Leveraging PIT tag data to better understand Northern Pikeminnow movement in the Columbia River basin

ABSTRACT Objective Though native to the Columbia River basin, Northern Pikeminnow Ptychocheilus oregonensis have been the subject of long-term population control. This began in response to their increased predation of anadromous juvenile salmonids as a result of habitat modification due to hydropower dam construction. The Northern Pikeminnow Management Program (NPMP) is a targeted harvest program with a sport reward fishery that has been operating in the Columbia and lower Snake rivers since 1991. As a result of this sport reward fishery, over 5.7 million piscivorous (≥200 mm FL) Northern Pikeminnow have been harvested for cash rewards. Each year, data are collected to monitor and evaluate the program; however, the program has come under scrutiny in recent years due to analytical methods that have not changed since the program’s inception. Northern Pikeminnow have been PIT-tagged through the NPMP since 2003 to estimate the program’s annual exploitation rate, with a goal of 10–20%, which feeds into an estimate of the program’s annual reduction in predation. The model used to estimate the annual exploitation rate assumes closed populations separated by dams, an assumption that relies on limited evidence generated largely before PIT tag technology was deployed in the region. After being tagged, most Northern Pikeminnow are never recaptured, but PIT tag antennas passively track their movement. Methods We combined data from the Columbia basin PIT Tag Information System database with detailed NPMP tagging and harvest data to measure Northern Pikeminnow movement throughout the Columbia River basin over two decades. Results Movement records reveal that individuals traveled at least as far as 968 km. Nearly 13% of Northern Pikeminnow tagged through the NPMP were detected in different river sections, separated by dams, from where they were initially tagged. Movement in tributaries and across main-stem dams reflects seasonal patterns aligned with Northern Pikeminnow upstream spawning migration and out-migration of juvenile salmonids. Conclusions Our increased understanding of Northern Pikeminnow movement informs the calculation of the exploitation rate, one of the key measures of NPMP success. Further, as the NPMP evolves over time, information on Northern Pikeminnow movement may also contribute to future management strategies, like focusing removals on river reaches in which immigration outpaces emigration.