Dry season habitat use of fishes in an Australian tropical river

The modification of river flow regimes poses a significant threat to the world's freshwater ecosystems. Northern Australia's freshwater resources, particularly dry season river flows, are being increasingly modified to support human development, potentially threatening aquatic ecosystems and biodiversity, including fish. More information is urgently needed on the ecology of fishes in this region, including their habitat requirements, to support water policy and management to ensure future sustainable development. This study used electrofishing and habitat survey methods to quantify the dry season habitat use of 20 common freshwater fish taxa in the Daly River in Australia's wet-dry tropics. Of twenty measured habitat variables, water depth and velocity were the two most important factors discriminating fish habitat use for the majority of taxa. Four distinct fish habitat guilds were identified, largely classified according to depth, velocity and structural complexity. Ontogenetic shifts in habitat use were also observed in three species. This study highlights the need to maintain dry season river flows that support a diversity of riverine mesohabitats for freshwater fishes. In particular, shallow fast-flowing areas provided critical nursery and refuge habitats for some species, but are vulnerable to water level reductions due to water extraction. By highlighting the importance of a diversity of habitats for fishes, this study assists water managers in future decision making on the ecological risks of water extractions from tropical rivers, and especially the need to maintain dry season low flows to protect the habitats of native fish.

Predicting the Influence of Streamflow on Migration and Spawning of a Threatened Diadromous Fish, the Australian Grayling Prototroctes Maraena

The development of effective strategies to restore the biological functioning of aquatic ecosystems with altered flow regimes requires a detailed understanding of flow-ecology requirements, which is unfortunately lacking in many cases. By understanding the flow conditions required to initiate critical life history events such as migration and spawning, it is possible to mitigate the threats posed by regulated river flow by providing targeted environmental flow releases from impoundments. In this study, we examined the influence of hydrological variables (e.g., flow magnitude), temporal variables (e.g., day of year) and spatial variables (e.g., longitudinal position of fish) on two key life history events (migration to spawning grounds and spawning activity) for a threatened diadromous fish (Australian grayling Prototroctes maraena) using data collected from 2008 to 2015 in the Bunyip-Tarago river system in Victoria. Our analyses revealed that flow changes act as a cue to downstream migration, but movement responses differed spatially: fish in the upper catchment showed a more specific requirement for rising discharge to initiate migration than fish in the lower catchment. Egg concentrations peaked in May when weekly flows increased relative to the median flow during a given spawning period. This information has recently been incorporated into the development of targeted environmental flows to facilitate migration and spawning by Australian grayling in the Bunyip-Tarago river system and other coastal systems in Victoria.

First detection of Edwardsiella ictaluri (Proteobacteria: Enterobacteriaceae) in wild Australian catfish

The bacterium Edwardsiella ictaluri is considered to be one of the most significant pathogens of farmed catfish in the United States of America and has also caused mortalities in farmed and wild fishes in many other parts of the world. E. ictaluri is not believed to be present in wild fish populations in Australia, although it has previously been detected in imported ornamental fishes held in quarantine facilities. In an attempt to confirm freedom from the bacterium in Australian native fishes, we undertook a risk-based survey of wild catfishes from 15 sites across northern Australia. E. ictaluri was detected by selective culturing, followed by DNA testing, in Wet Tropics tandan (Tandanus tropicanus) from the Tully River, at a prevalence of 0.40 (95% CI 0.21-0.61). The bacterium was not found in fishes sampled from any of the other 14 sites. This is the first report of E. ictaluri in wild fishes in Australia.

Influence of streamflow on spawning-related movements of golden perch Macquaria ambigua in south-eastern Australia

In this study, linkages were examined between movement and spawning behaviour for golden perch Macquaria ambigua in a lowland river by integrating acoustic telemetry and egg and larval drift sampling over 4 years. Movement was strongly seasonal, being most prevalent during the spawning season (spring to early summer), and occurred primarily downstream into the lower river reaches during elevated flows. A very strong association was found between the occurrence of spawning and long-distance M. ambigua movement. The results also revealed that targeted environmental water allocation can promote movement and spawning of this species. By integrating multiple analytical approaches and focusing on key life-history events, this study provides an improved picture of the life history and flow requirements of M. ambigua. The findings can help guide the development of effective environmental flow recommendations.

Spawning season movements of Macquarie perch (Macquaria australasica) in the Yarra River, Victoria

The Macquarie perch (Macquaria australasica) is a threatened fish species that inhabits rivers and impoundments in south-eastern Australia. Previous studies have shown that Macquarie perch in impoundments exhibit synchronised upstream spawning migrations to shallow, fast-flowing habitats in the lower reaches of inflowing streams. There has been little study of movement behaviours of entirely riverine populations of Macquarie perch despite this being the species’ natural habitat. Here, radio-telemetry is used to test the hypothesis that riverine populations exhibit synchronised migrations during the spawning season. Thirty Macquarie perch in the Yarra River, Victoria, a translocated population outside of the species’ natural range, were radio-tagged before the late spring–early summer spawning season and their movements followed over a 10-month period (May 2011 to February 2012). Tagged fish typically occupied restricted reaches of stream (<450 m). Sixteen of the fish undertook occasional upstream or downstream movements (~250–1000 m) away from their usual locations, particularly associated with large flow variations during the spawning season. There was no evidence of synchronised migratory behaviour or movement of multiple fish to specific locations or habitats during the spawning season. Whilst further research over more years is needed to comprehensively document the spawning-related behaviours of riverine Macquarie perch, our study demonstrates that management of riverine populations of this threatened species cannot necessarily be based on the model of spawning behaviour developed for lacustrine populations.

