Lake Trout

The lake trout, introduced to Yellowstone Lake, is one of several aquatic nonnative species having a significant detrimental effect on the park’s aquatic ecology.

© Jay Fleming

 

Lake trout (Salvelinus namaycush) are native to Canada, Alaska, the Great Lakes, New England, and parts of Montana. Lake trout were intentionally stocked in Lewis and Shoshone lakes in 1890 by the U.S. Fish Commission (a predecessor of today’s US Fish & Wildlife Service). The species was first documented in Yellowstone Lake in 1994. Evidence from chemical patterns in lake trout ear bones sampled in the late 1990s indicate that the initial stock originated from nearby Lewis Lake some time in the 1980s. Despite major efforts to remove them by gillnetting, the lake trout have had a significant ecological impact on the native Yellowstone cutthroat trout, an important food for other native animals. Lake trout differ from cutthroat trout as potential prey because they can grow larger, occupy deeper areas of the lake, and spawn in the lake instead of in shallow tributaries.

Description

• Dark gray body with white spots. Numerous spots on head.
• Deeply forked tail.
• Often white on the edge of fins.

Behavior

• Lake trout are voracious, efficient predators.
• Frequently live >25 years and grow very large. The Wyoming state record weighed 50 pounds (23 kg).
• Fall spawners (September/October). A 12-pound female could produce up to 9,000 eggs annually.
• About 30% of a mature lake trout’s diet is cutthroat trout. They can eat up to 41 cutthroat trout per year and can consume cutthroat trout up to 55% their own size.

Distribution

• Heart, Lewis, Shoshone, and Yellowstone lakes.

 

Lake Trout At a Glance

Description

• Dark-gray body with white spots; numerous spots on head.
• Deeply forked tail. 
• Often white on the edge of fins.

Behavior

• Lake trout are voracious, efficient predators.
• Frequently live >25 years and grow very large. The Wyoming state record weighed 50 pounds (23 kg).
•  Fall spawners (September/October). A 12- pound female could produce up to 9,000 eggs annually.
• About 30% of a mature lake trout’s diet is cutthroat trout. They can eat up to 41 cutthroat trout per year and can consume cutthroat trout up to 55% of their own size.

Background

• 1890s: The park stocked fish; lake trout were introduced to Lewis and Shoshone lakes.
• Lake trout were illegally introduced into Yellowstone Lake in the 1980s and 1990s.
• 1980s–1990s: Lake trout illegally introduced into Yellowstone Lake
• 1994: First verified lake trout caught in Yellowstone Lake.
• A mature lake trout can eat ~41 cutthroat trout per year.
• The cutthroat trout population in Yellowstone Lake is ~10% or less of historic highs.
• Many wildlife species, including the grizzly bear, otter, and bald eagle, depend on cutthroat trout for a portion of their diet.
• Most predators can’t catch lake trout because the trout live in deep water, spawn in the lake, and are large.

Current Status

• Gillnetting has removed more than 3.4 million lake trout since 1994.
• Anglers catch approximately 20,000 lake trout each year.
• Population models estimate a 71% decline in age 6+ lake trout since 2012.
• Lake trout recruitment remains strong. However, recent results indicate that is beginning to decrease as well.

Outlook

With continued aggressive control efforts, fisheries managers expect to reduce lake trout numbers and lessen impacts to cutthroat trout. Recent monitoring indicates Yellowstone cutthroat trout in Yellowstone Lake are starting to rebound and the lake trout population is in decline.

 

Population in Yellowstone Lake

Lake trout are a serious threat to the native Yellowstone cutthroat trout population and, as a result, the National Park Service (NPS) has worked to suppress the population. Although lake trout need energy-rich prey to continue to grow, they can persist for years with minimal food resources. Nonnative lake trout could decimate the native cutthroat trout population and then persist at high numbers on other foods, giving cutthroat trout no chance to recover. Lake trout also consume foods that have historically fed cutthroat trout in Yellowstone Lake, thereby making cutthroat trout recovery impossible until the lake trout population is suppressed.

 

Since 2009, the park has contracted a commercial fishing company to increase the catch of lake trout.

