Lionfish are just one of the predatory invasive fish species the National Park Service is battling/USGS
Invasive Fish in National Parks
Voracious, deadly invaders ply national park waters
By Rita Beamish
Aquatic ecologist Danny Boiano loves fish, grew up fishing, studied fish in Yosemite National Park for his master’s degree. So is it odd that today Boiano’s National Park Service job centers around eradicating entire fish populations?
Not when the fish are among the invasive species that infest national park waters. In Sequoia and Kings Canyon national parks, where Boiano works, non-native trout have upended the natural ecosystem, notably devouring mountain yellow-legged frogs and their young, contributing to the frog’s listing as an endangered species.
Boiano wants to return some glacier-carved lakes in California’s High Sierra to their natural state: fishless. That’s how they all were before humans introduced trout for anglers, as far back as 1870, when frogs were an abundant, key component of a vibrant mountain habitat.
Non-native trout have feasted on mountain yellow-legged frogs in Sequoia and Kings Canyon national parks/NPS file
To restore the natural balance, Boiano and colleagues, like peers across the park system, have to get rid of the harmful intruders.
The tally in Sequoia/ Kings Canyon so far: more than 90,000 trout removed, mainly using long gill nets, since 2001; 16 lakes fully cleared of fish -- among the 37 where Boiano and colleagues have initiated the effort. That’s out of 575 once-fishless lakes in these parks.
“On the landscape scale, it’s a small dent in the problem,” Boiano said. “But where we worked – it’s a huge difference where we were able to fully eradicate the fish.” In those lakes, the natural food web thrives again.
This work is but one finger in a Park Service dike seeking to hold back invasive swimmers that eat and outcompete native species for food, flourish without indigenous predators, and reproduce to the tune of thousands, even more than a million, eggs per year.
Non-native fish are pervasive, either introduced by people, escaped from ponds, or arriving via altered waterways, officials say. But no full inventory exists across the more than 240 national park units that have fishing. Suffice to say it’s a big problem, not just for parks but for the nation, said John Wullschleger, Park Service fish program lead in the Natural Resource Stewardship and Science Directorate’s water resources division.
“There’s an extremely high likelihood that every park with water probably has non-native fish,” he told the Traveler. “We’ve done a lot of damage to our native aquatic fish species by introducing non-native fish. And it’s expensive to document it just to find out what’s going on.”
By nature, it’s a complex issue: “The tailwaters of large dams on the Colorado River are cold and poorly suited for native fish, but can support trout and quality recreational fishing,” Wullschleger said. “However, those same trout become a problem when they move downstream and compete with or predate on native fish,” in the Grand Canyon and elsewhere.
Among the many species plaguing parks and their environs:
- The colorfully striped but venomous lionfish, an aquarium species from Indo-Pacific waters, was introduced to southeast U.S waters in the 1980s. A 2012 Park Service report warned lionfish “could cause irreversible changes” in coral reef habitats already stressed by global warming, fishing and pollution in Biscayne National Park and elsewhere. Lionfish corral native reef fish with their pectoral fins and inject venom with their sting. “Their rapid expansion threatens the very resources and values that parks were established to protect, and diminishes the quality of visitor experience,” the report said.
- Asian carp in the Mississippi River system can become veritable goliaths of up to 100 pounds, though most are much smaller. Thriving in polluted waters and able to hurdle small dams, they are known to jump into electrofishing boats and whack crew members monitoring them, the U.S. Geological Survey, reported as part of a Mississippi River project. Imported from Europe in the 1800s and later used to control nuisance vegetation in canals and elsewhere, the hardy bottom feeders have made their way to the Mississippi National River and Recreation Area, creeping north while feasting on plankton and the likes of snails.
- The dagger-toothed northern snakehead is so creepy in its land mobility that some call it Frankenfish. It’s believed to be in early invasion stages at Delaware Water Gap NRA, Upper Delaware Scenic and Recreational River, Rock Creek Park and potentially other parks. On land, “If it’s not too steep or not too far, they may be able to go from one water body to another,“ said Don Hamilton, chief of natural resources on the Upper Delaware. Comfortable in a variety of habitats, it scarfs up everything from amphibians to aquatic insects, depleting food for native fish.
- The parasitic sea lamprey is one of the worst non-native fish in the Great Lakes. Migrating from the ocean via manmade canals and locks, the sea lamprey threatens the lakes’ national parks – some of which have native lamprey -- and vast fishing industry. They prey on whitefish, salmon, lake trout and other prey so destructively that the United States and Canada created a cross-border compact in the 1950s to collaborate on control measures.
- Lake trout native to the Great Lakes, Canada, and parts of Montana and other introduced trout species are found in North Cascades, Glacier, Yosemite, Grand Canyon, Glen Canyon, Rocky Mountain, Sequoia and Kings Canyon, and Yellowstone. “There was just a lot of fish stocking” through much of the 20th century, said Boiano. “Fish stocking was accepted.”
That started changing, he noted, after research in the latter 20th century revealed non-native fish were drastically altering the food web. Osprey and eagle populations have nosedived around Yellowstone Lake where lake trout outcompete them for food. By 2010, the lake had maybe one osprey nest, down from 40 to 60 nesting pairs a decade earlier, said Todd Koel, leader of the park’s native fish conservation program.
Invaders Hungry And Lethal
One consistent descriptor characterizes invasive fish: voracious.
Non-native trout “are only limited by the size of their mouth” as to what they eat; even small ones can polish off tadpoles and little frogs, Boiano said.
