Ten years after netting operations began, fisheries biologists are finally reaching kill levels necessary to effectively suppress Yellowstone Lake’s lake trout population.

The netting, which is expected to remove 300,000 fish this year, is an integral part of the effort to restore populations of native cutthroat trout. About 500,000 catchable-sized lake trout, invasive and illegally introduced in the early 1990s, are believed to swarm the waters of the 139-square-mile high-elevation lake.

Boats netted and killed around 224,000 fish in 2011. But the total kill in the entire decade before was just 500,000.

“We want to get to where we’re removing more than half of the fish each year,” said Todd Koel, Yellowstone National Park’s fisheries supervisor.

That level of netting “will eventually drive the population to its knees,” Koel said.

Last week, Koel and Dave Hallac, chief of the Yellowstone Center for Resources, revealed the park’s lake trout operations up close.

The biologists started off at a diminutive two-foot wide creek that fed into the big lake. Historically, 60 such streams were destinations for cutthroat in the spring, when they run to spawn.

“We counted 8,000 cutthroat in a similar stream 12 years ago,” Koel said. “The other year it was just one.

“The ecological side of it can be devastating,” he said.

Because they run upriver and make themselves available to predators, cutthroat were once an important food source for grizzly bears, bald eagles, ospreys and river otters. Lake trout, conversely, spawn in the lakes and oftentimes in deep water — they’ve been documented at 100 feet down.

“The lake trout impact has been holistic — it’s been system-wide,” Koel said.

A large lake trout, Koel said, will devour 50 of the smaller cutthroat in a year.

Using Park Service and donated funds, Yellowstone has commissioned both gill netting and trap netting to kill one trout species in hopes of bringing back another.

Although the trap nets are responsible for just 7 percent of the total kill, they keep fish alive and allow contracted commercial fisherman to operate in waters where lake trout and cutthroat coexist. The majority of trap nets are set in Yellowstone Lake’s West Thumb.

“Today you’re getting a real look at the heroic efforts society is willing to take to restore an ecosystem,” Hallac said en route to a trap netting boat in the Thumb.

The biologist’s boat stopped when it reached the vessel of the Hickey Brothers, contracted commercial fisherman out of Door County, Wis. They usually net in Lake Michigan for whitefish and chubs.

Andy Stuth and Jake Junio, the fisherman aboard, were moving toward a trap net off of the single-treed Carrington Island.

Based on fish movement estimates, about a quarter million lake trout head to waters near the island each fall to spawn.

“Not bad for two nights,” Stuth said, pulling in a net that contained perhaps 150 to 200 fish including several cutthroat.

After the biologist’s boat departed, the fisherman would sex and measure the lake trout before slitting the swim bladders and tossing them back to rot.

Six fisherman, including volunteers and park employees, were working the National Park Service’s gill netting boat, the Freedom, last week. The Freedom crew was mechanically hoisting a monofilament gill net six feet tall and 300 feet long when Koel and Hallac’s boat pulled up.

The gill nets, set in clusters of three to six nets off of humps on the lake bottom, are tended seven days a week, said Pat Bigelow, a Park Service fisheries biologist. Twenty-five “gangs” of the nets were set around the lake last week.

Most of the fish tangled within the Freedom’s gill net were dead.

Bigelow said that the cutthroat catch was up this year, which is something of a mixed blessing.

“Most years, it’s about four cutthroat per 100 lake trout,” she said. “This year, it’s double that.”

Approximately half the cutthroat can be released and end up surviving, Bigelow said.

One mid-sized laker the Freedom pulled up last Wednesday afternoon had a red tag extending out if its back. The marker indicated the fish was equipped with transmitter, embedded by a surgeon, that’s used to tracked its movements.

Some 220 fish are currently affixed with the $400 transmitters.

“We don’t catch them very often — not every day,” Bigelow said. “We had one fish that we caught three different times.”

Because Yellowstone Lake is so big, the lake trout will never be eradicated, Koel said.

Rather, in the initial phase, Yellowstone plans to aggressively net the waters for the next five to six years and then transition to a lower-cost alternative treatment.

“We hope we can get to a maintenance level where we can spend less money and less effort,” Hallac said.

Yellowstone spent $2 million this year on cutthroat restoration initiatives and has $2.3 million budgeted for fiscal year 2013. That budget pays for six full-time and 14 seasonal employees working under Koel and also the contracted fishing boats.

“We hope we can get to a maintenance level where we can spend less money and less effort,” Hallac said.

A lower-cost treatment could harness data from the tagged fish, colloquially called “Judas fish.”

Data from the transmitters will be used to map lake trout spawning beds around Yellowstone Lake, said U.S. Geological Survey research biologist Robert Gresswell.

Fifty-two acoustic receivers distributed around the lake triangulate the location of each tagged fish, said Gresswell, who leads the effort to map lake trout movements.

“If a fish moves north, we know it,” Gresswell said, “if a fish moves west, we know it.”

Greswell, aboard the Hickey Brothers fishing boat, said he had been amassing fine-scale data for four years.

Treatment options at the spawning beds, believed to number about a dozen, are still up for discussion and have to be approved by the Park Service, Gresswell said.

“Right now, it looks like perhaps electricity is probably the most promising,” he said. “Really, what we’re talking about is something that can affect the embryos in the substrate.

“CO2 also works, but how we apply that is a big issue,” Gresswell added.

Fish poisons such as rotenone are also a possibility, he said.

Gresswell, known regionally as an expert on cutthroat, said that spawning bed treatments — although relatively unproven — hold promise to maintain Yellowstone Lake’s lake trout population at a low level.

“We’re in the process of hiring a full-time person to process the data,” Gresswell said. “We had over nine million detections over the winter, and those data have not hardly been analyzed at all.

“By winter, we will put visuals of where fish are moving online,” he said.

Next year, tentatively, the data will be put to use, Gresswell said.