Novel fishway entrance modifications for Pacific lamprey

Abstract Passage facilities at dams must accommodate a broad array of aquatic species to achieve full river connectivity. In an attempt to improve adult Pacific lamprey (Entosphenus tridentatus) passage, fishway entrance modifications were made at a large dam on the Columbia River in northwestern USA. The modifications consisted of a variable-width entrance weir and flow disrupters to create heterogeneity in water velocities near the bottom, without affecting attraction flows for adult salmonids (Oncorhynchus spp.) or alosids (Alosa sapidissima). Additionally, a lamprey passage structure (LPS) was installed to provide a lamprey-specific route from tailrace to forebay elevation (31 m). Passive integrated transponders and radio transmitters were used to monitor both Pacific lamprey and Chinook salmon (O. tshawytscha) movements. Fish use of the modified entrance was compared to that at a similar, but unmodified entrance before and after modifications. The entrance modifications resulted in increased velocity heterogeneity, but no measurable improvement in entrance efficiencies for lamprey or salmon. Lamprey successfully ascended the LPS, with annual counts ranging from 48 in 2010 to 3,851 in 2016 (0.2–8.9% of lamprey counted at the dam). Radiotelemetry indicated that tagged lamprey resumed upstream passage after LPS use and travelled at rates similar to those that used traditional fishway routes.