Most fish recruitment models consider only one or a few drivers in isolation, rarely include species’ traits, and have limited relevance to riverine environments. Despite their diversity, riverine fishes share sufficient characteristics that prediction of recruitment should be possible. Here we synthesize the essential components of fish recruitment hypotheses and the key features of rivers to develop a model that predicts relative recruitment strength, for all fishes, in rivers under all flow conditions. The model proposes that interactions between flow and physical complexity will create locations in rivers, at mesoscales, where energy and nutrients are enriched. The resultant production of small prey will be concentrated and prey and fish larvae located (through dispersal or retention) so that the larvae can feed, grow, and recruit. Our synthesis explains how flow and physical complexity affect fish recruitment and provides a conceptual basis to better conserve and manage riverine fishes globally.

Dispersal of fluvial freshwater fish larvae occurs commonly in heavily regulated rivers. Studies increasingly indicate that drifting young fish have an active component to their movement and so are capable of coping with the dynamic hydraulic forces typical of rivers. We investigated hydraulic–drift relationships of the young stages of fish over one breeding season along a gravel shore of the Austrian Danube using stationary drift nets from the first occurrence until the end of the seasonal peak (April–June 2008). Observed densities of families and developmental stages were related to the hydraulic parameters flow velocity (in three spatial directions), turbulent kinetic energy and water depth, derived from a three‐dimensional hydrodynamic model of the sampling site. We detected distinct responses of drifting young fish to several hydro‐physical factors under conditions (low light level, over‐critical currents), which were considered to cause washouts and passive drift. In general, drift densities decreased with water depth and flow velocity. Weak swimmers (e.g. bull head Cottus gobio and early‐stage cyprinid larvae) avoided turbulent flows, as their abilities to orientate and hold position may be limited. Early larval as well as early juvenile stages of cyprinids used lateral currents directing to the bank, potentially to drop out of the flow and reach inshore areas. Our study indicates that there are family‐specific and stage‐specific responses to hydraulic variables and that fish actively disperse, probably to minimize mortality and maximize successful dispersal. Copyright © 2013 John Wiley & Sons, Ltd.

The biomass of organisms of different sizes is increasingly being used to explore macroscale variation in food-web and community structure. Here we examine how invasive species and river flow regulation affect native fish biomass and fish community log10 biomass – body mass scaling relationships in Australia’s largest river system, the Murray–Darling. The log10 biomass – body mass scaling exponent (scaling B) of invasive fishes (95% CI: −0.14 to −0.18) was less negative than for native fishes (95% CI: −0.20 to −0.25), meaning that invasive species attained a higher biomass in larger size-classes compared to native species. Flow alteration and invasive common carp (Cyprinus carpio) biomass were correlated with severe reductions in native fish biomass ranging from −47% to −68% (95% CI). Our study provides novel evidence suggesting that invasive and native communities have different biomass – body mass scaling patterns, which likely depend on differences in their trophic ecology and body size distributions. Our results suggest that restoration efforts using environmental flows and common carp control has potential to boost native fish biomass to more than double the current level.

Widespread flooding in south-eastern Australia in 2010 resulted in a hypoxic (low dissolved oxygen, DO) blackwater (high dissolved carbon) event affecting 1800 kilometres of the Murray-Darling Basin. There was concern that prolonged low DO would result in death of aquatic biota. Australian federal and state governments and local stakeholders collaborated to create refuge areas by releasing water with higher DO from irrigation canals via regulating structures (known as 'irrigation canal escapes') into rivers in the Edward-Wakool system. To determine if these environmental flows resulted in good environmental outcomes in rivers affected by hypoxic blackwater, we evaluated (1) water chemistry data collected before, during and after the intervention, from river reaches upstream and downstream of the three irrigation canal escapes used to deliver the environmental flows, (2) fish assemblage surveys undertaken before and after the blackwater event, and (3) reports of fish kills from fisheries officers and local citizens. The environmental flows had positive outcomes; mean DO increased by 1-2 mg L for at least 40 km downstream of two escapes, and there were fewer days when DO was below the sub-lethal threshold of 4 mg L and the lethal threshold of 2 mg L at which fish are known to become stressed or die, respectively. There were no fish deaths in reaches receiving environmental flows, whereas fish deaths were reported elsewhere throughout the system. This study demonstrates that adaptive management of environmental flows can occur through collaboration and the timely provision of monitoring results and local knowledge.

Overbank floods in modified lowland rivers often inundate a mosaic of different land uses (e.g. forests, crops and pastures) on the floodplain. We used a glasshouse experiment to investigate dissolved organic carbon (DOC) and nutrient (TP, NH4+, NOx) releases, chemical oxygen demand (COD) and dissolved oxygen (DO) depletion in water following inundation of soil and vegetation from a lowland river floodplain in southern Australia. Six replicate samples of six intact soil and groundcover treatments were collected during summer; three from a forest (bare soil, wallaby grass and leaf litter) and three from an adjacent paddock (bare soil, wheat and ryegrass). Samples were placed in pots, inundated with river water over 16 days, and their leachates were compared with a river-water control. All vegetated groundcover treatments had significantly higher DOC and COD and significantly less DO at both Day 1 and Day 16 than did the soil-only treatments or the control. Leachates from paddock treatments were less coloured than those from forest treatments, despite having similar concentrations of DOC. Our findings imply that the inundation of any vegetation during summer floods can be a major source of DOC and a major contributor to DO depletion.