Use of Night Video to Enumerate Adult Pacific Lamprey Passage at Hydroelectric Dams: Challenges and Opportunities to Improve Escapement Estimates

Reliable estimates of adult Pacific lamprey Entosphenus tridentatus escapement are critically needed to improve management of this declining and ecologically important species. The longest time series of Pacific lamprey counts are from count stations at Columbia River basin dams designed to enumerate adult salmonids during the day, but many Pacific lamprey pass at night. To estimate their total escapement, we used video to monitor nighttime lamprey passage in combination with daytime counts at two count stations at Bonneville Dam and two at The Dalles Dam in 2007–2008. We examined relationships among day and night counts and evaluated the potential for using expansion factors to estimate total escapement from past and future daytime count data. As expected, daytime counts systematically underestimated total lamprey passage, and day and night counts were positively correlated in most comparisons. Unexpectedly, ratios of night: day counts varied widely among sites and years because patterns of upstream and downstream movements past count stations varied. We highlight challenges associated with enumerating cryptic and nocturnal species, such as Pacific lamprey, the potential impact of species‐specific behaviors on enumeration efforts, and the importance of appropriate count station location and structure for video monitoring of fish passage. Received August 9, 2011; accepted April 27, 2012