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Floodplain inundation in rivers is thought to enhance fish recruitment by providing a suitable spawning environment and abundant food and habitat for larvae. Although this model has not previously been tested in Australian rivers, it is often extrapolated to fishes of the Murray-Darling Basin. Fortnightly sampling of larvae and juveniles was conducted in the unregulated Ovens River floodplain during spring–summer of 1999 (non-flood year) and 2000 (flood year). The only species that increased in larval abundance during or shortly after flooding was an introduced species, common carp (Cyprinus carpio). Additionally, the peak abundance of larvae on the floodplain occurred during a rapidly declining hydrograph under low flow conditions in isolated billabongs and anabranches. The low use of the inundated floodplain for recruitment contradicts previous models. We propose a model of the optimum environmental conditions required for use of the inundated floodplain for fish recruitment. The model suggests that the notion of the flood pulse alone controlling fish recruitment is too simplistic to describe all strategies within a system. Rather, the life history adaptations in the fauna of the system and aspects of the hydrological regime such as duration and timing of inundation will control the response of a river's fish fauna to flooding.

1. Patterns in abundance and distribution of larval fish in a heavily regulated and a mildly regulated Australian lowland river were compared over four breeding seasons to gain some insight into how river regulation affects fish populations. 2. Larvae from a total of 13 species from nine families were recorded from the two rivers. The mildly regulated Broken River supported twice as many species as the heavily regulated Campaspe River. The two rivers shared three introduced species but only two native species. The dominant species in the Campaspe was not found in the Broken River. 3. The two most abundant species in the Campaspe were classified as `opportunists'. They are small, short‐lived species, which spawn for up to 9 months, encompassing extremes in temperature and flow. The extended spawning period may place a subset of larvae in optimal conditions for recruitment and is hypothesised as being the key to the success of these species. 4. Most species spawned each year, despite large interannual variation in flow and temperature conditions. Poor recruitment over several decades, rather than a failure to spawn, is considered the most likely explanation for differences in the larval fish faunas between the two rivers. 5. The highly regulated section of the Campaspe River downstream of the regulating impoundment is thought to provide suboptimal habitat conditions for larvae relative to the less regulated downstream sections. 6. The timing of occurrence of larvae of the dominant species varied by breeding season and may be the result of flexibility in the timing of spawning.

Summary 1. This paper introduces, and summarises the key messages of, a series of papers that emanated from a symposium on the Role of Drought in the Ecology of Aquatic Systems, held in Australia in 2001.2. Defining drought hydrologically is problematic because the return times, intensity, duration and long‐term trends in low‐flow periods are specific to regions and times. Droughts may instead be referred to as ‘significant low‐flow periods’, many of which have been replaced by ‘anti‐drought’ conditions in rivers as they are used increasingly as irrigation conduits.3. Droughts can be divided into those that cause predictable, seasonal press disturbances and less predictable, protracted ‘ramp’ disturbances. However, while droughts may be ‘ramp’ disturbances, their effects on aquatic biota are most likely to be ‘stepped’ when geomorphological or hydrological thresholds are crossed, causing abrupt changes in biological community structure and ecosystem processes.4. Physical, morphological, physiological or behavioural refugia confer resistance or resilience to riverine populations and communities that experience drought conditions. The physical and chemical parameters associated with refugia habitats and their formation, influence population parameters within, and interactions among, species and can have protracted reproductive consequences, even well after the cessation of the drought.5. Fish, invertebrate and plant populations and assemblages seem to recover rapidly from drought. Most studies of the effects of drought, however, have arisen fortuitously and have involved relatively short temporal, and small spatial, scales. Innovative approaches, such as microsatellite DNA analyses, can reveal that the effects of drought may be profound and long‐lasting, resulting in population bottlenecks and altering the course of the evolution of species.6. During periods of drought, decreases in inputs of dissolved organic carbon, nitrogen and phosphorus may lead to carbon limitation to microbial metabolism, resulting in autotrophic production being favoured over heterotrophic production.7. Long‐term climate trends, as indicated by palaeoecological evidence, suggest that, at least for Australia, droughts are likely to occur more frequently in the future. Anthropogenic effects on climate are likely to exacerbate this.8. It is important that drought is seen for what it is: a natural extreme of the flow continuum, with flooding at the other extreme. Thus, despite the potential for dramatic impacts on aquatic biota and the negative social connotations associated with such events, drought must be incorporated into river management plans.

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Alterations to the natural hydrologic regime in regulated rivers can disrupt cues that initiate the maturation and spawning of riverine fish, or they can change the conditions which are suitable for the recruitment of larvae into juvenile populations. Observations of fish larvae have the potential to provide insights into the effects of flow regulation, showing whether it has had a greater impact on fish by preventing spawning or by reducing or eliminating recruitment. We investigated historical and current records of native fish in the highly regulated Campaspe River and the moderately regulated Broken River, Murray-Darling Basin, Australia, and compared these with the results from sampling of fish larvae over three consecutive years to assess the likely impact that river regulation has had on fish populations in lowland Australian rivers. Of the 12 native species of fish that have been recorded historically from the Campaspe River, eight still occur, generally in low abundance, but only three of these were recorded as larvae in this experiment. From recent records, ten native fish species are extant in the Broken River from a suite of 15 that have been recorded there; of these, nine were collected as larvae. The presence of Murray cod larvae in this river was a significant finding. Thus, the less regulated Broken River is in a much healthier state than the Campaspe River. The results of sampling in both rivers indicated that most species of fish spawned each year, despite high inter-annual variation in antecedent hydrological conditions. This suggests that for the species present in these two rivers, the hydrology (pattern of daily discharge) during the winter and spring preceding breeding was unlikely to be a cue for final maturation and spawning. These findings are only preliminary, but they may show that river regulation has had more of an impact on post-spawning recruitment than on prevention of spawning. This has important implicatons for the remediation of the effects of river regulation, with targeting of recruitment processes and the factors influencing these, a priority.

Current-mediated downstream dispersal by the early developmental stages of fish in rivers is a common phenomenon. Knowledge of patterns and processes in the dispersal, or 'drift', of young fishes provides important information on spawning location and spawning success, habitat use, movement paths and flow-ecology relationships more generally, all of which are critical for effective river conservation and management. But despite the importance of such information, our understanding of the patterns and processes of the drift of the early life stages of riverine fishes is limited. Furthermore, riverine fish drift research has tended to occur in isolation from movement studies of other organisms, limiting its integration with higher level concepts and theory. This manuscript reviews the literature on the dispersal of young fishes in running waters. Relevant studies from all climatic zones and geographical regions are investigated, with particular attention given to the types and life history stages of fishes that drift and the seasonal and diel patterns of drifting. We then consider how fish enter the drift and their mode of drifting, attempting to reconcile a long-running discussion, under what we call the 'active-passive conundrum'. We argue that, aside from eggs, the early stages of fish are not exclusively either passive or active drifters, but usually a mixture of the two, which we term 'actipassive' drift. Finally, we evaluate existing knowledge in the context of a general conceptual framework for movement ecology, identifying gaps in our understanding of the roles of internal state, navigation capacity, motion capacity, external factors and internal factors in influencing the dispersal process.