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 reg ulated and diverted since 1888 by the construction of 29 dams 7 m or more i n height and another 52 smaller structures to supply water for the populati ons of Sydney, Wollongong and the Blue Mountains. This study identifies dif ferences in fish assemblages between reaches of the river system affected b y dams, flow diversion and regulation, and rivers unmodified for water supp ly. 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 r eaches of the Nepean and Hawkesbury rivers. Corresponding upper, slopes and lowland reaches were sampled in two unregulated tributaries. Fish were als o sampled within the impoundments. Seven fish assemblages were identified b y multivariate analyses, which revealed a separation of assemblages in habi tats affected by dams or flow alteration. Fish within the lowland reaches o f all three rivers formed a single assemblage, which became progressively f ragmented with increasing distance upstream, and with increasing difference s 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 corn were more abu ndant in unregulated reaches while Retropinna semoni, Philypnodon grandicep s and Anguilla reinhardtii were more abundant in regulated reaches. Charact eristic 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 cat chment. Sustainable development of water resources in the catchment require s a more detailed analysis of biotic responses to river flows, accounting f or confounding factors. Copyright (C) 1999 John Wiley & Sons, Ltd.