Yellowstone’s Supporting Cast: Cutthroat Trout

EDITOR’S NOTE: This article is part of a MoJo series that goes beyond bison, bears and wolves to spotlight Yellowstone’s often-overlooked wildlife.

A battle is raging beneath the surface of Yellowstone Lake. In one corner are native cutthroat trout; in the other are nonnative lake trout. On their own, cutthroats — their name not withstanding — are underdogs. But the National Park Service is on their side, leading a robust conservation program that’s helping the home team make a remarkable recovery.

Cutthroat trout are one of a dozen fish species native to Yellowstone National Park’s waters, including Yellowstone Lake — the largest high-elevation lake in North America. For ages, cutthroats ruled the lake’s 137 square miles, their population once topping 3.5 million. But that dominance ended in the mid-1990s, when nonnative lake trout mysteriously arrived and quickly devastated cutthroat numbers.

Knowing the damage lake trout had caused elsewhere, NPS staff moved quickly to protect the cutthroats — an angler favorite and a crucial food source for other species, including grizzly bears. Through intensive lake trout suppression efforts, the native fish are now rebounding to numbers not seen in decades, said Todd Koel, leader of the park’s Native Fish Conservation Program.

Threatening an icon

Cutthroat trout are striking fish with long, slender bodies shaded in yellow, brown, gray, and green. Their backs and tails are peppered with variable black spots, and a vivid red slash at the jawline gives the species its distinctive name.

Some cutthroats spend their entire lives in a single waterway, while others — like those in Yellowstone Lake — migrate to tributaries to spawn, NPS notes. A single female can lay a few hundred to several thousand eggs, though only a small fraction survive.

In these shallow streams, adult cutthroat trout are easy prey for bears, eagles and otters. Nearly 20 bird and mammal species feed on the nutrient-rich fish, making them a cornerstone of Yellowstone’s ecosystem. But in recent years, that balance has been under threat.

In 1994, fishermen discovered nonnative lake trout in Yellowstone Lake. Park officials initially suspected an intentional release and even hung wanted posters offering a $10,000 reward for information, Koel said. Their concern was rooted in experience.

Cutthroat trout are one of a dozen fish species native to Yellowstone National Park’s waters, including Yellowstone Lake — the largest high-elevation lake in North America. For ages, cutthroats ruled the lake’s 137 square miles, their population once topping 3.5 million.

From the late 1800s until the 1930s, park managers stocked fishless waters across Yellowstone with nonnative fish — including lake trout — for recreational purposes. By the time the practice ended in the 1950s, more than 300 million nonnative fish had been introduced into the park, though Yellowstone Lake was not among the release sites.

How lake trout arrived remains a mystery. Someone could have transported them from another waterway, or they may have swum through connecting streams and tributaries, Koel said. Either way, their arrival spelled trouble for cutthroats — and every species that depends on them.

The much larger lake trout prey heavily on cutthroats. A single adult can consume more than 40 cutthroats a year, quickly ravaging the native population, according to Yellowstone Forever, the park’s nonprofit educational and funding partner. But that’s not their only advantage.

Preferring cooler temperatures, lake trout thrive far below the lake’s surface, out of reach of bears, birds and other predators. They also live longer — 25-30 years on average — and reproduce at a much higher rate, says the U.S. Fish and Wildlife Service, with a single female laying up to 20,000 eggs.

Turning the tide

Without intervention, lake trout would easily overtake cutthroats — and nearly did. By 2012, despite early mitigation efforts, the lake trout population in Yellowstone Lake had swelled to 800,000, and more than 90 percent of the cutthroats were gone. As their numbers plummeted, the species that relied on them were forced to look elsewhere for food.

That’s when Yellowstone Forever pledged $1 million annually to help save Yellowstone Lake’s native trout. Combined with NPS funding, the support gave Koel’s team the resources to expand a two-part suppression program — gillnetting and egg destruction. 

Each season, from mid-May through mid-October, about a dozen contracted gillnetters deploy thousands of miles of nets across the lake’s deep waters, removing and killing hundreds of thousands of lake trout each year. Since 1995, they’ve culled nearly 5 million nonnatives from the lake, NPS reports.

In the fall, another team of biologists and fishermen focuses on lake trout egg destruction. From helicopters, they airdrop tens of thousands of pounds of soy-based pellets over spawning sites. The pellets quickly sink and adhere to rocks, depleting oxygen as they decompose — a process that has reduced mature lake trout by nearly 80 percent since 2012, according to Yellowstone Forever.

Recent models confirm the program is working, and cutthroat populations have rebounded to pre-lake trout numbers. While the fight may never fully be over, for now, the natives are ahead. “We’ve rebuilt what people tell me is one of the best — if not the best — trout fly fisheries,” Koel said. “We’re winning this battle.”