Rapid Migration and High Survival of Adult Pacific Lampreys in Reservoirs

Abstract Dams and reservoirs present diverse hazards for riverine fishes, including slowed migration, reduced or blocked access to essential habitats, and direct and delayed mortality risks. Many of these hazards are evident for Pacific Lamprey Entosphenus tridentatus in the heavily dammed Columbia River basin, where the species’ distribution and abundance has sharply declined. In a 3‐year study, we used acoustic telemetry to investigate behavior and apparent survival of 784 adult Pacific Lampreys in Columbia River reservoirs and turbulent dam tailraces, two environments where previous monitoring efforts have been somewhat ineffectual. We released Pacific Lampreys upstream and downstream from Bonneville Dam to test whether Pacific Lamprey passage at one dam negatively affected upstream dam and reservoir passage metrics (i.e., delayed effects on migration rate or reach passage success) and whether upstream‐released groups migrated further than their downstream‐released counterparts. Overall, results showed that Pacific Lampreys migrated very rapidly (median rates = 50–62 km/d) in reservoir reaches and slowly (1–3 km/d) in reaches with tailraces or fishways. Pacific Lampreys also had much higher passage success in reservoir reaches (reach × year estimates = 0.93–0.99) than in reaches with tailraces or fishways (most estimates = 0.58–0.82). We found little statistical evidence for negative delayed effects of dam passage. Pacific Lampreys released in the Bonneville forebay were far more likely to pass upstream dams (~26–57%) than were those released in the Bonneville tailrace (~20–36%). We conclude that tailraces and fishways constrained the upstream distribution of adult Pacific Lampreys, that reservoirs were not associated with high mortality during active upstream migration, and that translocation past individual dams can increase the upstream migration distance of individuals and population distribution. Areas of continuing uncertainty include behavior and survival during overwintering (i.e., prespawn holding) and the potential delayed effects of tailrace and fishway passage on Pacific Lamprey fitness.