The effects of Balranald Weir on spatial and temporal distributions of lower Murrumbidgee River fish assemblages
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The effects of Balranald Weir on spatial and temporal distributions of lower Murrumbidgee River fish assemblages |
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Objectives:
- To document differences in the composition of fish assemblages upstream and downstream of Balranald Weir over different seasons.
- To investigate whether small native fish are adversely affected by a low level weir.
The proliferation of dams and weirs has significantly reduced migratory opportunities for Australian native fish species. However,most studies of inland fish have focussed on large species that are recreationally and commercially important. Consequently, littleis known of the migratory requirements of small fish or how they might be affected by barriers such as low level weirs.
This study focused on identifying the effects of Balranald Weir on fish of the Murrumbidgee River. Fish were sampled by boat electrofishing from upstream and downstream regions at three monthly intervals between April 2000 and September 2002. Sampling yielded 11,960fish from 13 species. Small fish species such as western carp gudgeons, crimson spotted rainbowfish and flyspecked hardyheads were caughtin large numbers immediately downstream of Balranald Weir, despite previously being considered non-migratory. In contrast, few juvenilesof much larger species such as golden perch, Murray cod or silver perch were sampled either above or below the Balranald Weir. Thissuggests that their recruitment may be naturally low in this section of the Murrumbidgee River.
The composition of fish assemblages varied considerably among seasons and between upstream and downstream regions. Alien fishspecies such as goldfish, carp and redfin perch accumulated downstream of Balranald Weir during summer. Seasonal changes in the relativeabundance of bony bream, Australian smelt, crimson-spotted rainbowfish and common carp lowered species diversity downstream of BalranaldWeir.
Overall, the effect of Balranald Weir on fish assemblages of the lower Murrumbidgee River was greatest during summer and autumn, whenfish were trying to move upstream. The dominance of small fish from downstream samples suggest that fishways constructed in the MurrayDarling Basin should be designed to allow passage for small individuals and species and not just for the adults of large species.