Trophic Ecology of Nonnative Northern Pike and their Effect on Conservation of Native Westslope Cutthroat Trout

Abstract

Westslope Cutthroat Trout Oncorhynchus clarkii lewisi in Coeur d’Alene Lake, Idaho, have declined in recent years; 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 by using a bioenergetics modeling approach. Fish were sampled monthly from March 2012 to May 2013 using pulsed-DC electrofishing and experimental gillnetting in four bays. Northern Pike catch rates from electrofishing were generally low but increased slightly each season and were highest in the southern portion of the lake; catch rates from gillnetting were approximately 50% higher during the two spring sampling periods compared with the summer and fall. Seasonal growth and food habits of 695 Northern Pike (TL = 16.2–108.0 cm; weight = 24–9,628 g) were analyzed. Diets primarily consisted of kokanee O. 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. Total Westslope Cutthroat Trout biomass consumed by Northern Pike (2008–2011 year-classes) across all seasons sampled was estimated to be 1,231 kg (95% CI = 723–2,396 kg), and the total number consumed was 5,641 (95% CI = 3,311–10,979). The highest occurrence of Westslope Cutthroat Trout in Northern Pike diets was observed during spring. Thus, reducing Northern Pike predation on Westslope Cutthroat Trout would be one tool worth considering for conserving Westslope Cutthroat Trout populations in Coeur d’Alene Lake.

Received March 7, 2014; accepted September 22, 2014

ACKNOWLEDGMENTS

We thank C. Brown, M. Castaneda, W. Field, B. Harper, E. Hendrickson, J. Fredericks, D. Jolibois, I. Lee, N. Porter, J. Sanchez, T. Schill, C. Smith, J. Smith, M. Stanger, C. Watkins, S. Whitlock, and T. Wilson for assistance with field research. We also thank A. Vitale (Coeur d’Alene Tribe) and J. Fredricks (Idaho Department of Fish and Game) for assistance in planning and implementing this study. We are grateful to N. Bean (Colville Tribe) for insights on the physiological parameters he developed for bioenergetics modeling of larger Northern Pike. C. Muhlfeld, M. Wiest, F. Wilhelm, and two anonymous reviewers provided helpful comments on an earlier version of this manuscript. Funding for the project was provided by the Coeur d’Alene Tribe. Additional support was provided by the Idaho Cooperative Fish and Wildlife Research Unit. The unit is jointly sponsored by the U.S. Geological Survey, University of Idaho, Idaho Department of Fish and Game, Coeur d’Alene Tribe, and Wildlife Management Institute. This project was conducted under the University of Idaho's Institutional Animal Care and Use Committee Protocol 2011-43. The use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.