Response of downstream moving European eel (Anguilla anguilla) to hydrodynamic conditions created by channel constrictions

The efficacy of bypass channels that provide safe routes of passage for downstream moving fish at river infrastructure can be compromised if fish avoid the hydrodynamics encountered at the entrance. Using an open-channel flume, avoidance exhibited by European eel (Anguilla anguilla) was investigated in response to: (1) hydrodynamics created by a smooth tapered constriction under three discharge regimes (low, medium, high); and (2) a smooth tapered and sharp angled constriction under the low discharge. Although eel exhibited avoidance on encountering the smooth tapered constriction, the prevalence and magnitude of response was not influenced by flow acceleration, the hydrodynamic factor often considered responsible for deterring fish. Eel more frequently rejected the sharp angled compared to the smooth tapered channel by retreating upstream, delaying downstream progress. This was likely due to flow recirculation (i.e. the occurrence of flow reversal upstream and downstream of the constriction) that was generated only at the sharp angled entrance. Flow accelerated at both constriction geometries, suggesting that recirculation had a greater influence on behaviour and subsequent rate of downstream movement. Minimising areas of flow recirculation, a seldom considered factor, as well as rates of acceleration at bypass entrances is likely to enhance their efficiency.