Australian Bass Research Articles

Ontogenetic milestones of chemotactic behaviour reflect innate species-specific response to habitat cues in larval fish

The distribution and connectivity of marine populations are largely dependent on biophysical factors affecting pelagic larval dispersal between spawning at adult spawning sites and settlement to juvenile nursery habitats. Behaviour and swimming ability of pelagic larvae are increasingly understood to influence patterns of dispersal, but it is unclear which sensory cues are involved and when during ontogeny these abilities first develop. Here we studied the early ontogenetic development of responses to olfactory cues from coastal and estuarine waters in larvae of two temperate estuarine-associated fish species, Australian bass, Macquaria novemaculeata, and mulloway, Argyrosomus japonicus, to determine when olfaction begins to influence dispersal. Olfactory responses to habitat-associated cues were not present when larvae first transitioned from nonswimming to swimming (indicated by flexion of the notochord), but emerged after ca. 7 days in a species-specific manner that was consistent across different cohorts. Based on general additive models (GAMs), age (in days posthatch) best explained the ontogenetic pattern in both species. The emergence of chemotactic responses coincides with an exponential increase in swimming endurance reported for these species. This suggests the existence of ontogenetic milestones during larval development that, once reached, trigger active influence on dispersal. Salinity and pH did not influence choice behaviour after these ontogenetic milestones; however, the presence of cues generated by seagrass harvested from the estuary habitat elicited strong responses in fish larvae consistent with species-specific habitat preferences, indicating an important role for aquatic vegetation in driving these behaviours.

Is climate change driving recruitment failure in Australian bass Macquaria novemaculeata in southern latitudes of the species range?

Flow regimes have been fundamentally altered in many of the world’s river systems. There is a need to restore components of natural flow variability to protect freshwater biodiversity. The Australian bass is a long-lived, catadromous percichthyid endemic to coastal drainages of south-eastern Australia. Little is known of the timing and magnitude of flows considered important for recruitment and growth of individuals in southern latitudes of the species range. Herein we generate nearly 50 years of otolith-derived recruitment and growth histories for Australian bass and relate these to hydrologic, climatic and demographic variables. We found that younger Australian bass grew fastest in years following a cooler spring, whereas the growth of older fish exhibited little response to temperature. Australian bass year class strength was positively related to high flows in spring and in a negative curvilinear way to spring temperature. A lack of suitable natural flows in spring may be responsible for little to no recruitment of Australian bass in the Genoa River over the past two decades. Based on our findings, the continuing trend of lower rainfall and higher temperatures, coupled with river regulation in some systems in the region, will likely result in depressed juvenile growth and prolonged periods of recruitment failure over the medium to longer term, with significant implications for the viability of populations.

Behavioral interactions under multiple stressors: temperature and salinity mediate aggression between an invasive and a native fish

Interspecific aggression is a critical determinant of the success and competitive superiority of many invasive over native species. While single abiotic stressors can alter aggression levels, the manner in which multiple stressors may alter the strength and outcome of interspecific interactions and hence the invasion potential of a species is still poorly understood, even though multiple stressors are prevalent in many ecosystems. Furthermore, multiple stressors may interact to produce synergistic or antagonistic effects on individual level behaviors, thereby modulating invasive-native species interactions in unexpected ways. Here we examined the effect of two key abiotic stressors in freshwater ecosystems-temperature and salinity-on interspecific aggression between the invasive eastern mosquito fish (Gambusia holbrooki) and juveniles of the native Australian bass (Macquaria novemaculeata). Under controlled laboratory conditions, individuals were exposed to low or high salinity levels (15 and 35‰), and low or high temperatures (21 and 28 °C), and the frequency of interspecific aggressive behaviors was scored. The effect of temperature and salinity on interspecific aggression was antagonistic for both M. novemaculeata and G. holbrooki. While elevated temperature promoted aggression, elevated salinity partially or entirely negated this effect. Moreover, regardless of temperature, M. novemaculeata was more aggressive than G. holbrooki under elevated salinity. In addition to this, the native displayed more aggression to smaller than larger heterospecifics when exposed to elevated salinity alone, while G. holbrooki showed no such preference. These results highlight the importance of considering the interplay between multiple abiotic stressors and behavioral interactions between invasive and native species, combined with the modulating effect of species-specific and size based responses to those stressors.

