Windows of Opportunity: Passage of Adult Pacific Lamprey at Yakima River Diversion Dams

Throughout the Pacific Northwest, Pacific lamprey (Entosphenus tridentatus) are in decline. While the causes of this decline are uncertain, studies have shown that hydropower and diversion dams often impede lamprey spawning migrations. Such delays may reduce the time and energy available for adults to complete spawning migrations, increase predation risk, and negatively impact reproduction. The Yakima River has numerous irrigation diversion dams, and the effects of these dams on adult migration are unknown. We used radio telemetry to evaluate Pacific lamprey approach and passage metrics at six diversion dams on the lower Yakima and Naches Rivers from 2011 to 2014. Dam passage efficiencies ranged from 0% at Roza Dam to 82% at Wapato Dam. Seasonal passage efficiencies were generally greater at the upstream (Sunnyside, Wapato, and Cowiche) than downstream (Wanawish and Prosser) dams. Cumulative passage over multiple dams declined exponentially; few tagged lampreys passed three or more successive dams. Our results indicate that Yakima River diversion dams substantially impact the upstream migration of Pacific lamprey. Movement information described in this study is being used to improve adult Pacific lamprey passage in the Yakima River system, and can provide a useful case study for other basins throughout the West Coast.