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.

Understanding the causal mechanisms that determine recruitment success is critical to the effective conservation of wild fish populations. Although recruitment strength is likely determined during early life when mortality is greatest, few studies have documented age-specific mortality rates for fish during this period. We investigated age-specific mortality of individual cohorts of two species of riverine fish from yolksac larvae to juveniles, assaying for the presence of a “critical period”: A time when mortality is unusually high. Early life stages of carp gudgeons (Hypseleotris spp.) and unspecked hardyhead (Craterocephalus stercusmuscarum fulvus)—two fishes that differ in fecundity, egg size and overlap between endogenous and exogenous feeding—were collected every second day for four months. We fitted survivorship curves to 22 carp gudgeon and 15 unspecked hardyhead four-day cohorts and tested several mortality functions. Mortality rates declined with age for carp gudgeon, with mean instantaneous mortality rates (-Z) ranging from 1.40–0.03. In contrast, mortality rates for unspecked hardyhead were constant across the larval period, with a mean -Z of 0.15. There was strong evidence of a critical period for carp gudgeon larvae from hatch until 6 days old, and no evidence of a critical period for unspecked hardyhead. Total larval mortality for carp gudgeon and unspecked hardyhead up to 24 days of age was estimated to be 97.8 and 94.3%, respectively. We hypothesise that life history strategy may play an important role in shaping overall mortality and the pattern of mortality during early life in these two fishes.

In the world's rivers, alteration of flow is a major driver of biodiversity decline. Global warming is now affecting the thermal and hydrological regimes of rivers, compounding the threat and complicating conservation planning. To inform management under a non-stationary climate, we must improve our understanding of how flow and thermal regimes interact to affect the population dynamics of riverine biota. We used long-term growth biochronologies, spanning 34 years and 400,000 km 2 , to model the growth dynamics of a long-lived, apex predator (Murray cod) as a function of factors extrinsic (river discharge; air temperature; sub-catchment) and intrinsic (age; individual) to the population. Annual growth of Murray cod showed significant, curvilinear, life-stage-specific responses to an interaction between annual discharge and temperature. Growth of early juveniles (age 1+ and 2+ years) exhibited a unimodal relationship with annual discharge, peaking near median annual discharge. Growth of late juveniles (3+ to 5+) and adults (>5+) increased with annual discharge, with the rate of increase being particularly high in adults, whose growth peaked during years with flooding. Years with very low annual discharge, as experienced during drought and under high abstraction, suppress growth rates of all Murray cod life-stages. Unimodal relationships between growth and annual temperature were evident across all life stages. Contrary to expectations of the Temperature Size Rule, the annual air temperature at which maximum growth occurred increased with age. The stage-specific response of Murray cod to annual discharge indicates that no single magnitude of annual discharge is optimal for cod populations, adding further weight to the case for maintaining and/or restoring flow variability in riverine ecosystems. With respect to climate change impacts, on balance our results indicate that the primary mechanism by which climate change threatens Murray cod growth is through alteration of river flows, not through warming annual mean temperatures per se.

Studies in the Northern Hemisphere have shown that mussels play important roles as benthic‐pelagic couplers in freshwater systems, transferring filtered material, nutrients, and energy from the water column to sediments, through biodeposition and excretion. However, we know little of the functional roles of species of the Southern Hemisphere Hyriidae, the second most diverse family of the Unionoida. The aims of this study were to determine the biodeposition and excretion rates of nutrients and organic matter of the hyriid Alathyria jacksoni in an unregulated Australian river and test experimentally if the physical structure and biodeposition of A. jacksoni influenced sediment nutrients, organic matter, periphyton concentrations and meiofauna densities. In a lowland Australian river in late summer over 4 weeks, we measured biodeposition and excretion rates of A. jacksoni, and conducted a mesocosm experiment, during which we compared the effects of live mussels, dead mussel shells (shams), and controls without mussels on nutrients, organic matter, and meiofauna invertebrate densities in the sediment. Mean (± SE) mass of biodeposition was 70.85 ± 3.20 mg mussel−1 hr−1 and the relative rates of inorganic and organic biodeposition were 60.9 ± 3.1 and 9.9 ± 0.8 mg mussel−1 hr−1, respectively. Organic matter was significantly greater in the sediment of the mesocosms with live mussels than in that of the control mesocosms. There was also a trend of greater mean sediment chlorophyll‐a and total nitrogen concentrations in mussel treatments than in the controls. No significant difference was detected in sediment total phosphorous between mussel and control enclosures. The valves of live mussels had significantly higher amounts of organic material and chlorophyll‐a concentrations than the valves of sham mussels. There was a consistent trend of higher meiofauna densities collected in the benthos of the mussel enclosures than the control enclosures for total density and densities of Rotifera, Copepoda, Cladocera, Diptera, Oligocheata, Nematoda, and Ostracoda; this trend was near significant for total density; and significant for densities of Ostracoda, Rotifera, Cladocera, and Nematoda. Using our biodeposition and excretion rate estimates, and estimates of population size of A. jacksoni in the Lower Ovens River, we estimated that in a typical 1‐km reach in a month over summer, A. jacksoni would be biodepositing approximately 4.8 kg/day of organic matter (dry mass), 137 g N/day and 19 g P/day, and excreting 21 g N/day and 3.7 g P/day. Our findings are similar to those for Northern Hemisphere families of the Unionoida and serve to highlight the roles mussels play in processing of organic matter and cycling of nutrients in freshwater ecosystems.