Effects of a flow-gauging weir on the migratory behaviour of adult barbel, a riverine cyprinid

Barbel Barbus barbus exhibited substantial movements in the River Nidd between March and July 1994, with some individuals moving nearly 20 km upstream, although there was considerable individual variation. Most upstream movement occurred during May. Fifteen of the 23 fish tracked attempted to pass the weir, and of these six were successful. Barbel tended to approach the weir around dusk and dawn, reflecting observed patterns of localized activity, or at night, but crossed the weir only at night. Those fish which crossed the weir moved substantial distances upstream to spawn, while those that were unsuccessful moved down-stream. The weir delayed the net upstream progress of all adult barbel. Successful traversal was not closely related to flow or water temperature conditions. The study emphasizes the impact of relatively minor obstructions on the natural migrations of barbel.