Factors affecting juvenile galaxiid fish passage at culverts

Abstract

Alteration of urban streams by culverts is common throughout the world and is known to impede freshwater fish movement. The main objectives were therefore to investigate how different culvert characteristics (e.g. height, length, angle, perched v. non-perched) and habitat variables (e.g. water velocity, riverbed gradient) affect the passage success of juvenile inanga (Galaxias maculatus), a weak-swimming migratory fish. We determined whether passage of juvenile G. maculatus could be improved by installing a ramp at thirteen replicate in situ culverts in Nelson City, New Zealand. As expected, none of the 400 fish tested were able to pass undercut (i.e. perched) culverts before ramp installation (0% passage), compared with the 250 fish (65% passage) that were able to pass non-perched culverts. Significantly more fish (44% passage) reached perched culvert outlets when aided by the ramp. Passage success with the ramp depended on its length and angle, with fewer fish passing longer or steeper ramps. To provide upstream passage for weak-swimming species such as juvenile (fork length < 60 mm) G. maculatus, ramp length and angle should not exceed three metres and 20 degrees, respectively. With passage requirements known and mitigation processes in place, urban fish populations might be able to be sustained.