Researchers discover general impacts of lake trout invasion in Yellowstone

Zooplankton, cutthroat trout, river otter, osprey, bald eagles, bears, and probably elk are among the creatures whose numbers, diet and behavior have been altered due to the presence of invasive fish, according to an article published today in the scientific literature. newspaper Progress of science.

The authors include Lusha Tronstad, a research scientist at the Wyoming Natural Diversity Database at the University of Wyoming; Todd Koel, Jeffrey Arnold, Kerry Gunther, Doug Smith and Patrick White, National Park Service scientists; and John Syslo of the Montana Cooperative Fishery Research Unit at Montana State University.

Scientists analyzed data covering more than four decades, from 1972 to 2017, and concluded that the impact of lake trout in Yellowstone Lake – particularly the decline of native cutthroat trout – has impacted the lake, its tributaries and the surrounding ecosystem.

"Our study illustrates the potential impact of a single invasive predatory species on otherwise untouched ecosystems," the researchers wrote, reinforcing Yellowstone National Park's efforts to suppress the lake trout population in the area. Yellowstone Lake.

Monitoring over the last three decades has shown a dramatic decline in the number of native cutthroat trout inhabiting Yellowstone Lake and spawning in its tributaries, largely as a result of the illegal introduction of lake trout in the 1980s – which feeds on trenches and breeds depths inaccessible to bears, birds and other animals.

As a result, according to the new document:

• The size of the zooplankton in the lake increased because fewer cutthroat trout were present to consume them. This has resulted in an increase in the clarity of the lake water, a likely factor of a slight increase in lake surface water temperature in the summer.
• Fewer spawning brook trout in tributaries reduce the transport of nutrients such as ammonium in these streams.
• The density and success of the Osprey has greatly diminished because they can not attack Lake Trout, which lives at inaccessible depths. In fact, while there were an average of 38 Osprey nests at Yellowstone Lake from 1987 to 1991, only three were seen from 2013 to 2017. And these birds left the lake to forage in the Great Lakes Snake River, at least five miles away.
• Bald eagles have changed their diet to compensate for the loss of cutthroat trout. Despite this change in diet, the average number of bald eagle nests on Yellowstone Lake increased from 11 in 2004-2008 to eight in 2013-17. And nesting success dropped from 56% in 1985-1989 to zero in 2009, rebounding to 70% in 2013-2017 as eagles found alternative food sources. The eagle at Yellowstone Lake was seen more frequently than ever before. feeding crested loons, trumpet swans and young white pelicans, which could also contribute to the decrease in the number of birds.
• Otters, which mainly fed on cutthroat trout, began feeding on suckers and amphibians. Although there is no estimate of the number of otters in Yellowstone Lake and its tributaries before the introduction of lake trout, the 2008 estimate – an otter for eight miles from shore – is among the lowest ever recorded for a river otter population.
• Black bears and grizzlies, for whom cutthroat trout spawning had been a major source of food, had to look for alternatives. The predicted number of grizzly bears' annual losses of gorgonians has increased from 20,910 in the late 1980s to 2,266 in the late 1990s to only 302 in the late 2000s.

Despite the decline of cutthroat trout, the abundance of grizzly bears in Yellowstone National Park has remained stable.

"The frequency of occurrence of grizzly bears and black bears on tributary spawners and the use of cutthroat trout as a food resource have been significantly reduced as a result of lake trout invasion." was a localized displacement, and their populations were not otherwise affected because only bears The neighboring Yellowstone Lake lost the cutthroat trout breeding as a food resource, "wrote Researchers. "Since bears are omnivorous generalists, they could use other foods."

These "other foods" probably include elk calves. Each spring, thousands of wintering elk on lower elevation lands migrate into the interior of Yellowstone, and elk calves born in the Yellowstone Lake area are vulnerable to predation, particularly during the winter. first weeks after birth.

"In 2007-2009, grizzlies had gone to other prey and the proportion of cutthroat trout in their diet had dropped to zero," the researchers wrote. "Elk accounted for 84% of all ungulates consumed by bears in the Yellowstone Lake area, suggesting that lake trout had some indirect negative impact on migratory elk that used this area when trout spawning cutthroat was rare. "

Researchers say the reintroduction of wolves to Yellowstone from 1995 – the year following the discovery of lake trout in Yellowstone Lake – has not played a role in changing bear diets in the region. Yellowstone Lake. While grizzlies attack elk calves mainly during the first days after birth, wolves and other predators kill elk calves later in summer and winter.

Scientists also refute the idea that the massive Yellowstone fires in 1988 may be responsible for the decline of cutthroat trout.

"We do not attribute significant losses of cutthroat trout to forest fires, as the recruitment of young cutthroat trout into spawning streams, process most likely to be affected by fires." of forest, remained vigorous more than ten years after 1988, "they wrote. "Cutthroat trout recruitment declined sharply as the population of lake trout increased, and forest fires had a negative impact on bears, bald eagles, ospreys, and birds. 39, other cutthroat trout consumers, these impacts would also have been documented elsewhere, outside the Yellowstone Lake Ecosystem, which was not the case ".

Scientists have noted that over the past 24 years, the gillnet gillnet has begun to assist the cutthroat trout population of Yellowstone Lake.

"Juveniles are recruiting back into the cutthroat trout population, and after being away for many years, spawning adult spawning trout are returning to some of the smaller tributaries, resulting in increased use of these fish courses. water, "they wrote. "However, Ospreys have not yet responded to the recent increase in prey numbers for cutthroat trout and the results of trophic level restoration efforts with and between the Yellowstone Lake watershed ecosystems remain unclear. uncertain. "

Explore further:
According to a study, a trout invasion behind the decline of Yellowstone elk

More information:
T.M. Koel el al., "The invasion of predatory fish induces effects within and through the ecosystem in Yellowstone National Park," Progress of science (2019). DOI: 10.1126 / sciadv.aav1139, http://advances.sciencemag.org/content/5/M/eaav1139