Effect of slope and headpond on passage of American shad and blueback herring through simple Denil and deepened Alaska steeppass fishways

Abstract

Passage and transit time of adult upstream-migrant American shad Alosa sapidissima and blueback herring A. aestivalis were investigated in standard Denil and Alaska steeppass fishways with variable slope and headpond under semicontrolled conditions. Percent of American shad passed per unit time (percent passage) increased with temperature, while time required to ascend from the fishway entrance to the exit (transit time) decreased with increasing temperature for both species. Increasing fishway slope decreased percent passage of American shad, regardless of fishway type. Higher fishway slope decreased percent passage of blueback herring in the steeppass fishway only. Low headpond enhanced percent passage of American shad in the Denil fishway, but decreased percent passage of American shad in the steeppass fishway. Headpond level had no effect on percent passage of blueback herring in either fishway. Because headpond level in the steeppass fishway affected percent passage of American shad but not the smaller blueback herring, the relatively small cross-sectional area of the steeppass fishway operated at low headpond may inhibit passage of larger species such as American shad. Transit time of American shad decreased with both increasing slope and high headpond, and American shad increased through-water swimming speed under these conditions. American shad appeared to regulate their swimming speed through Denil-type fishways below maximal values. Blueback herring ascended the fishways at speeds comparable to those of American shad. Shorter transit times were not associated with increased percent passage. Turbulence and air entrainment may influence percent passage of American shad more than longitudinal water velocity.