MAGNITUDE, FREQUENCY AND DURATION OF INSTREAM FLOWS TO STIMULATE AND FACILITATE CATADROMOUS FISH MIGRATIONS: AUSTRALIAN BASS (MACQUARIA NOVEMACULEATA PERCIFORMES, PERCICHTHYIDAE) I. V. REINFELDS, a * C. T. WALSH, b,c D. E. VAN DER MEULEN, d I. O. GROWNS e AND C. A. GRAY d a NSW Ofﬁce of Water, PO Box 53, Wollongong, NSW 2500, Australia b Department of Primary Industries Batemans Bay Fisheries Centre, PO Box 17, Batemans Bay, NSW 2536, Australia c University of Wollongong, School of Biological Sciences, NSW 2522, Australia d Department of Primary Industries Cronulla Fisheries Research Centre, PO Box 21, Cronulla, NSW 2230, Australia e NSW Ofﬁce of Water, PO Box U245, Armidale, NSW 2351, Australia ABSTRACT The migratory response and behaviour of catadromous Australian bass with regard to hourly mean river ﬂows and water temperatures was assessed over 15 months. Fish movement was assessed using a 75-km passive acoustic telemetry array in the regulated Shoalhaven River below Tallowa Dam, NSW, Australia. The majority (62%) of downstream pre-spawning migrations from freshwater to estuarine habitats were stimulated by a series of ﬂow pulses from April to September, but a proportion of ﬁsh (38%) commenced downstream migrations under regulated baseﬂow conditions after a sustained decrease in water temperature to below 15  C in late autumn. Equal numbers of ﬁsh undertook post-spawning upstream return migrations during ﬂow pulses and during regulated baseﬂow conditions, with regulated baseﬂow migrants exhibiting a preference for dusk–dawn passage through freshwater pool–rifﬂe sequences. The median magnitude of ﬂow pulses at the time of commencement of downstream and upstream freshwater migrations by Australian bass was not large, equivalent to natural (in the absence of river regulation) ﬂows equalled or exceeded for 56% and 48% of time, respectively. There was no evidence for increased numbers of migrants with increasing ﬂow pulse magnitude, with individual ﬁsh ignoring some ﬂow pulses but responding to subsequent events. In regulated rivers, the release of more frequent ﬂow pulses with peak magnitudes approximating the natural 50th ﬂow duration percentile may be more effective in stimulating greater numbers of Australian bass to undertake pre-spawning and post-spawning migrations between freshwater and estuarine habitats than the release of a single, larger event. The propensity of Australian bass to also undertake spawning migrations under regulated baseﬂow conditions emphasizes the need for provision of baseﬂow regimes in regulated rivers that can facilitate migrations by large bodied ﬁshes. Copyright © 2011 John Wiley & Sons, Ltd. key words: Macquaria novemaculeata; migration; environmental ﬂow; ﬂow pulse; acoustic telemetry Received 12 September 2011; Accepted 21 October 2011 INTRODUCTION The natural ﬂow paradigm surmises that the magnitude, frequency, duration, timing and rates of change of ﬂow are the principal variables regulating riverine ecological pro- cesses (Poff et al., 1997). These hydrological characteristics provide key stimulatory cues for ﬁsh to undertake spawning migrations (Lucas and Baras, 2001). Flow regulation by dams, however, can suppress or remove these cues through the capture of ﬂoods and smaller freshes and the subsequent release of downstream ﬂows that may be less variable than the natural ﬂow regime (Gehrke and Harris, 2001). Although the importance of increases in river ﬂow as a stimulatory cue for ﬁsh migrations is well-known (e.g. Mallen-Cooper, 2000; Benson et al., 2007; Heublein et al., 2009; Sykes et al., 2009), only limited information is avail- able on the magnitude, frequency, duration, timing and rates of change of ﬂow events that will stimulate and facilitate spawning migrations by ﬁsh (Murchie et al., 2008; Enders et al., 2009). Even for the Atlantic salmon (Salmo salar), one of the most extensively studied ﬁsh with regard to miti- gation of the effects of river regulation, a recent review of ﬂow management for this species found that further infor- mation is needed on discharge requirements during critical life–history stages (Enders et al., 2009). In particular, the magnitude of river spates required for optimal salmon migration and survival and the amplitude of spring ﬂooding necessary to initiate downstream migrations were identiﬁed as areas requiring further research (Enders et al., 2009). The need for further research on ﬁsh behaviour and response to rises and falls in river levels and ﬂow rates, in concert with changes in water temperatures, was emphasized in a recent review of ﬁsh responses to modiﬁed ﬂow regimes in the study conducted by Murchie et al. (2008). *Correspondence to: I. V. Reinfelds, NSW Ofﬁce of Water, PO Box 53, Wollongong, NSW 2500, Australia. E-mail: ivars.reinfelds@water.nsw.gov.au RIVER RESEARCH AND APPLICATIONS River Res. Applic. 29: 512–527 (2013) Published online 28 November 2011 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/rra.1611 Copyright © 2011 John Wiley & Sons, Ltd.