In the fall of 2018, officials with the Alaska Department of Fish and Game and their partners celebrated what they thought was a milestone: an end to the infestation of invasive northern pike in the Kenai Peninsula.
Their laborious program – they thought – had ridden the peninsula of the salmon-gobbling species that has wreaked havoc on the natural runs that are important to commercial and sport fishers, as well as to the overall ecological system.
“We were all excited, you know. We spent, like, 15 years eradicating them off the peninsula. it was like this big, monumental moment,” said Kristine Dunker, a biologist who coordinates the Alaska Department of Fish and Game’s program addressing invasive northern pike.
The celebration proved short-lived.
A week later, Fish and Game officials got a report from an angler who was fishing in a remote lake on the northern part of the peninsula. The angler, who went to a site in the Kenai National Wildlife Refuge called Vogel Lake, said he had caught a northern pike there. Fish and Game officials, armed with nets, came to Vogel Lake the following spring and confirmed that invasive pike were in the lake.
It was the first documented case in North America of invasive northern pike, considered an exclusively freshwater fish, swimming in an estuary, an area where freshwater and saltwater meet.
“I think everybody’s jaw dropped,” said Matthew Wooller, the University of Alaska Fairbanks scientist who is a lead author of a newly published study describing the discovery.
The Vogel Lake discovery was followed in 2022 by discoveries of invasive pike in two more estuaries: Westchester Lagoon and Campbell Lake in Anchorage, events also analyzed in the study, which is coauthored by Dunker and others.
Wooller, a professor at UAF’s College of Fisheries and Ocean Sciences and director of the university’s stable isotope laboratory, used chemical analysis to show that the pike that colonized Vogel Lake got there after swimming in the marine saltwater of Cook Inlet.
“They’re actually a freshwater fish, so to show evidence that they spent some time in an estuarine environment is very surprising. It’s not something they should want to be spending time doing.” Wooller said.
It is also worrying, he said.
“If they can kind of dip into the marine environment, go up another river, dip into marine environment, go up another river, that’s an additional way of invading,” he said.
Wooller’s isotope analysis tracked the chemical fingerprints the environment left on the otoliths, tiny ear bones that record fish life histories in layers.
He noted a particularly striking part of the study’s findings: the record imprinted on the otolith of a big female pike pulled from Vogel Lake, a fish that was possibly the first pike pioneer to the site. It clearly showed the chemical signs of a freshwater environment followed by a marine environment, followed by a freshwater environment. Small young pike in Vogel Lake that were presumably the female’s offspring did not have the saltwater marking on their tiny ear bones.
“It doesn’t take much to put two and two together,” he said.
Northern pike are native to much of the state, a fixture in freshwater systems north of the Alaska Range. Known for providing both tasty food and tough fights at the ends of fishing lines, pike are a favorite target among avid anglers. Wooller’s father is one; several years ago, he traveled from Britain to Alaska specifically to fish for northern pike and did so at the Minto Flats area of the Interior, “a total pike-fishing heaven,” said Wooller, who does not share his father’s passion.
But northern pike should not be in Southcentral Alaska, scientists say. Reasons for that go back to the end of the last Ice Age, when Southcentral Alaska retained its glacial coverage much longer than did more northern areas, said Wooller, who is an expert in ancient ecosystems.
The invasion started in the 1950s, when someone released pike from Minto Flats into remote Bulchitna Lake in what is now the Matanuska-Susitna Borough. After the initial illegal introduction, floodwaters spread the pike around, and effects cascaded.
Dunker said there are good reasons for the ban on moving fish between water bodies, which was illegal even before statehood. That is especially the case when it comes to northern pike, she said.
Pike are voracious predators, eating not just salmon, trout and other fish but occasionally rodents and birds, Dunker said. The evidence is seen when pike are captured. Their stomachs can be found stuffed with juvenile salmon and other prey.
“They’re top predators and they can completely take out the entire population of salmon. We’ve seen that time and time again. And trout as well. And in worst case scenarios they take out the sticklebacks too,” along with species that may not be targeted by anglers but are nonetheless ecologically important, Dunker said.
And while they are not native to Southcentral Alaska, the region contains plenty of pike-friendly habitat: shallow, slow-moving and plant-filled freshwater bodies, often packed with the kind of fish that pike eat.