NPS

Taking action

In 1995, after confirming lake trout were successfully reproducing in Yellowstone Lake, the NPS convened a panel of expert scientists to determine the likely extent of the problem, recommend actions, and identify research needs. The panel recommended that the park suppress lake trout in order to protect and restore native cutthroat trout. The panel also indicated that direct removal efforts such as gillnetting or trapnetting would be most effective but would require a long-term, possibly perpetual, commitment.

Lake trout gillnetting begins as ice is leaving the lake in spring and continues into October. Gillnet operations remove lake trout from the population and also provide valuable data—population estimates, age structure, maturity, and potential new spawning areas—leading to more effective control. Incidental catch of native cutthroat trout is minimized by fishing deeper waters not typically used by cutthroat trout.

As initial gillneting efforts expanded, the number of lake trout removed from the population also increased. This suggested the lake trout population was continuing to grow. In 2008 and 2011, scientific panels were convened to re-evaluate the program and goals. The panel concluded netting is still the most viable option for suppressing lake trout. Both reviews also indicated a considerable increase in suppression effort would be needed over many years to collapse the lake trout population.

Starting in 2009, the park contracted a commercial fishing company, to increase the take of lake trout through gillnetting. From 2011 to 2013, they also used large, live-entrapment nets that allow removal of large lake trout from shallow water while returning cutthroat trout to the lake with little mortality.

Anglers are encouraged to fish for lake trout, and are required to kill all lake trout caught in Yellowstone Lake and its tributaries. Of the total lake trout removed from Yellowstone Lake, anglers have removed approximately five percent.

 

Lake Trout Response to Increased Suppression Efforts

More than 3.4 million lake trout have been removed from Yellowstone Lake since 1994. In 2019, National Park Service and contracted crews captured more than 282,000 lake trout—the majority of which were two-year-old fish. Catches of lake trout in the larger meshes decreased in 2019 compared to similar effort in 2018, indicating a further population decline. Catch rate in these meshes has dropped dramatically from 3.2 lake trout per net night in 2012 to less than 0.8 lake trout in 2019.

The number of lake trout caught during distribution surveys remained relatively constant from 2010-2017, ranging from 360-618 fish annually. Annual catches declined in 2018 and 2019 to 263 and 240 fish, respectively, the lowest numbers observed since 2010. Annual mean total length from 2010-2019 ranged from 313-344 millimeters (12-13.5 in.).

Increases in catch can reflect increased efficiency, increased abundance, or both. Improvements in gear, knowledge, and experience can lead to more efficient removal, despite a decreasing population. Hence, independently monitoring the effectiveness and effects of suppression activities, as well as updating population models, is an important aspect of the program.

Population modeling has shown that the lake trout population expanded through 2012, but increased gillnetting has begun to reduce lake trout numbers and biomass (total weight) in Yellowstone Lake, particularly in older age classes. Abundance models show no population growth for lake trout age 2–3 years, a decrease in fish 3–5 years, and a 71% decrease 6 years and older. Overall lake trout abundance has decreased 32% across all ages since the population peaked in 2012. Total annual mortality rates have been steadily increasing over the past several years and have exceeded 50% in three of the last four years. In addition, the total biomass of lake trout removed annually has increased from 0.5 kilograms per hectare to over 4.0 kilograms per hectare. If this trend continues, models predict a population crash within the next 5 to 10 years.

 

Discovery of New Species in Yellowstone Lake

On August 22, 2019 a gill net set in 158 feet of water northeast of Stevenson Island pulled in one fish of a new species (Coregonus sp). not previously known to exist in Yellowstone Lake. At this point, what is known is this was a cisco, but the exact species or strain is still being determined genetically. It was a 3 year old immature female, and based on otolith microchemistry, likely hatched in Yellowstone Lake. It is not likely this species arrived in Yellowstone naturally, as the nearest source populations are in northern Montana. There are no existing waterways between Yellowstone Lake and any known cisco populations.

In its native range, cisco are a preferred prey item for lake trout where the two species overlap. They prey mostly on aquatic invertebrates and tend to reside at mid-water depths.

 

Research guides methodology

In 2010, Yellowstone developed the Native Fish Conservation Plan. This adaptive management plan guides efforts to recover native fish and restore natural ecosystem functions based on scientific assessment.

In 2011, the National Park Service and the US Geological Survey launched a movement study to target lake trout embryos in spawning beds and identify general and seasonal movement patterns. The results helped gillnet operators to target lake trout more efficiently.

