Northern pike are now traversing salt water to invade freshwater ecosystems in Southcentral Alaska, as indicated by a recent study. This marks the first documented case of northern pike journeying through estuaries—areas where river freshwater mingles with saltwater from the ocean—to establish new habitats in North America.
Northern pike are now traversing salt water to invade freshwater ecosystems in Southcentral Alaska, as indicated by a recent study.
A team from the University of Alaska Fairbanks and the Alaska Department of Fish and Game made this finding by sampling and analyzing small ear stones known as otoliths from northern pike captured in the area. This is the first documented evidence of northern pike using estuaries as routes to expand into new territories across North America.
This revelation sheds light on the ongoing spread of northern pike throughout Southcentral Alaska. Originally a native species to Interior and Western Alaska, northern pike were introduced unlawfully to the Susitna River basin in the 1950s. Since that time, this predatory fish has become established in over 150 lakes and rivers within the region.
Previously, it was believed that the expansion of northern pike was restricted to freshwater pathways or through human introductions.
“They are freshwater fish, and it was assumed that Cook Inlet acted as a marine barrier preventing their movement from one watershed to another,” explained Matthew Wooller, a professor at the UAF College of Fisheries and Ocean Sciences and the study’s lead author.
Wooller, who also oversees the Alaska Stable Isotope Facility at UAF, spearheaded efforts to track pike movements by examining otoliths collected by ADFG starting in 2019. The strontium isotopes found in otolith layers correlate with chemical signatures in various waterways, revealing where a fish has traveled throughout its life.
“Strontium levels differ based on geology and location,” Wooller noted. “If pike are transitioning between watersheds, we can identify that by analyzing the strontium in their otoliths.”
The research identified three pike from distinct locations whose isotopic signatures aligned with water from upper Cook Inlet, indicating they had been in the inlet at some point. These fish were caught in freshwater areas linked to Cook Inlet: Campbell Lake and Westchester Lagoon in Anchorage, and Vogel Lake on the Kenai Peninsula. This finding emphasizes the significant challenge of curbing the spread of northern pike in the region, suggesting that waterways connected to the ocean where pike have been eradicated could become reinfested.
As highly efficient predators, pike threaten native fish populations, including salmon, when they invade new areas.
The realization that these fish are utilizing estuaries “adds yet another reason to consider northern pike as a prime example of a powerful invasive species,” said Peter Westley, an associate professor of fisheries at UAF who has researched northern pike in both their native and introduced environments for over a decade.
While the findings are concerning, they also pave the way for more strategic actions against the invasive fish.
“Confirming that northern pike can navigate this path provides us with the information necessary to focus on preventing their spread and safeguarding valuable habitats,” remarked Parker Bradley, an ADFG biologist specializing in invasive species.
Kristine Dunker, who leads an ADFG initiative to manage invasive northern pike in Southcentral Alaska, stated, “The findings will aid in directing resources toward monitoring regions without pike that are at a higher risk of invasion.”
“This discovery marks a significant advancement in both our scientific understanding of northern pike ecology in North America and in our management strategies for invasive northern pike in our area,” Dunker added.
In addition to Wooller, Bradley, Dunker, and Westley, other contributors to the study included Karen Spaleta from UAF and Robert Massengill, who previously worked with ADFG.
