Summary

The construction of dams and weirs has been identified as a major factor in widespread declines of Australian native fish. Scientists have previously argued that a potential reason for these declines is reduced survival of larval fish, but no studies have attempted to identify specifically how survival is affected by the presence of dams and weirs. Two types of weir, under-shot (sluice) and over-shot, are used on Australian freshwater streams and each design is associated with different downstream flow conditions. Over-shot weirs discharge water over the crest creating a plunging flow effect where water falls either vertically or down a sloping spillway. Releases from under-shot weirs are always from the bottom of the weirpool and water travels from an area of high pressure immediately upstream to an area of lower pressure downstream.

A number of recent studies have determined that several species of native fish in the extensive Murray-Darling River system regularly disperse downstream. Two important recreational species, Murray cod and golden perch, have a dispersal phase where larvae drift downstream for several days. The present study investigated if downstream passage through low-level undershot and overshot weirs was associated with increased injury and mortality in 1,600 larval Murray cod and golden perch. Experiments were conducted in an experimental channel at the Narrandera Fisheries Center, which contained a 1m high dropboard-regulated weir past which a flow of 6Ml/day was maintained. During the experiments, the weir was set up to operate in an undershot, overshot or no weir (control) mode, and the responses of larvae were recorded.

Passage through an undershot weir caused the death of 95% of golden perch and 52% of Murray cod larvae. In contrast, mortality of larvae resulting from passage through an over-shot weir was no different from handling and no weir controls. These results strongly suggest that under-shot weirs could be significantly reducing the survival of Murray cod and golden perch, in Australian river systems. It is recommended that water management authorities consider the replacement of undershot gates with overshot designs due to the substantial threat posed to the survival of larval fish in the Murray-Darling Basin.