Galaxias maculatus (īnanga in New Zealand or common jollytail in Australia) is a small, diadromous fish native to New Zealand, Australia, Tasmania, Lord Howe Island, Chatham Island, Chile, Argentina, and the Falkland Islands. Īnanga as a species is generally marginally catadromous, living in lowland fresh waters and migrating downstream to tidal reaches to spawn in autumn. Larvae spend about 5 months at sea before migrating back into fresh water as 40-50 mm juveniles.
Barriers to migration can severely curtail habitat availability, so knowledge of their swimming ability is critical to designing and remediating structures under road crossings to allow upstream īnanga passage. We evaluated the effects of substrate type, flow rate, and slope on īnanga passage over artificial ramps using five substrate types (smooth and corrugated surfaces, herring-bone baffle, Promax PolyfloTM culvert pipe, and the core of the plastic-moulded ground-drainage product MiraDrainTM). These were tested on slopes of 3 and 5% and flows of 4 and 6 L s-1. Slope, fish size, and substrate type each had a significant influence on the mean distances travelled up the ramps. MiraDrainTM was the most effective substrate type, enabling the greatest swimming distances to be achieved, and a maximum vertical elevation of 0.39 m. The results of this study demonstrate the potential for ramps to provide fish passage past small in-stream barriers, and the need to refine ramp slopes and lengths to meet the specific barrier remediation needs. The use of MiraDrainTM to remediate fish passage through structures with overhangs and free fall is now in practiced in New Zealand.