Balancing non-native trophy pike with struggling westslope cutthroat trout in Coeur d’Alene Lake

By on February 25, 2015

Northern pike illegally introduced into Idaho’s Coeur d’Alene Lake have become a popular sport fishery for the big, toothy predators. But pike may also be taking a bite out of the lake’s struggling population of native westslope cutthroat trout. A recent study of how many trout the pike are eating and when could help turn the tide.

The westslope cutthroat have long 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 were returning as adults. The rate of return was eight to 12 times lower than in comparable systems.

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 this month in the journal North American Journal of Fisheries Management took a closer look at whether that was the case.

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, which allowed them to track growth of individual fish and angler harvest.

A non-native northern pike is processed by a research crew on Coeur d’Alene Lake. (Courtesy John Walrath)

A non-native northern pike is processed by a research crew on Coeur d’Alene Lake. (Courtesy John Walrath)

They also flushed the pikes’ stomachs to get data on how much of which species they were eating, according to lead author John Walrath, a regional fisheries biologist with the Wyoming Game and Fish Department. 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.

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 apparently 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 researchers tagged northern pike to track growth, movement and angler pressure. (Courtesy John Walrath)

The researchers tagged northern pike to track growth, movement and angler pressure. (Courtesy John Walrath)

The results showed that pike were consuming the most westslope cutthroat trout in spring, but were mostly frequently eating sub-adults. 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. But the sub-adults may be following the adults as far as the bays, but don’t continue upstream because aren’t yet mature enough to spawn. That leaves them to linger in the pike’s 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. But it is clear that pike are eating cutthroat, and the study did produce 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 in 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.”

 

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