Baumgartner, L. and Reynoldson, N., 2007. Fish communities of the Nepean River in the vicinity of Pheasants Nest Weir. NSW Department of Primary Industries – Fisheries Research Report Series No. 15. ISSN 1449-9959. 18pp.

Summary

The NSW government is committed to improving environmental flows in many of the regulated rivers in NSW. This involves increasing the amount of water released from storage dams to reverse past degradation of the ecological condition of these rivers. However, our poor understanding of the relationship between changes in flow and changes in fish communities restricts our ability to predict accurately how fish communities will respond to different environmental flow regimes.

The Sydney Catchment Authority is in the process of delivering improved environmental flows to the Nepean River and has recently initiated a fish passage rehabilitation program. The works involve modifications to weirs and the investigation of design options for the construction of future fishways. Environmental flows will increase opportunities for fish migration, increase flooding frequency of lowland regions, restore a more natural flow and provide improved spawning opportunities for freshwater animals.

The aim of this brief study was to collect baseline data, using electrofishing, on the fish communities in the Nepean River system prior to the reinstatement of environmental flows from the upper dams in the system. The data also allowed an assessment of the impact of Pheasant’s Nest Weir on fish of the Nepean River.

The preliminary data confirmed that dams and weirs were probably impacting fish communities of the Nepean River system. Few migratory species were sampled upstream of Pheasant’s Nest Weir and some potential migratory species had accumulated downstream. Weirs downstream of Pheasant’s Nest also restricted the upstream passage of species such as bullrout, bully mullet and freshwater mullet. Dozens of fish were individually tagged and some of the tagged bully mullet were subsequently caught by anglers substantial distances downstream in the estuarine reaches of the Hawkesbury system. This short assessment has provided a good baseline estimate of fish assemblages near weirs in the Nepean River system but a more comprehensive study is now required to allow the development of appropriate management responses.