Effect of water acceleration on downstream migratory behavior and passage of Atlantic salmon smolts and juvenile American shad at surface bypasses

Behavior and passage rate of smolts of Atlantic salmon Salmo salar and juvenile American shad Alosa sapidissima were compared between a standard (sharp‐crested) and a modified surface bypass weir that employs uniform flow velocity increase (1 m·s⁻¹·m⁻¹ of linear distance). Within the first 30 min after release, significantly more smolts passed the modified weir than the standard weir, but no differences in passage rate between weir types were noted for juvenile American shad. More Atlantic salmon smolts and juvenile American shad were passed by the modified weir in groups of two or more than were passed by the standard weir. Mean lengths of passed and nonpassed smolts were not significantly different between weir types, but American shad passed by the sharp‐crested weir were significantly smaller than nonpassed fish. Most individuals of both species that passed the modified weir maintained positive rheotaxis and strong swimming throughout the length of the weir. In addition to acceleration, visual cues may be an important factor in avoidance behaviors near bypass entrances. The observed reduction of delay time before passage and maintenance of school integrity may facilitate appropriate timing of emigration and enhance passage survival.