In 2013, NPS and Montana State University conducted a mark/recapture study of lake trout in Yellowstone Lake. In order to estimate population size, 2,400 lake trout were tagged and released back into the lake. More than 86% of tagged fish were recaptured. Results produced an estimate of the number of lake trout present in the lake: 367,650 fish greater than 210 millimeters (8.3 in.) long.

The mark-recapture study also helped estimate rates of capture for four size classes. This effort removed 72% of lake trout 210–451 millimeters (8.3– 17.8 in.) in length, 56% for fish 451–541 millimeters (17.8–21.3 in.) long, 48% for fish 541–610 millimeters (21.3–24.0 in.) long, and 45% for fish more than 610 millimeters (>24.0 in.) long . These results supported previous estimates and highlighted the difficultly in catching older, mature lake trout, which eat the most native cutthroat trout and have the highest reproductive success.

 

Future of Lake Trout Control

With current technology, lake trout probably cannot be eliminated from Yellowstone Lake. However, ongoing management of the problem can significantly reduce lake trout population growth and maintain the cutthroat trout population, which is a critical ecological link between Yellowstone Lake and its surrounding landscape.

Reducing the lake trout population to a level that will have only minor impacts to the cutthroat trout population is predicted to take place in the near future—provided we maintain current high levels of suppression effort. Given the high reproductive potential of this lake trout population, it will rebound immediately and dramatically if we reduce the suppression effort.

Present research efforts by Yellowstone National Park biologists and collaborating scientists at the USGS Montana Cooperative Fishery Research Unit and Montana State University focus on killing lake trout embryos on spawning sites. So far, only 50 acres of lake trout spawning areas have been identified by tracking tagged fish, visual observations during SCUBA diving, and remote observations with underwater cameras.

One technique uses lake trout carcasses from the gillnetting operation to kill lake trout embryos. Carcasses are shredded and deposited on the rocky areas where lake trout naturally spawn. Natural decomposition of the shredded fish decrease the dissolved oxygen in the areas around the embryos, making the spawning site inhospitable. Tests showed almost complete mortality of lake trout embryos at the treatment site within a couple of weeks. This technique shows great potential for increasing the effectiveness of the suppression program. A similar technique uses pellets of vegetative materials which, limited research has shown, are less of an attractant to bears, easier to handle, and just as effective at killing embryos. These options will be explored further in the coming field season.

 

Resources

Bigelow, P.E. 2009. Predicting areas of lake trout spawning habitat within Yellowstone Lake, Wyoming. Doctoral dissertation, University of Wyoming, Laramie, WY.

Gresswell, R.E. 2009. Scientific review panel evaluation of the National Park Service lake trout suppression program in Yellowstone Lake, August 25–29, 2008: Final report, October 2009, Edited by USGS Northern Rocky Mountain Science Center. Bozeman, MT.

Koel, T.M., P.E. Bigelow, P.D. Doepke, B.D. Ertel, and D.L. Mahony. 2005. Nonnative lake trout result in Yellowstone cutthroat trout decline and impacts to bears and anglers. Fisheries 30(11):10–19.

Koel, T.M., P.E. Bigelow, P.D. Doepke, B.D. Ertel, and D.L. Mahony. 2006. Conserving Yellowstone cutthroat trout for the future of the Greater Yellowstone Ecosystem: Yellowstone’s Aquatic Sciences Program. Yellowstone Science 14(2).

Middleton, A.D., T.A. Morrison, J.K. Fortin, M.J. Kauffman, C.T. Robbins, K.M. Proffitt, P.J. White, D.E. McWhirter, T.M. Koel, D. Brimeyer, and W.S. Fairbanks. 2013. Grizzly bears link non-native trout to migratory elk in Yellowstone. Proceedings of the Royal Society B 280:20130870.

Munro, A.R., T.E. McMahon, and J.R. Ruzycki. 2006. Where did they come from?: Natural chemical markers identify source and date of lake trout introduction in Yellowstone Lake. Yellowstone Science 14(2).

Wyoming Water Project. 2014. Science supporting management of Yellowstone Lake fisheries: Responses to frequently asked questions. Trout Unlimited: Lander, WY.

Ruzycki, J.R., D.A. Beauchamp, and D.L. Yule. 2003. Effects of introduced lake trout on native cutthroat trout in Yellowstone Lake. Ecological Applications 13:23–37.