Remediating small-scale migratory fish barriers with floating fish ramps

Instream barriers are a well-documented stressor for diadromous fish species, and can be numerous and costly to remediate. In New Zealand, previous experiments have investigated the swimming ability of indigenous migratory fish over small ramps, but this work has not led to the development of cost-effective ramps that can be deployed by river managers. This study built on previous research and investigated the effect of ramp surface type on the swimming ability of two species of diadromous fish; inanga (Galaxias maculatus) a weak swimmer, and redfin bully (Gobiomorphus huttoni) a moderate climber, in an effort to inform the design of a floating plastic ramp that could be installed at low head instream barriers. Three phases of tank trials in the Hawke’s Bay tested different ramp substrates, including rock climbing holds, and small and large raised Miradrain® cusps. The results indicated that a linear arrangement of small cusps provided an optimal surface for swimming species such as inanga while redfin bully passage rates did not differ between the surfaces tested. The addition of spat rope reduced velocities and increased depth on all surfaces, and increased passage rates for inanga on ramps with large cusps, and redfin bullies on both cusp sizes. Fish surveys were conducted at 4 lowland streams in Hawke’s Bay in order to characterise fish communities, investigate the impact of barriers on their distribution, and evaluate the success of ramp installations in facilitating passage. Fish communities were impacted by barriers to varying degrees depending on the climbing ability of different species present. Inanga and redfin bully abundances were lower above the barriers, whereas eel abundances were similar between upstream and downstream reaches. Rotomoulded plastic floating ramps lined with small cusps were installed at barriers on 2 streams, the Awanui and the Irongate. Inanga were found above the barrier in the Awanui Stream 12 months after a ramp was installed, but fish did not pass over the wooden weir on the Irongate Stream. Buoyancy issues with the Irongate ramp are thought to have reduced its utility and potentially hindered fish passage. The buoyancy issues have since been overcome, and further monitoring is required to assess whether fish passage is consequently improved. These results show that floating rotomoulded ramps can be an effective and cost-effective tool for remediating small scale instream barriers to migratory fish.