Northern pike are sneaking through the ocean to invade new lakes

Alaska’s northern pike, long thought to be confined to freshwater, are now crossing saltwater to invade new areas in Southcentral Alaska.

Scientists previously believed that marine barriers, like the Cook Inlet, prevented their spread. However, new research shows that pike are using estuaries to move between watersheds.

This discovery is significant because it means northern pike can reinvade lakes and rivers where they were previously removed. It also challenges long-held beliefs about how these invasive fish spread, forcing scientists to rethink their strategies for controlling their expansion.

The finding highlights the urgent need for new measures to protect native fish species, like salmon, from the growing threat of northern pike.

Northern pike are traveling

Researchers from the University of Alaska Fairbanks (UAF) and the Alaska Department of Fish and Game (ADFG) made this finding by analyzing otoliths – tiny ear stones from pike.

These structures record the chemical composition of the water where a fish has lived, offering a timeline of its movements.

“Strontium varies according to geology and location,” said Matthew Wooller, a UAF fisheries professor and the study’s lead author. “If pike are moving between watersheds, you can pick it up by analyzing strontium in the otoliths.”

By comparing otolith samples collected since 2019, the team found three pike with chemical signatures matching upper Cook Inlet. These fish were caught in freshwater habitats connected to the inlet, including Campbell Lake, Westchester Lagoon, and Vogel Lake.

Formidable invasive species

Northern pike naturally belong in Interior and Western Alaska, where they have lived for thousands of years. However, in the 1950s, people illegally introduced them to the Susitna River basin in Southcentral Alaska.

Since then, these highly efficient predators have spread rapidly, establishing themselves in more than 150 lakes and rivers. Their presence has had a major impact on native fish populations, particularly salmon, which they hunt and eat.

For a long time, scientists believed that northern pike could only spread through connected freshwater systems or by human introduction.

Many thought Cook Inlet – a large body of saltwater separating different watersheds – would act as a natural barrier, preventing the fish from moving between river systems.

However, the latest study has proven otherwise, showing that pike are capable of traveling through estuaries, where freshwater and saltwater mix, to invade new areas. This challenges previous assumptions and makes controlling their spread even more difficult.

Northern pike use estuaries to spread

This new information raises concerns that waterways previously cleared of pike could be reinvaded through ocean-connected routes.

“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 fisheries associate professor.

Implications for management

Although the discovery is concerning, it provides scientists and resource managers with a clearer understanding of how northern pike spread.

With this knowledge, they can develop more targeted strategies to monitor high-risk areas, block migration pathways, and prevent re-invasion.

This focused approach will help protect native fish species and limit the long-term impact of this invasive predator on Alaska’s freshwater ecosystems.

“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.

Significance of the study

Kristine Dunker, who coordinates ADFG’s invasive pike management program, emphasized the importance of directing resources to high-risk areas.

“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.

The fact that pike can use estuaries to travel between watersheds means they are far more adaptable than scientists previously believed.

This creates new challenges for resource managers, who must now account for saltwater pathways in their efforts to contain and remove pike from vulnerable freshwater systems.

If left unchecked, these invasive fish could continue to spread and threaten native species, like salmon, making effective management strategies more urgent than ever.

The study is published in the journal PLOS ONE.

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