Various species of Asian carp, such as this Bighead carp, threaten native fisheries in the Mississippi River and potentially the Great Lakes/USGS
“Common carp will eat nearly anything, from vegetation and detritus to aquatic insect larvae and crawfish,” the Park Service states.
“There’s no end to the species they will eat,” Vanessa McDonough, Biscayne National Park supervisory wildlife biologist, said of lionfish.
All of that appetite reduces biodiversity and diminishes food to sustain natives, with cascading food-web impacts on larger mammals, birds and snakes that feed on native prey.
Fighting The Invaders
Parks, states, and federal agencies “are expending millions of dollars, time and effort controlling and/or mitigating the effects on non-native fish in and around parks across the country,” Wullschleger said. “In many parks those efforts have been going on for decades and are likely to continue into the foreseeable future.”
Efforts vary from aggressive gill netting, to chemical treatments, to study and monitoring modes where options are limited or unclear.
In the Great Lakes, a picicide is used against sea lamprey.
To keep Asian carp from invading the Great Lakes, agencies have been developing a range of barriers and control techniques, including a Wisconsin and Minnesota project on the Mississippi using electric currents and underwater speakers to herd them near La Crosse, Wisc. The Park Service asks boaters to trailer around, instead of opening, Mississippi locks that can let carp get through above Twin Falls.
To deal with northern snakeheads on the Delaware, “we don’t have a lot of good options,” said Hamilton. “We’re in a monitoring mode right now, trying to understand how widespread it might be. We’re telling anglers to kill any they catch and report them to us and not move them around.”
Delaware Water Gap and Middle Delaware National Scenic River have reports of snakeheads, said resource management chief Kara Deutsch, “but we don’t have information on how many.” Park officials are conferring with Pennsylvania agencies. “We have not had the ability to take management actions for this species and would need to work collaboratively with state biologists to perform electro-fishing or other control measures if deemed appropriate,” Deutsch said.
Northern snakeheads are another one of the predatory invasive fish species the National Park Service is battling/USGS
Lionfish Meet “Spear And Kill”
In Biscayne, lionfish face a “spear and kill” denoument: More than 10,700 have been vanquished by spear-wielding staff and volunteer divers since 2010, said McDonough. Divers target reef and shipwreck hangouts where the lionfish dine on native fish, including those that keep algae from overgrowing the reefs.
“Nothing proved to be as efficient as just putting divers in the water,” compared to trapping and other methods, McDonough said.
Lionfish range throughout Caribbean and Gulf waters, making full eradication unlikely. Their numbers declined after 2017’s Hurricane Irma, McDonough said, adding, “We have a pretty good hold on them. We are just maintaining.”
Assault On Lake Trout
To take down non-native trout, Western parks like Sequoia and Kings Canyon string gill nets across lakes, and use electrofishing to stun them.
The nets catch about 300,000 a year in a major removal effort at Yellowstone Lake, where lake trout have been gobbling up the smaller cutthroat trout, a native species that is threatened and an important food source for grizzlies, birds, and other wildlife. The $2.8 million-a-year suppression program has reduced lake trout by 80 percent since 2012, Koel said.
Yellowstone National Park's iconic Yellowstone cutthroat trout have been under attack from nonnative lake trout/NPS, Jay Fleming
“We are winning this war,” Koel said. “It’s not something that is going to be an abrupt crash in lake trout and a fast recovery of the cutthroat and the other animals that depend on the cutthroat. … It’s more of a long-term sustained pressure decline.” Any reduction in gill-netting, though, “could set us back by years,” he added.
At Glacier National Park, gill-netting targets invasive lake and brook trout that have caused “substantial declines” in native bull trout in west-side lakes, said Chris Downs, the park's aquatic and physical science programs leader.
“We are seeing positive signs in our gill-netting efforts, but it requires a sustained long-term effort, possibly over decades, to be effective,” Downs said. Tens of thousands have been removed from Quartz and Logging lakes since 2009, but resources are scant to tackle all compromised lakes, he said. The park thus is moving bull trout to isolated lakes upstream of waterfalls that invaders can’t surmount, and also constructing fish barriers to block them.
“Eradication is the goal, but long-term suppression may be the reality of where we are,” Downs said. “They may always be there, but we keep them at a low enough level that they don’t take over.”
Sustaining A Long Effort
More than three decades after stocking was discontinued at Sequoia and Kings Canyon, as well as neighboring Yosemite, the hybrid rainbow-golden trout, eastern brook trout, and some brown trout remain in the high country, altering lake basins that feed outlets to water California agriculture and quench urban thirst.
Slurping up large-bodied zooplankton and invertebrates like mayflies, the fish have reduced energy in the food web. Devouring tadpoles as well as the yellow-legged frogs’ insect-and-flies diet, they gave the frogs scant chance when a deadly amphibian fungus blasted into the Sierra Nevada starting in the 1970s. In their historic range, more than 92 percent of frog populations have disappeared, park officials say.
Boiano now estimates that by 2050, trout removal in 110 lakes will be completed. It’s a long road, but he’s seen the “National Geographic” scene at fish-free lakes: Frogs are repopulated, providing food for natural predators like snakes, weasels and water shrews. Clouds of insects buzz above the water. Tadpoles wriggle by the thousands. Birds and bats swoop. The environment teems with interconnected life.
“We can’t eradicate the non-native fish from the entire high country,” Boiano said. But, “(I)f we remove enough populations, we can have a bunch of lakes that are fishless, that can allow for the natural dynamic to play out.”
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