Invasive pike use marine corridors to colonize new Alaska territory

Researchers at the University of Alaska Fairbanks and the Alaska Department of Fish and Game made the discovery by collecting and analyzing tiny ear stones called otoliths from northern pike caught in the region. It's the first known documentation that northern pike are traveling through estuaries, where fresh water from rivers mixes with the ocean, to colonize new territory in North America.

The discovery offers new insights into the ongoing spread of northern pike throughout Southcentral Alaska. A native species in Interior and Western Alaska, northern pike were illegally introduced to the Susitna River basin in the 1950s. Since then, the predatory fish has become established in more than 150 lakes and rivers in the region.

Until now, the spread of northern pike was thought to be limited to freshwater corridors or illegal introductions by people.

"They're a freshwater fish, and it was thought that Cook Inlet represented a marine barrier stopping them from moving from watershed to watershed," said Matthew Wooller, a professor at the UAF College of Fisheries and Ocean Sciences and lead author of the paper.

Wooller, who is also director of the Alaska Stable Isotope Facility at UAF, led the team's efforts to reconstruct the movements of pike by analyzing otoliths collected by ADFG since 2019. The composition of strontium isotopes in the layers of an otolith can be matched with chemical signatures in various waterways, showing where a fish traveled during its life.

"Strontium varies according to geology and location," Wooller said. "If pike are moving between watersheds, you can pick it up by analyzing strontium in the otoliths."

The study found three pike from three separate locations with isotopic signatures matching upper Cook Inlet water, suggesting they had occupied the inlet at some point. Those fish were caught in freshwater habitats that connect to Cook Inlet: Campbell Lake and Westchester Lagoon, both in Anchorage, and Vogel Lake on the Kenai Peninsula. The discovery highlights the steep challenge of limiting the spread of northern pike in the region. It suggests that ocean-connected waterways where northern pike have been eradicated may become reinvaded.

As efficient predators, pike impact native fish species such as salmon when they invade new territory.

The newfound realization that the fish are moving through estuaries "is just one more reason that northern pike are a poster child of what makes a formidable invasive species," said Peter Westley, a UAF associate professor of fisheries who has studied northern pike in their native and introduced ranges for over a decade.

While concerning, the new research also could lead to more targeted action against the invasive fish.

"Confirming northern pike can use this pathway gave us the information we needed to now focus on preventing this spread and protecting valuable habitats," said Parker Bradley, an ADFG invasive species biologist.

Kristine Dunker, who coordinates an ADFG program to manage invasive northern pike in Southcentral Alaska, said "the findings will help direct resources toward monitoring areas without pike that are at the highest risk of invasion.

"This discovery has been a step forward, both scientifically with our understanding of northern pike ecology in North America and also for our invasive northern pike management here at home," Dunker said.

Along with Wooller, Bradley, Dunker and Westley, contributors to the paper included Karen Spaleta at UAF and Robert Massengill, formerly at ADFG.