DO FISH LOCKS HAVE POTENTIAL IN TROPICAL RIVERS? IVOR G. STUART, a * ANDREW P. BERGHUIS, b PETER E. LONG a and MARTIN MALLEN-COOPER c a Queensland Fisheries Service, Post Ofﬁce Box 6014, Rockhampton, Queensland 4700, Australia b Queensland Fisheries Service, Post Ofﬁce Box 1143, Bundaberg, Queensland 4670, Australia c Fishway Consulting Services, 8 Tudor Place, St Ives Chase, NSW 2075, Australia ABSTRACT This study evaluated attraction and passage of native ﬁsh through an automated ﬁsh lock on the tropical Fitzroy River in north-eastern Australia. In 69 samples (24 h each) taken at the exit and entrance of the ﬁsh lock, 17 ﬁsh species and 13 402 individuals were collected, at a maximum rate of 3317 ﬁsh per day. During low river ﬂows, the ﬁsh lock transferred a broad size range of ﬁsh (35–710 mm long), though migratory biomass was small. Removal of a vertical ﬁsh-crowding device did not affect the passage rate. Netting studies and observations of ﬁsh migrating below the weir suggested that the entrance was poorly located during high ﬂows and another ﬁshway near the spillway would enhance ﬁsh passage. The ﬁsh lock was inoperative for 48% of the time, due to mechanical and software failure, and a narrow operational range unsuited to the variable hydrology. Nevertheless, these design issues are site speciﬁc and reﬂect that ﬁsh lock technology is in its infancy in Australia. The operational reliability of the ﬁsh lock is now greatly improved but further work is needed to optimize the automatic cycling. To accommodate the inherently variable hydrology of lowland tropical/sub-tropical rivers into ﬁshway design, we highlight important research needs for ﬁshways and migratory ﬁsh communities. Fish locks are often considered a less favourable ﬁsh passage option but with the operational reliability issues partially resolved, they appear to have considerable potential for tropical river systems with low minimum ﬂows and low biomass; with further research and design, they may have wider application. Copyright # 2007 John Wiley & Sons, Ltd. key words: Australia; Queensland; ﬁsh passage; ﬁshway; migration; potamodromous; Fitzroy River; Eden Bann Weir Received 1 February 2006; Revised 16 July 2006; Accepted 2 August 2006 INTRODUCTION In the USA, Europe, Ireland, Russia and South America ﬁsh locks, lifts and trap and transport systems have been used to improve ﬁsh passage at medium (>7 m high) and high barriers (>15m high; Pavlov, 1989; Clay, 1995; Lucas and Baras, 2001; Oldani and Baigun, 2002). Most high ﬁsh passage systems only target a few important commercial species and, in contrast to tropical ﬁshes, these usually have a well-deﬁned migration season (Clay, 1995; Lucas and Baras, 2001). Fish locks have often been unsuccessful, mainly due to their limited capacity to pass a large biomass and because some ﬁsh fail to enter and exit. In France, for these reasons, ﬁsh locks are rarely considered when constructing ﬁsh passage facilities (Larinier et al., 2002). Presently, there are eight ﬁsh locks in Australia with six of these on barriers up to 15 m high in the tropical/ sub-tropical coastal rivers of the north-east. The design and operation of Australian ﬁsh locks has largely been based on overseas speciﬁcations, although maximum water velocities and turbulence have been reduced to allow for the swimming abilities of native ﬁsh (Mallen-Cooper, 1992; Stuart and Berghuis, 2002). To date, few studies have assessed the effectiveness of ﬁsh locks, particularly for passing the diverse and abundant ﬁsh communities present in large tropical river systems. In northern Australia, tropical rivers are characterized by long periods of zero ﬂow during the dry season (May to October), where even large rivers may contract to waterholes, followed by less predictable but intense summer and RIVER RESEARCH AND APPLICATIONS River Res. Applic. 23: 269–286 (2007) Published online in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.979 *Correspondence to: Ivor G. Stuart, Arthur Rylah Institute for Environmental Research, Post Ofﬁce Box 137, Heidelberg, Victoria 3084, Australia. E-mail: Ivor.Stuart@dse.vic.gov.au Copyright # 2007 John Wiley & Sons, Ltd.