Yellowstone Lake is the largest body of water in Yellowstone National Park, covering 136 square miles. It is of great interest to the United States Geological Survey, as part of the Yellowstone supervolcano lies below the basin of the lake. The area caused a brief spell of concern in 2004, when the ground under the lake started to rise significantly creating a bulge.
However, it was a more recent find that that has alarmed researcher.
In March this year, a new study revealed how non-native lake trout has heavily affected animals in the park.
Zooplankton, cutthroat trout, river otters, osprey, bald eagles, bears and, elk are among the creatures whose numbers, diet and behaviour have been altered as a result of the presence of the invasive fish.
Scientists analysed data spanning more than four decades, from 1972 to 2017, and concluded that the impact of lake trout in Yellowstone Lake – in particular, the decline of native cutthroat trout – has cascaded across the lake, its tributaries and the surrounding ecosystem.
Our study illustrates the potential impact of a single, invasive predatory species
Lusha Tronstad
The study was spearheaded by Lusha Tronstad from the Wyoming Natural Diversity Database.
She said: “Our study illustrates the potential impact of a single, invasive predatory species on otherwise pristine ecosystems.”
Monitoring over the past three decades has shown a dramatic drop in the numbers of native cutthroat trout that inhabit Yellowstone Lake.
This is believed to have been caused by the illegal introduction in the Eighties of lake trout – which prey on cutthroats and spawn at depths inaccessible to bears, birds and other animals.
Despite the cutthroat trout decline, grizzly bear abundance throughout Yellowstone National Park has remained stable.
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Dr Tronstad explained: “Grizzly and black bear frequency of occurrence on spawning tributaries and use of cutthroat trout as a food resource were greatly reduced following the lake trout invasion.
“However, this was localised displacement, and their populations were not otherwise affected, because only bears with home ranges neighbouring Yellowstone Lake lost spawning cutthroat trout as a food resource.
“Since bears are omnivore generalists, they could make use of other foods.”
“By 2007-2009, grizzly bears had shifted to alternative prey, and the proportion of cutthroat trout in their diet had declined to zero.
“Elk then accounted for 84 percent of all ungulates consumed by bears in the Yellowstone Lake area, suggesting lake trout had some level of indirect, negative impact on migratory elk using this area when spawning cutthroat trout were rare.”
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However, Dr Tronstad did note that the extensive gill-netting of lake trout over the past 24 years has begun to help the cutthroat trout population in Yellowstone Lake.
She continued: “Juveniles are again recruiting to the cutthroat trout population.
“After being absent for many years, spawning adult cutthroat trout are returning to some of the smaller tributaries, and bear use of these streams has increased as a result.
“Ospreys, however, have not yet responded to the recent increases in cutthroat trout prey.
“The outcome of restoration efforts to trophic levels with and across ecosystems in the Yellowstone Lake watershed remains uncertain.”
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