Movement and behaviour of adult American shad (Alosa sapidissima) and sea lamprey(Petromyzon marinus) were monitored by closed-circuit video at several locations within amodified Ice Harbor fishway. American shad ascended and descended the fishway exclusively bysurface weirs, while sea lampreys used both surface weirs and submerged orifices. Upstreammovement of American shad during the day was higher than at night at both lower and middlefishway observation sites. Peak downstream movement of American shad at both locations wasassociated with decreasing light levels in the evening. Sea lampreys moved primarily at night atthe lower and middle fishway sites. Mean daily passage efficiency was low (1% for Americanshad, -2% for sea lamprey) at the lower fishway surface weir, but passage efficiency at the middlefishway surface weir was moderate (70% for American shad, 35% for sea lamprey) . High watervelocity, air entrainment, and turbulence of the modified Ice Harbor fishway design appeared toinhibit American shad and sea lamprey passage by disrupting upstream migratory motivation andvisual and rheoiaclic orientation.