizzly bears has declined from more than 20,000 in the late 1980s to just over 300 in the late 2000s. By 2007-2009, that number of cutthroat trout eaten by the bears had dropped to zero. 

The researchers noted “this was a localized displacement” for the bears, and that their population has remained stable.

"Since bears are omnivore generalists, they could make use of other foods,” they wrote.

“Other foods” includes elk calves. The researchers noted that the number of young elk eaten by bears suggests the invasive lake trout has also had an indirect impact on migratory elk populations.

In recent years, scientists have looked at gillnetting as a possible solution to the problems caused by lake trout, and to attempt to recover the native cutthroats.

The study’s authors include scientists from the Wyoming Natural Diversity Database at the University of Wyoming, five National Park Service scientists, and a researcher from Montana State University’s Cooperative Fishery Research Unit.

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