Asteriscus v. lapillus: comparing the chemistry of two otolith types and their ability to delineate riverine populations of common carp Cyprinus carpio

The chemical composition of common carp Cyprinus carpio asteriscus (vaterite) and lapillus (aragonite) otoliths from the same individual and reflecting the same growth period was measured to (1) determine whether there are differences in the uptake of trace metals (Mg:Ca, Mn:Ca, Sr:Ca and Ba:Ca ) and Sr isotope ratios ((87)Sr:(86)Sr) in co-precipitating lapilli and asterisci and (2) compare the ability of multi-element and isotopic signatures from lapilli, asterisci and both otolith types combined to discriminate C. carpio populations over a large spatial scale within a river basin. Depth profile analyses at the otolith edge using laser-ablation inductively coupled plasma mass spectrometry showed that asterisci were enriched in Mg and Mn and depleted in Sr and Ba relative to lapilli, whilst (87)Sr:(86)Sr values were nearly identical in both otolith types. Significant spatial differences among capture locations were found when all trace element and Sr isotope ratio data were aggregated into a multi-element and isotopic signature, regardless of which otolith type was used or if they were used in combination. Discriminatory power was enhanced, however, when data for both otolith types were combined, suggesting that analysis of multiple otolith types may be useful for studies attempting to delineate C. carpio populations at finer spatial or temporal scales.

Genetic analysis of threatened Australian grayling Prototroctes maraena suggests recruitment to coastal rivers from an unstructured marine larval source population

Population genetic variation of Australian grayling Prototroctes maraena was examined to determine whether the dispersal strategy of this amphidromous species favours retention of larvae and juveniles in close proximity to their natal river, or mixing of populations via marine dispersal. Variation in microsatellite and mitochondrial DNA markers was unstructured and differentiation was indistinguishable from zero across four coastal rivers spanning approximately one-quarter of the continental range of the species. This result indicates that the marine larval and juvenile phase probably facilitates extensive gene flow among coastal rivers and agrees with a previous analysis of otolith chemistry that suggested larvae probably move into the sea rather than remain in estuaries. It appears likely that the dispersal strategy of P. maraena would enable recolonization of rivers that experience localized extinction provided that connectivity between freshwater habitats and the sea is sufficient to permit migration and that enough source populations remain intact to support viability of the wider population.

Nuptial coloration varies with ambient light environment in a freshwater fish

Visual signals play a vital role in many animal communication systems. Signal design, however, often varies within species, raising evolutionarily important questions concerning the maintenance of phenotypic diversity. We analysed nuptial colour variation within and among nine populations of southern pygmy perch (Nannoperca australis Günther) along an environmental light gradient. Within populations, larger males were redder and blacker, and better-condition males were blacker. Among populations, red colour was positively correlated with the amount of orange-red light present, suggesting that males are likely optimizing signal conspicuousness by producing proportionally larger and redder patches in broad spectrum environments with more orange-red light. Signal contrast, in this regard, is maximized when red colour, appearing bright because of the prevalence of red wavelengths, is viewed against the water-column background. Together, our results are concordant with the sensory drive hypothesis; selection favours signal adaptations or signal plasticity to ensure communication efficacy is maximized in different light environments.

Transgenerational marking of freshwater fishes with enriched stable isotopes: a tool for fisheries management and research

A promising new method of marking larval freshwater fishes with enriched stable isotopes by means of injecting the maternal parent with the marking agent was investigated. The (138)Ba:(137)Ba ratios in the otoliths of larval golden perch Macquaria ambigua were compared to determine the effect of injecting female broodstock with different dosages of enriched (137)Ba at various times before spawning. There was 100% mark success in the progeny of fish injected with 20 microg g(-1) of enriched (137)Ba 24 h before inducing spawning with hormones and 40 microg g(-1) administered at the same time as inducement of spawning. Injection of 40 microg g(-1) enriched (137)Ba 21 days before spawning resulted in only 81% mark success and suggests rapid elimination of barium in M. ambigua. Injection with enriched (137)Ba did not significantly affect the fertilization rate, number of fertilized eggs or hatching rate compared with long-term hatchery records. These results suggest that transgenerational marking is an effective and affordable means of mass-marking larval fishes. Thousands of larval fishes can be permanently marked with a unique artificial isotopic mark via a single injection into the maternal parent, thus avoiding the handling of individual fishes or having to deal with chemical baths. Because no single mark or tagging method is suitable for all situations, transgenerational marking with enriched stable isotopes provides another method for researchers and managers to discriminate both hatchery-reared and wild fishes.