Do low-head riverine structures hinder the spread of invasive crayfish? Case study of signal crayfish (Pacifastacus leniusculus) movements at a flow gauging weir

Increasing legislative drivers demand the removal or modification of riverine barriers to enhance habitat connectivity for fish; however there is also concern that greater connectivity will hasten the spread of aquatic invaders such as the signal crayfish (Pacifastacus leniusculus). Passive Integrated Transponder (PIT) telemetry was used to assess passage of signal crayfish (n=392) over a typical low-head riverine structure, a flow gauging weir, during a 17 month period. Sixty percent of tagged crayfish were detected, with greatest crayfish activity associated with high water temperatures and long day lengths. The study weir reduced upstream crayfish movements, with 45% less passages than in the control stretch with no weir, but there was no difference in the downstream direction. We found sex and size related differences in crayfish movement patterns with male crayfish more likely to successfully ascend the weir, and larger crayfish to descend the weir. Although increased fluvial connectivity will benefit migratory fish species, we suggest that the removal or modification of even quite minor, low-head structures such as the one investigated could hasten the upstream spread of signal crayfish.