Predation by northern squawfish (Ptychocheilus oregonensis) has been documented to be significant on emigrating juvenile salmonids near juvenile bypass outfalls at hydroelectric dams on the Columbia River. Criteria for siting juvenile fish bypass outfalls to reduce predation were developed using locational data from radio-tagged northern squawfish in The Dalles Dam trailrace, Columbia River. Radio transmitters were surgically implanted in 164 northern squawfish in 1993 and 1994, and their movements and distribution were monitored. Position estimates of northern squawfish were compared with data from a physical hydraulic model of the dam to estimate water velocities where northern squawfish were located. Eighty-two percent of northern squawfish position estimates were in water velocities ≤110 cm/s in 1993 and ≤90 cm/s in 1994. Fish locations were usually associated with water depths ≤10 m (84% in 1993 and 82% in 1994); 90% were within 110 m of the shore or dam structure in 1993, and 86% were within 80 m in 1994. In a related study at John Day Dam, Columbia River, where the juvenile bypass outfall is located 40 m from shore, water depth is 10 m and water velocities typically exceed 75 cm/s, only 13 of 1443 (0.9%) contacts on radio-tagged northern squawfish were located within 200 m of the bypass outfall. We recommend that new or modified juvenile bypass outfalls on the Columbia River be located in water velocities of ≥100 cm/s, ≥75 m from the shore or dam structure, and in water ≥10 m deep.
