Upstream passage of Galaxias maculatus and Galaxias brevipinnis: comparative behavioural assessment for improved fish pass design

Abstract

1. Customized fish passage solutions enhance the upstream passage of fish through low-head instream structures. These solutions are designed based on the behaviour of benchmark species, with the hope that they will also perform well for other species. Across species, suitability can be supported by establishing similarity in passage behaviour. However, for small-bodied fish species, such comparative behavioural assessments are largely missing. 

2. With respect to spoiler baffle-based passage design for Southern Hemisphere small-bodied fish species, Galaxias maculatus are typically considered as a benchmark species. In order to characterise the upstream passage of G. maculatus with a view to extending it to other species, here the behaviour of G. maculatus and Galaxias brevipinnis were recorded for three different spoiler baffle designs. Hydrodynamic preferences during upstream swimming and resting, including pathway assessment, were analysed.

3. Both species show increased preference to swim at locations with streamwise velocities similar to the known sustained swimming capacity threshold of G. maculatus, whilst resting locations align with minimal streamwise velocities. Preferred pathways were observed to be influenced by the streamwise water velocity and turbulent kinetic energy, with both species showing increased preference to move towards locations with similar streamwise velocities, by avoiding locations with higher turbulent kinetic energy. 

4. The study further shows that both species' passage behaviour is more dependent on space availability than individual behaviour alone. The spatial distribution of fine-scale hydrodynamics influences observed swimming and resting behaviour, with both species showing similar passage behaviour for all three tested designs. 

5. Synthesis and applications: Similarities, for both passage behaviour and pathway selection, suggest that spoiler baffle-based passage designs are a suitable management option for both species. Both species were observed to use different fine-scale hydrodynamic regions for resting and subsequent swimming. Among the patches, patch P2 provided increased resting and low-velocity swimming cells for both species, which are shown to be important ecological indicators of successful passage applications, supporting further evaluation for possible application of culvert retrofitting, allowing for varying flow rates, slopes and fish species.