Yellowstone National Park officials are hoping they’re getting the upper hand in the life-and-death battle. But the National Park Service doesn’t expect to entirely rid the lake of unwelcome lake trout.
Lake trout prey on cutthroats. There once were an estimated 4 million cutthroats in Yellowstone Lake and its tributaries, but that number probably is down to around 400,000 now — one-tenth of what it was.
Cutthroats have been here for the last 10,000 years and are crucial to Yellowstone’s ecosystem. Bears, osprey, eagles, otters and other animals feed on cutthroats that spawn in the spring. Lake trout stick to the deep lake water, where predators rarely catch them, said Todd Koel, supervisor for the Yellowstone fisheries program.
A weir on Clear Creek feeding the east side of the lake counted 70,000 to 80,000 spawning cutthroats per year in the past. That number is down to nearly zero now, said David Hallac, Yellowstone Center for Resources chief.
The weir began counting fish in 1945, but washed out in 2008.
A low-impact, high-technology structure is being built on Clear Creek this summer and is expected to begin counting trout next year, Hallac said.
Lake tout, on the other hand, have been flourishing.
Lake trout were introduced to the then fish-less Lewis Lake (about 10 miles southwest of Yellowstone Lake) in 1890. There has been official documentation of lake trout in Yellowstone Lake since 1994. Park Service employees began gill-netting lake trout in 1995.
An angler may have introduced lake trout to Yellowstone Lake, Koel said.
“There were likely 500,000 catchable lake trout in the lake this spring,” Koel said. “The hope is that we kill more this summer than will enter into the population by next year. When we get to the level of effort that this occurs ... we will eventually drive the population to its knees.”
Last year, around 224,000 lake trout were netted and killed.
There are indicators that cutthroats are recovering. Large numbers of small cutthroats are being caught in the gill nets, but nearly all can be released safely. And cutthroats were seen in spawning streams again. “It’s encouraging to see those signs,” Hallac said.
This year, no lake trout weighing more than 20 pounds have been caught — a fortunate indicator for cutthroats. A 20-pound lake trout can eat 50 cutthroats per year.
Lake trout also compete with native trout for freshwater crustacea, Koel said.
Although gill nets are employed to snag lake trout, cutthroats get caught too. But with care, cutthroats can be safely removed from the gnarly nets.
The service is intensifying efforts to eliminate lake trout. Those include attacking spawning beds and adding another boat next year.
“There will be a third very large contract gill-netting boat on the lake next year,” Koel said.
On Wednesday, Koel led a handful of reporters to a narrow brook just west of the Lake Clinic. Tall grass and trees shaded the stream, and like a big blue saucer, Yellowstone Lake could be seen a few hundred yards distant.
Gill nets snag fish by the gills. Most gill nets are set at a depth of 150 to 160 feet where lake trout swim, Koel said.
Trap nets, at a 30- to 50- foot depth, are deployed where cutthroats linger around lake trout. The fish are funneled into the trap unscathed. Male lake trout ensnare themselves on the outside of the trap because they are attracted to females within. “This is the peak of lake trout spawning right now,” Koel said.
Aboard a Hickey Brothers boat contracted to catch lake trout, a trap net is hauled up the side like a black steel mesh waterfall. The nets are brimming. Big seething lake trout slap the net like fly swatters smacking a table.
The net was cast two days ago. Hickey employee Andy Stuth estimated about 150 to 200 lake trout were caught in the net.
Stuth and his partner, Jake Junio, pitch the fish in containers nonchalantly like merchants at a fish market. Each lake trout will be measured and its sexual maturity recorded before being killed, Stuth said.
About two cutthroats die for every 100 lake trout caught in gill nets, Koel said.
If a significant number of lake trout can be removed, then a maintenance program can be adopted to keep lake trout numbers in check, Koel said.
Dr. Bob Gresswell of the U.S. Geological Survey Northern Rocky Mountain Science Center, hops aboard from a Park Service boat.
Gresswell heads up the effort to eliminate lake trout eggs, another major focus of the lake trout population control program. For four years, biologists have been hunting the means to augment gill nets. If they can pinpoint spawning sites, they can employ equipment to kill eggs.
To that end, radio tags were surgically implanted in the bellies of 220 lake trout. Fifty-two receivers on the lake triangulate the trouts’ journeys, Gresswell said.
Carrington Island, in the West Thumb area, resembles a cartoon strip’s rendition of a desert island. But instead of a lone palm tree, one pine defies the rocky islet, which couldn’t be more than 12 feet long. The island may be minuscule, but it’s a major spawning bed.
Carrington is one of two confirmed spawning areas. Of the 140 lake trout fitted with tags last year, 70 cruised the island.
“Judging from last year, that’s where most of the action is right now,” Gresswell said.
Killing eggs can decrease gill-netting if it becomes less economical due to fewer fish being netted, Gresswell said.
Electricity is the most promising egg-eliminating method on Yellowstone Lake because fish biologists have experience working with electricity. A rod could be driven into the lake bed. An electrical charge could be applied to the sediment, and the rods would conduct the electricity into the sediment or substrate where the eggs are, Gresswell said.
Tentatively, Gresswell wants to try it next year.
“The Park Service hasn’t agreed to it yet,” Gresswell said.
Aboard a service boat, a big spool hauled in a gill net where employees and volunteers untangled, then examined lake trout before killing and tossing the fish over the side.
Heather Paddock, a recent graduate of the University of California, Santa Barbara, deftly unsnarled a big lake trout entwined in the net, while Pat Bigelow, Yellowstone fishery biologist, shows a radio transmitter. Lately, they’ve been attaching little wires on the radio fish to make it easier to spot them.
“We’ve had one (transmitter) fish three times in three different trap nets,” Bigelow said.
All lake trout caught by anglers in Yellowstone lake are to be killed. If they catch fish with transmitters in their bellies, anglers are asked to return the transmitters to Park Service personnel.
The service has committed $1 million from service funding and another $1 million from the Yellowstone Park Foundation to its cutthroat trout restoration program this year. The East Yellowstone Chapter of Trout Unlimited, National Parks Conservation Association and Greater Yellowstone Coalition are helping, too. “All of these groups have really stepped up to the plate,” Hallac said.