Can a low gradient vertical-slot fishway provide passage for a lowland river fish assemblage?
Protection of biodiversity and ecosystem processes is a priority for natural resource management in Australia. Dam construction in Australian rivers has led to a dramatic decline in native fish populations and aquatic biodiversity by reducing opportunities to complete important spawning and dispersal migrations. In 2001, the Murray-Darling Basin Commission (MDBC) initiated a program to improve fish passage to over 2000 km of the Murray River, from the sea to Hume Dam, by constructing 14 new fishways.
A monitoring and assessment program was established to determine if the reinstatement of passage was providing benefits to fish of the Murray-Darling Basin. The first new fishway (a vertical-slot design) was completed in 2003 at Lock 8 in the mid reaches of the Murray River. The fishway was constructed on a conservative slope (1:32), with low velocities and turbulence. It is the first of its kind in the Murray-Darling Basin. A detailed study was undertaken to determine whether it successfully provided passage for native fish.
Fish migrating through the fishway were trapped over 57 days between November 2003 and November 2005. The fishway passed large numbers (>50,000 fish), high diversity (13 species), and a wide size-range of fish (40 mm to 1000 mm long). These initial results suggested that the fishway adequately provided passage for all target fish at the site. However, large numbers of even smaller fish (< 31 mm long) were unable to ascend the fishway, including an unexpectedly high abundance of small carp gudgeons (< 50 mm long), a species previously considered non-migratory.
The study demonstrated that the low-gradient fishway at Lock 8 was effective at providing passage for an entire fish assemblage comprising the full range of sizes above 40 mm. The attempted migrations of large numbers of small-bodied species suggest, however, that the construction of future fishways should be designed to pass an even greater size range of fish.