Fish Community Response to Rapid Colonization and Subsequent Suppression of Northern Pike: A 15-year Study of Box Canyon Reservoir, Washington

The expansion of Northern Pike outside its native range into the western states of North America is a growing problem due to negative effects on fish communities. Illegally introduced Northern Pike were first detected in Box Canyon Reservoir (Pend Oreille River), in northeast Washington, in 2004. By 2010, the Northern Pike population was estimated at more than 10,000 individuals. Between 2012 to 2019, the Kalispel Tribe of Indians conducted gill net suppression, which reduced the Northern Pike population by 97.3% by number and 98% of their biomass by 2019. From 2004 to 2019, four standardized surveys were conducted to track changes in the Box Canyon Reservoir fish community. All but two species (Brown Trout and Smallmouth Bass) experienced significant declines in relative abundance during Northern Pike colonization and intensive suppression. There was a 55.5% reduction in native species biomass (primarily Northern Pikeminnow, Peamouth Chub, Largescale Sucker, Longnose Sucker) during the course of the 15 year study. Conversely, biomass of nonnative species (Largemouth Bass, Smallmouth Bass, Brown Bullhead, Black Crappie, Pumpkinseed Sunfish and Yellow Perch) increased 31% over the same period. While biomass of Northern Pike was reduced drastically by 2019, native species exhibited no signs of recovery. Most nonnative species declined from 2004-2011, during Northern Pike colonization, but rebounded during the response period following intensive Northern Pike suppression from 2014-2019. Brown Trout and Smallmouth Bass were present at low abundance in 2004, and continued to increase in abundance throughout the study. As an apex predator, Northern Pike had a significant impact on the fish community of Box Canyon Reservoir in a relatively short period of time (7 years) and likely acted as a catalyst to increase the rate of replacement of native species by nonnative species.