Northern pike are native to Interior and Western Alaska, but in the 1950s, they were illegally introduced into the Susitna River basin in Southcentral Alaska. Since then, the fish have established themselves in over 150 lakes and rivers in the region.
The spread of northern pike (esox lucius) 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 [a body of water linking fresh and saltwater in Southcentral Alaska] represented a marine barrier stopping them from moving from watershed to watershed," says Matthew Wooller, a professor at the University of Alaska Fairbanks (UAF) College of Fisheries and Ocean Sciences.
However, a new study – led by Wooller – now challenges that assumption, revealing that pike can indeed travel through coastal regions to reach new freshwater habitats.
Evidence of this came through the analysis of a chemical element (known as strontium) in small ear stones (otoliths) found in the fish's inner ear.
"Strontium varies according to geology and location," explains Wooller. "If pike are moving between watersheds, you can pick it up by analysing strontium in the otoliths."
By studying the otoliths from specimens caught in freshwater bodies, the team was able to trace the movements of these fish and confirm their passage through saltwater – it revealed that some pike had occupied waters connected to the Cook Inlet.
Why does this matter?
The findings underscore the difficulty of controlling the spread of northern pike, says Wooller, and highlights the risk that previously eradicated populations may return.
Northern pike are known to be highly effective predators, and their presence in new habitats can have significant negative impacts on native species, particularly salmon.
While the authors say the discovery is concerning, it also provides valuable information on both scientific understanding of northern pike’s ecology and practical management of their spread in Alaska.
Parker Bradley, an Alaska Department of Fish and Game (ADFG) invasive species biologist, noted that the findings will help focus efforts to prevent further spread and protect vulnerable habitats. Kristine Dunker, who leads ADFG’s invasive northern pike management program, adds that the discovery will help direct resources towards monitoring areas most at risk of invasion.
Find out more about the study: Estuarine dispersal of an invasive Holarctic predator (Esox lucius) confirmed in North America
Main image: the Turnagain Arm, one of two narrow branches at the north end of the Cook Inlet/Getty
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