Role of Olfaction and Vision in the Behavior of American Shad (Alosa sapidissima) Homing to the Connecticut River from Long Island Sound

Twenty-two adult American shad (Alosa sapidissima), captured, sensory-impaired, sonic-tagged, and released adjacent to and 10 km west of the Connecticut River, were tracked in Long Island Sound in 1972. Both anosmic and blind/anosmic shad oriented into the tidal current and altered swimming speeds in relation to changes in tidal current velocity as do unimpaired fish. They did not, however, exhibit the consistency in these behavioral responses that resulted in westerly displacement in unimpaired shad. Blind shad exhibited neither of these behavior patterns but did tend to affect a westerly displacement in Long Island Sound.Dart-tagged adult shad that were sensory-impaired and released without displacement in the same areas of Long Island Sound in 1971 and 1972 homed less successfully to the Connecticut River than did unimpaired dart-tagged shad released in the same years. Anosmic dart-tagged shad located the Connecticut River less successfully than unimpaired shad. Blind shad successfully located the river from 10 km distance but failed to do so when released adjacent to the river. Blind/anosmic shad failed to locate the river from either release site.It was concluded that an olfactory–rheotaxic mechanism forms the basis of the location of the Connecticut River by shad. The preferred direction of displacement appears to be established by olfactory clues indicative of the Connecticut River whereas the orientation along the migratory path is maintained by reference to the rate and direction of tidal currents. Vision alone cannot account for this rheotaxic response.