Summary

Migratory obstruction caused by dams and weirs often leads to declines in numbers and diversity of freshwater fish. Fishways are commonly constructed to restore migration pathways for fish. Although many different designs have been trailed in Australia, the Denil fishway is a relatively inexpensive option with substantial potential for large-scale application. A Denil fishway can be simply described as an open channel containing a series of upstream-sloping ‘U’-shaped baffles. The baffles create a low-velocity zone close to the fishway floor, which allows fish to ascend. Denil fishways have been constructed worldwide and provide passage for many species, with varying degrees of success.

A Denil fishway was constructed at the offtake regulator on the Edward River in 1998. The installation is unusual because the fishway was actually designed and constructed for another site in NSW, Brewarrina Weir. Fisheries managers have previously questioned the effectiveness of such an installation because the operating range for which it was designed (based on Darling River flows) does not match the flow conditions experienced in the Edward River. This study was undertaken to address these concerns, by assessing fish passage through the structure. Electrofishing was also used to determine any differences between fish communities upstream and downstream of the regulator.

A total of 222 fish (7 species) and 5,036 freshwater shrimp (1 species) were sampled from the Denil fishway during the study period. Surprisingly, small-bodied species such as Australian smelt, flyspecked hardyhead and western carp gudgeon largely dominated fishway catches. Many species also accumulated downstream of the structure suggesting not all species were able to successfully gain passage. Several operational issues severely compromised fishway operation, including a limited ability to cope with water level fluctuations and an inappropriate location of the entrance.

The study also investigated whether fish passage was required at the nearby Gulpa Creek regulator. Electrofishing surveys determined that both small and large-bodied species (from 13-516mm) accumulated downstream of the structure. These results suggest that a fishway is needed at this site that should provide passage for a wide range of species and size classes. The most suitable design would be a low-gradient vertical slot fishway provided it is appropriated sited and is constructed to criteria that permits the passage of both small and large-bodied species.