Walrath, J. D., M. C. Quist, and J. A. Firehammer. 2015. Trophic ecology of nonnative northern pike and their effect on conservation of native westslope cutthroat trout. North American Journal of Fisheries Management 35:158-177.
Westslope Cutthroat Trout Oncorhynchus clarkii lewisi in Coeur d’Alene Lake, Idaho have declined in recent years and predation by Northern Pike Esox lucius, a nonnative sport fish, is thought to be a causative mechanism. The goal of this study was to describe the seasonal food habits of Northern Pike and determine their influence on Westslope Cutthroat Trout in Coeur d’Alene Lake using a bioenergetics modeling approach. Fish were sampled monthly from March 2012 to May 2013 in four bays using pulsed-DC electrofishing and experimental gill nets. Electrofishing catch rates for Northern Pike were generally low, but increased slightly each season and were highest in the southern portion of the lake. Northern Pike catch rates using gill nets were approximately 50% higher during the two spring sampling periods compared to the summer and fall. Seasonal growth and food habits were analyzed from 695 Northern Pike varying from 162 to 1,080 mm in total length and 24 to 9,628 g in weight. The diet of Northern Pike primarily consisted of Kokanee Oncorhynchus nerka, Westslope Cutthroat Trout, and Yellow Perch Perca flavescens. Results of a bioenergetics model estimated that Westslope Cutthroat Trout represented approximately 2-30% of the biomass consumed by age 1-4 Northern Pike. The highest occurrence of Westslope Cutthroat Trout in Northern Pike diets occurred during spring. Thus, reducing predation of Westslope Cutthroat Trout by Northern Pike might be a useful tool for conserving Westslope Cutthroat Trout populations in Coeur d’Alene Lake.