Effects of Body Size and River Environment on the Upstream Migration of Adult Pacific Lampreys

Dams in the Columbia River basin present significant obstacles to declining populations of anadromous Pacific lampreys Lampetra tridentata. Mitigation efforts have focused on fine-scale improvements in passage at individual dams, but there is an increasing need for basinwide estimates of survival and escapement. We developed a half-duplex passive integrated transponder (PIT) tag monitoring array at five Columbia and Snake River dams to evaluate adult lamprey migrations. We tagged 3,598 lampreys over 3 years and calculated the rates of main-stem escapement through 15 river reaches. From these data, we assessed the relative effects of lamprey size, river discharge, water temperature, and migration timing on upstream passage. The results indicated high attrition as lampreys progressed upstream. In each year, about one-half of the fish passed one dam, 28-33% passed two (dams, 17-19% passed three dams, 4-5% passed four dams, and about 1% passed the first dam on the Snake River (five dams and >300 km upstream from their release sites). In most reaches, upstream passage was strongly size dependent, the largest lampreys being two to four times more likely to pass than the smallest fish. Lamprey size was more predictive of passage than were the river discharge, temperature, or migration timing variables. These findings suggest that adult Pacific lamprey migration is affected by physiological constraints and that effective mitigation for the difficult passage conditions at dams should include size-related considerations.