Grating Size Needed to Protect Adult Pacific Lampreys in the Columbia River Basin

Abstract

AbstractAdult Pacific lampreys Lampetra tridentata routinely pass through picketed leads and diffuser gratings at lower Columbia River hydropower dams and enter areas where they can be delayed, injured, or killed. We assessed the bar spacing needed to exclude adult lampreys that enter the fishways at Bonneville Dam (river kilometer (rkm) 235). The experimental fish used (n = 242) ranged from 53 to 79 cm in total length and from 282 to 800 g in weight. All were able to swim through a 2.5‐cm vertical gap, 47% passed through a 2.2‐cm gap, and none passed through a gap of 1.9 cm or less. In dewatering simulations, none of the additional 50 Pacific lampreys tested were able to pass through a 1.9‐cm diffuser grating, while 86% were able to pass through a traditional 2.5‐cm grating. Based on these results, we conclude that bar spacing of 1.9 cm or less is needed to exclude most adult Pacific lampreys in the Columbia River drainage. An initial field test of the 1.9‐cm grating at John Day Dam (rkm 347) confirmed these results.