More flow in a regulated river correlates with more and earlier adult lamprey passage, but peak passage occurs at annual low flows

Artificial obstructions such as dams are a key limiting factor or threat to adult Pacific lamprey (Entosphenus tridentatus) attempting to access upstream spawning habitats. Nevertheless, lamprey counted during dam passage (dam counts) is useful for monitoring abundance trends of these fish. We describe the trends in lamprey dam counts during 2005–2020 at Leaburg Dam (6.7 m height) on the McKenzie River (Oregon, USA). Despite similar flow volumes at each of two fish ladders, most lampreys in most years passed Leaburg Dam via the right bank fish ladder (vertical slot design) rather than the left bank fish ladder (half Ice Harbor). Counts ranged between 32 and 176 lampreys per year (median = 71.5 individuals). Akaike Information Criterion revealed the best fit General Additive Model (GAM) that described the cumulative proportion of lamprey passage included ‘year’, ‘day of the year’ and ‘river flow’, as opposed to other GAMs that included fewer of these variables or that included ‘water temperature’. Lamprey generally began passing during consecutive days of decreasing river flows, with most passing during annual low flows during June–August each year. In addition, total annual dam counts were strongly correlated with the sum of mean daily river flows. Thus, higher annual river flows correlate with earlier and more lamprey passage, but peak passage occurs at annual low flows. Mean daily water temperature ranged between 7.8 and 14.9°C during lamprey passage.