The invasion in the Matanuska-Susitna region may be too far gone to reverse, particularly along the Susitna River drainage. But to the Department of Fish and Game, it is important to keep the Kenai Peninsula pike-free, as well as the western side of Cook Inlet. That is both to protect the region’s economically valuable natural runs of salmon and other fish and because there are numerous remote freshwater bodies where any pike infestation would be difficult to spot quickly and even more difficult to address.
The Vogel Lake discovery was particularly ominous because of the lake’s connection to a web of other remote freshwater systems, all of which could be susceptible to secondary invasions triggered by the Cook Inlet crossers.
That left managers “faced with the reality that if they’re widespread through the system, there may not be an option to eradicate anymore,” Dunker said.
While northern pike cannot survive for long in saltwater, Cook Inlet is evidently tolerable for short periods, according to the study findings. The inlet, especially in spring and summer when meltwater from snow and ice is flowing, is less salty than other marine waterbodies. And it is a short swim from the Matanuska-Susitna Borough, where pike are entrenched in numerous rivers and lakes, to Anchorage or the northern part of the Kenai Peninsula, where Vogel Lake is located.
Dunker noted that it is only about 20 miles from the Susitna River basin, an example of a heavily infested Matanuska-Susitna area, to the Kenai Peninsula point that holds Vogel Lake. Westchester Lagoon and Campbell Lake are even closer, she said.
“In a low-salinity situation, if it’s the right time of year and the tides are strong, it probably propels the fish along,” she said.
In retrospect, Dunker and Wooller said, the proof that pike had swum across the inlet should not have been too surprising. There are fishers’ tales of finding pike in their salmon nets in the inlet. And European colleagues at a 2016 conference warned Dunker that Cook Inlet, like the Baltic Sea, was probably already a passageway.
Getting rid of invasive pike is a multifaceted and sometimes difficult process. It starts with public education and awareness, which the Department of Fish and Game and other agencies conduct. State, federal and tribal agencies have at times encouraged Alaskans to catch all the invasive pike they can – with the proviso that none are returned alive to the water, as that is illegal.
Efforts beyond those are labor-intensive. Fish and Game officials and their partners have spent countless hours netting invasive pike in various water bodies, but that is considered an incomplete technique that misses a lot of fish. More drastic steps include the draining of lakes or other water bodies, which can be difficult logistically, or applications of a fish-killing chemical called rotenone.
https://pubchem.ncbi.nlm.nih.gov/compound/Rotenone
In Alaska, lake draining has been done only rarely, as it is usually impractical, Dunker said. The invasive species program has relied more on rotenone, though applications are carefully considered and planned, she said.
Rotenone is sometimes best employed in areas where the invasive pike have eaten through the natural fish supply. That can be seen in pikes’ stomachs.
“They’re kind of little swimming traps to tell us what’s in an environment,” Dunker said. “And when we only find water bugs like dragonfly larvae or other insects like that, that tells us how messed up that lake environment is,” she said.
Ultimately, rotenone was what was used in Vogel Lake. The project there, conducted in 2021, was especially complicated, as Dunker describes it.
Pike had been in the lake for only about a couple of years, so there was still a thriving population of coho, rainbow trout and other native fish that needed to be rescued, she said. Fish and Game workers corralled as much native fish as they could, scooped them up and helicoptered them to a new location – activity allowed only through special permitting, she noted. They also installed a weir to block any future pike colonization.
Beyond that, there is ongoing research to better understand how and when pike might be swimming across Cook Inlet. Dunker and her scientific colleagues are examining the limits of their saltwater tolerance, the time it takes for marine environments to be imprinted on otoliths and other questions. They will be monitoring footage from video cameras installed at Westchester Lagoon and Campbell Lake, among other sites. Additionally, there are rotenone applications planned this year for some road-accessible sites in the Matanuska-Susitna Borough that are intended to limit the further spread of pike there.
The successful eradication at Vogel Lake, with all its complexities, was a relief to the department. But this time around, celebrations are more muted.
“Right now, we can say that to our knowledge the Kenai Peninsula is pike-free. But we have to be very vigilant in our monitoring, and be ready,” Dunker said.