Tidal barriers located at the interface between estuarine and freshwater habitats receive large numbers of diadromous fish migrating upstream after spawning and early life-history development in estuarine waters. These sites receive the highest biomass of upstream migrating fish of any barrier within a catchment, commonly dominated by juveniles less than 15 mm long.
Fishways at these sites have traditionally been designed to pass juvenile to adult size fish, but these targets are often well above the sizes of the post larval migrants that actually need to be passed. These early juveniles are weak swimmers with limited energy reserves, so even low turbulence, low velocity fishways can halt passage when they must repeatedly ascend multiple cells, leaving large cohorts delayed below barriers and vulnerable to predation.
Recent monitoring at tidal barriers in Queensland and New South Wales has highlighted the limitations of traditional fishway designs for passing this size and biomass. At the Fitzroy Barrage, existing vertical slot and cone fishways pass thousands of fish but have not resulted in sustained upstream populations of diadromous species. Similarly, monitoring at Seaham Weir has shown that the vertical slot fishway passes fish under conventional downstream flow, but massively greater numbers move during reverse flow when high tide creates an upstream hydraulic gradient.
Although these fishways can all pass thousands of fish under conventional operation, this may still represent only a small proportion of the juvenile biomass at these sites.
These findings indicate that successful passage at tidal barriers requires a shift in design and management objectives. Fishways should be assessed not only by whether they pass large number of fish, but whether they pass enough of the smallest and most abundant migrants to maintain upstream populations. Future designs should prioritise very low-energy fishways, fish locks or reverse-flow opportunities and biomass-scale recruitment monitoring.