The population of cutthroat trout in Yellowstone Lake is increasing. The history of this native fish includes a collapsing population and now, a second chance.
For thousands of years, a large population of cutthroat trout lived in Yellowstone Lake. Each spring, they migrated up feeder streams to spawn. While in these streams, they provided a valuable food source for many wildlife species.
Dozens of bald eagles and osprey nests lined the lake and streams. These birds of prey took advantage of this food source to raise their young. Black bears and grizzly bears traveled to these streams each spring to feast on cutthroat. Otters and white pelicans were common along the lake edge for the same reason.
During the 1950s-60s, 250,000 to 300,000 cutthroat trout were being caught by anglers each year. In the 1970s, 80s, and early 90s, about 142,000 angler days occurred annually on the lake. Almost 350,000 tourists visited Fishing Bridge annually. This bridge crossing the lake outlet was a popular place to fish and observe cutthroat trout.
The situation changed in the 1980s, when someone illegally released lake trout into Yellowstone Lake. In the late 1800s and early 1900s, the NPS had stocked park streams and lakes with rainbow, brown, brook, and lake trout, as well as Yellowstone cutthroat trout. Lake trout were introduced into nearby Lewis and Shoshone Lakes, but not in Yellowstone Lake.
The illegally introduced fish probably came from one of these nearby lakes. The first reported lake trout was caught in the lake in 1994. Since then, lake trout numbers have soared at the expense of cutthroat trout. A mature lake trout can eat approximately 41 cutthroat trout per year. Lake trout often live 20–25 years and grow huge. Due to this predation, the cutthroat trout population in Yellowstone Lake has dropped to 10% or less of historic high numbers.
Compounding the damage, lake trout do not migrate up streams to spawn but reproduce deep in the lake. In some feeder streams, cutthroat numbers have dropped by 99%.
Clear Creek has been monitored since 1945. In 1978, 70,000 cutthroats entered the stream to spawn. That number fell to 538 fish by 2007. Osprey and bald eagle numbers have plummeted along the lake shore. Bears do not travel to the lake nearly as much. Grizzly bears have replaced cutthroat in their spring diet with elk calves. This has furthered the drop in numbers of park elk. Fishermen literally stopped fishing for the few remaining cutthroats and visitation to Fishing Bridge dropped drastically.
Soon after the discovery of lake trout in Yellowstone Lake, the NPS began a program of reducing lake trout numbers. Gill netting operations are now removing large numbers of lake trout each summer. Currently, about $2 million is spent annually on lake trout suppression in Yellowstone Lake.
Much of the funding comes from private donors that want to restore the cutthroat trout population in the lake. Four contractor boats and two Park-operated boats work the lake from spring to fall. Gillnetting has removed more than 3.14 million lake trout since 1994.
Lake trout are popular with fishermen. They come from around the country to catch the large lake trout in Yellowstone Lake. Anglers catch approximately 20,000 lake trout each year. Park rules state fishermen must release any cutthroats caught. They cannot return live lake trout. Lake trout must be kept or killed.
After 24 years of gill-netting operations, the NPS is starting to succeed in reducing the lake trout population. Over 400,000 lake trout were removed from Yellowstone Lake in 2017, while only 297,000 were removed in 2018 and 282,960 fish between May and October of this year. The number of lake trout caught in nets continues to decline, from 4.4 per net in 2017, and 3.1 per net in 2018, to just 2.9 per net in 2019. Abundance models show no population growth for lake trout age 2–3 years, a decrease in fish 3–5 years, and a 70% decrease in fish 6 years and older.
Gill-netting is expensive and other ways of killing lake trout embryos are being tried. Lake trout have been fitted with acoustic tags and released, leading researchers to the location of spawning beds on the lake bottom. researchers have learned these spawning sites are relatively small and well defined. Caught lake trout carcasses have been shredded and the remains dropped in the lake over these spawning beds. Rotting carcasses have reduced the dissolved oxygen in the bottom water, killing the eggs. Since then, an organic soy and wheat pellet has been found that will accomplish the same feat.
As lake trout numbers decline, cutthroat trout numbers are rebounding. More cutthroats are being observed traveling up feeder streams. A few grizzly bears are returning to these streams to eat cutthroat trout.
Reducing the lake trout population is working, and leading to more cutthroat trout in the lake. Lake trout will likely never be completely removed from Yellowstone Lake. Suppression activities may always be a part of lake management, but it appears that less expensive suppression techniques may be substituted for netting operations.
In the long term, the NPS feels Yellowstone cutthroat trout can be restored in Yellowstone Lake and resume their important ecological roles within the greater Yellowstone ecosystem.