Within‐catchment effects of flow alteration on fish assemblages in the Hawkesbury–Nepean River system, Australia

Streamflow in the Hawkesbury–Nepean River system has been progressively regulated and diverted since 1888 by the construction of 29 dams 7 m or more in height and another 52 smaller structures to supply water for the populations of Sydney, Wollongong and the Blue Mountains. This study identifies differences in fish assemblages between reaches of the river system affected by dams, flow diversion and regulation, and rivers unmodified for water supply. Fish were sampled in the upper-reach tributaries above five major dams, in slopes reaches within 1 km downstream of the dams, and in the lowland reaches of the Nepean and Hawkesbury rivers. Corresponding upper, slopes and lowland reaches were sampled in two unregulated tributaries. Fish were also sampled within the impoundments. Seven fish assemblages were identified by multivariate analyses, which revealed a separation of assemblages in habitats affected by dams or flow alteration. Fish within the lowland reaches of all three rivers formed a single assemblage, which became progressively fragmented with increasing distance upstream, and with increasing differences between regulated and unregulated rivers. Gambusia holbrooki and Anguilla reinhardtii were more abundant in the upper reaches of unregulated rivers, whereas Galaxias brevipinnis and Retropinna semoni were abundant in upper reaches above dams, but not recorded in upper reaches of unregulated rivers. In slopes reaches, Gambusia holbrooki and Gobiomorphus coxii were more abundant in unregulated reaches while Retropinna semoni, Philypnodon grandiceps and Anguilla reinhardtii were more abundant in regulated reaches. Characteristic species in lowland reaches included Macquaria novemaculeata, Mugil cephalus, Philypnodon grandiceps, Cyprinus carpio and Potamalosa richmondia. However, the true effects of flow alteration in the system are confounded by geomorphic and climatic variation and other disturbances within the catchment. Sustainable development of water resources in the catchment requires a more detailed analysis of biotic responses to river flows, accounting for confounding factors. Copyright © 1999 John Wiley & Sons, Ltd.