Approximately 315,800 YNP lake trout were killed in 2015

It appears the National Park Service is slowly turning the tide against lake trout in Yellowstone Lake, but the war has yet to be won.

Lake trout were somehow transported to Yellowstone Lake, possibly from illegal stocking in Lewis Lake and they devour native Yellowstone cutthroat. The species was first noted in the lake in the mid 1990s. Soon after, the National Park Service launched a campaign to suppress lake trout by gill-netting.

Gillnets tangle fish by the gills, making escape nearly impossible.

About one-half of the 1.7 million lake trout killed since 1994 were captured in the past three years with gillnets, according to the park’s Native Fish Conservation Program, Yellowstone Fisheries & Aquatic Sciences 2012-14.

Results suggest that lake trout abundance increased from 129,382 in 1998 to 809,858 in 2012, but then decreased to 485,468 in 2014, according to the report.

Approximately 315,800 lake trout were killed in 2015, said Pat Bigelow, Yellowstone fishery biologist. It was a good year for dispatching lake trout. Hickey Brothers, the commercial fishing contractor, and Park Service crews enjoyed a very efficient summer season.

“Lake trout expansion in Yellowstone Lake could be curtailed by inflicting an annual total mortality of 60 percent,” according to the report. “However, it is uncertain how long this mortality level must be maintained to meet lake trout suppression goals.”

The 2014 mortality rate was around 70 percent, Bigelow said.

The methods that target lake trout embryos and/or larvae on spawning sites hold the greatest promise, according to the report. Tiny Carrington Island in the West Thumb area, was documented as a major spawning area in the late 1990s.

Bigelow said she believes gillnets are most effective. Big lake trout can be killed with nets and supplemented with other methods such as electroshock and suction-dredging on spawning beds, she said.

In 2011, a telemetry study was initiated to locate spawning grounds and track lake trout, according to the report. Transmitters were implanted in 159 lake trout, and 40 stationary acoustic receivers were deployed in the lake. Over 90 percent of tagged trout moved into West Thumb during spawning migrations. Over 50 percent of tagged trout were detected at least once near the Carrington Island spawning site.

The National Park Service has been working with the Montana Cooperative Fishery Research Unit of the U.S. Geological Survey to develop electroshock methods to kill lake trout embryos.

The prototype array was used on Montana’s Swan Lake in 2013, and nearly 100 percent of lake trout embryos were killed to a depth of about 8 inches in the gravel, according to the report.

For two years, electrical grids have been used to shock eggs and embryos around Carrington, Bigelow said.

A suction dredge was used on Yellowstone Lake spawning grounds for four days in October 2014, removing approximately 5,075 viable eggs, according to the report.

Electroshocking and suction-dredging are still in the experimental phase and Bigelow said she is not sure how effective those measures will be at this time.

Cutthroat increase

The average catch of cutthroat trout per distribution net increased from 22 in 2012 to 26 in 2013 and 31 in 2014, according to the report.

Distribution nets are exactly what the name suggests — they estimate a species’ distribution.

The desired conditions for Yellowstone Lake are an average of at least 40-60 spawning cutthroat trout observed per stream visit to its 11 tributaries.

There are around 60 tributaries around the lake, the aforementioned 11 are around Lake and Grant villages, Bigelow said. More cutthroats are now spawning the tributaries, but not in huge numbers. “It’s an incremental increase.”

The National Park Service is spending around $2 million annually on suppression, research and monitoring with funding help from the Yellowstone Park Foundation and Trout Unlimited. Of the $2 million, $1.5 million is allocated to suppression.

Eliminating lake trout is not going to happen overnight. “People expect results fast and we’re not going to get them fast,” Bigelow said.