Northern pike illegally introduced into Idaho’s Coeur d’Alene Lake have become a popular sport fish but have wreaked havoc against westslope cutthroat populations. Pike may be taking a bite out of the lake’s struggling population of native westslope cutthroat trout. The specific impacts were measured in a study that looked at how many trout pike are eating and what the public can do to help protect native species.
Westslope cutthroat have historically been culturally important to the Coeur d’Alene Tribe, which in the mid-2000s launched an intensive study of the species in the lake and some of its tributaries where adult trout return to spawn. They found a very small percentage of the juvenile fish that hatch in the streams returned as adults. Compared to other similar systems, the rate of return was eight to 12 times lower.
One suspected cause was the lake’s non-native northern pike, which in spring congregate to spawn in the same shallow bays near mouths of tributaries where juvenile cutthroats exit their birth streams. The hypothesis: That time of overlapping habitats made the young trout especially vulnerable to hungry pike. A study published online in the journal North American Journal of Fisheries Management took a closer look at whether the overlapping habitats really harmed cutthroats or if the damage could be attributed to another variable.
Pike were captured through non-lethal gillnetting and electroshocking in four of Coeur d’Alene Lake’s bays. The crew measured and tagged the fish so they could be identified later. The tagging system allowed them to track fish growth and angler catch rates.
John Walrath, a regional fisheries biologist with the Wyoming Game and Fish Department and lead author of the study, shared that the researchers also flushed the pikes’ stomachs to get data on how much of which species they were eating. Walrath, who was with the Idaho Cooperative Fish and Wildlife Research Unit at the time of the research, said they could usually identify the partially digested remains down to the species. But when all they had were bones, it was tough to tell the difference between the cutthroats, rainbow trout and kokanee salmon that all swim in the lake.
While the pike were clearly eating things naturally found in the lake, their diet also appeared to be linked to human intervention. Along with yellow perch, bluegill and other fish species, Idaho salamanders turned up in a few pike stomachs. So did the occasional jig in fish that had broken off anglers after being hooked. And a few pike were “chock-full” of preserved Pacific herring that anglers had presumedly dumped from their bait supplies, Walrath said.
“It’s not something that they just found one here and one there,” he said. “They found a pile of them and gobbled them up.”
The researchers took all of their pike age, weight, growth, diet and temperature data and plugged it into a computer model, which produced a seasonal day-by-day breakdown of how much of each species the pike were eating. Walrath said they could convert that into an estimate of what size fish the pike were eating as well.
The results showed that pike consumed the most westslope cutthroat trout in spring but most frequently ate juvenile fish. According to Walrath, their new hypothesis is that juveniles leaving the tributaries don’t stop in the bays but quickly head out into the lake. And adults passing through the bays from the main lake head straight upstream to spawn and return to the lake in the same manner. However, juvenile cutthroat may be following the adults as they return to the bay and getting caught in between the lake and swimming upstream because they cannot yet spawn. Unfortunately, this hesitation leaves them vulnerable to being eaten by northern pike as they linger in their typical feeding grounds.
Even with an estimate of how many westslope cutthroat the northern pike are eating in these bays, it’s difficult to know how big the impact is on Coeur d’Alene Lake’s trout because the overall population size isn’t well understood. Nevertheless, it is clear that pike are eating cutthroat, and the study produced some information that could help alleviate the problem. They found that pike tended to stick to one particular area without moving around that lake, suggesting that pike could potentially be netted from bays near cutthroat streams and relocated to areas with less habitat overlap. Managers are considering that option now, Walrath said.
Even though pike are non-native to the lake, a plan that benefits cutthroat without eradicating the predators is important for Coeur d’Alene Lake. The pike population hasn’t exploded, and some still grow larger than 40 inches, making them one of the go-to targets for trophy anglers.
“The northern pike as a sport fishery are really important,” Walrath said. “But there are also a lot of people around there that remember when westslope cutthroat trout used to be quite numerous, and they would like to see that again.”
The IDFG continues to control the Northern Pike population through gillnetting and maintains a stable predator population. TheIDFG works closely with the Coeur D’Alene Tribe to protect native species. Since the state has begun monitoring and controlling pike populations, there has been an increase in cutthroat trout populations. The IDFG reports that “Nearly 500 adult cutthroat ascended Lake Creek in 2019, the largest spawning run observed in more than 20 years.”
The state and Coeur d’Alene Tribe are hopeful that these trends will continue.