Migration patterns and estuarine aggregations of a catadromous fish, Australian bass (Percalates novemaculeata) in a regulated river system

Catadromous fish species require adequate flows to migrate between fresh and saltwater habitats to reproduce. However, artificial barriers and flow alteration affect fish populations by reducing habitat connectivity and disrupting movement cues. In regulated rivers, it is critical that migratory flow requirements are quantified to optimise water allocation for multiple users. In the present study, we assessed the migratory timing, flow and estuarine aggregation requirements for Australian bass (Percalates novemaculeata). Over 2 years, 66 bass were tracked using an acoustic receiver array in the Logan River (Qld, Australia). Bass performed large-scale downstream movements in response to elevated winter flows (40 and 108m3 s–1), which facilitated migration to the lower estuary, where salinity conditions were appropriate for spawning. Bass migrations occurred only when gonads were mature, despite large flows providing opportunities for movement outside this period. Experimental flow releases from an impoundment (2.1m3 s–1) during winter did not elicit a migratory response. Connectivity between upstream and estuarine habitats was reduced by the presence of instream weirs, with downstream movement across weirs occurring only when sufficient flow magnitude was achieved (>76.1m3 s–1). These findings are relevant for water resource managers formulating environmental flow rules for catadromous fish species in systems with multiple instream artificial barriers.

Effects of fish and macrophytes on phytoplankton and zooplankton community structure in a subtropical freshwater reservoir

Excessive macrophyte biomass and cyanobacterial blooms associated with eutrophication and possibly exotic fish frequently compromise freshwater systems. In this study, 20 large (∼3.2 m 3 ), replicated enclosures were used to investigate the effects of piscivorous Australian bass ( Macquaria novemaculeata ), planktivorous gambusia ( Gambusia holbrooki (Girard)), benthivorous carp ( Cyprinus carpio ), and macrophyte removal on water quality, as well as trophic interactions within the enclosures. Fish effects on reservoir water quality were carp > gambusia > bass. Cryptomonads spp. (54,083 cells/mL) and Anabaena spp. (47,983 cells/mL) increased significantly (63 and 23 fold, respectively) in carp enclosures, possibly because of physiological adaptation to low light, high turbidity, total phosphorus concentrations (TP) and low TN: TP ratios (N-limitation); a consequence of carp benthic grazing. Carp and gambusia caused an unconventional shift from smaller to medium sized zooplankton (e.g. Boeckella sp., Bosmina meridionalis ), possibly a result of copepod nauplius grazing. In the subtropical system studied, fish-induced nutrient recycling appears more important to the outcome of bio-manipulation than grazing impacts. Macrophyte harvesting unexpectedly decreased phytoplankton biomass linked to declines in Euglenophyta and diatoms ( Asterionella spp.). Cyanophyta ( Oscillatoriales spp./ Anabaena spp.) increased in response to macrophyte harvesting and was consistent with findings on European lakes that Cyanophyta abundance tends to be higher in the absence of macrophytes. Results indicate exotic fish removal, nutrient loading control and macrophyte conservation is important in these aquatic systems to maintain high water quality.

Using otolith microchemistry to differentiate between stocked and unstocked Australian bass (Percalates novemaculeata)

Understanding the contribution of stocking to fish populations is an essential part of any program to increase fish numbers and abundance. However, subsequent assessments that aim to quantify the success of fish stocking are rarely considered. Juvenile-sized (<200mm) Australian bass (Percalates novemaculeata), from the upper Snowy River, southern New South Wales (NSW), Australia (assumed to be stocked), wild mature-sized (>400mm) fish from estuarine areas of the Snowy River, juveniles (15–30mm) from a commercial Australian bass hatchery facility known to have supplied fish for past stockings in the Snowy River, and wild fish (>400mm) considered as controls from the Clarence River in northern NSW were used for otolith microchemical analysis. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was used to measure Sr:Ca and Ba:Ca ratios at the otolith core and across radial transects to verify natal origins of fish and more specifically, to confirm the origin of fish caught in the river that were assumed to have originated from a recent stocking program. Otolith core Sr:Ca ratios of juvenile fish collected from the upper Snowy River and hatchery fish were not significantly different suggesting they originated from the same source. Significant differences were found in core Sr:Ca and Ba:Ca between known origin hatchery fish and wild fish. Significantly higher Sr:Ca ratios were found at otolith cores compared to the edges in fish collected from the upper Snowy River, suggesting early life was spent in more saline conditions prior to stocking compared to wild fish. Otolith transect data showed a consistent decrease in Sr:Ca ratios and a simultaneous increase in Ba:Ca ratios at approximately 17–59mm from the core, corresponding with the size at which these fish were stocked. This study has demonstrated that otolith microchemistry can successfully distinguish stocked fish from wild recruits and reveals the timing of natural migrations or artificial translocations.

Body size mediated effects of multiple abiotic stressors on the growth and social behaviour of an estuarine fish, Australian Bass (Macquaria novemaculeata)

Anthropogenic climate change is expected to result in dramatic shifts in the abiotic conditions within estuaries, including an elevation of temperature and salinity levels. Even so, few studies have addressed the impacts of multiple abiotic stressors on the behaviour and life history of key estuarine species, such as those of biological and commercial importance. Here we used a cross-factored experimental design to tease apart the effects of temperature and salinity on intraspecific aggression and growth rates of a native estuarine fish species, the Australian bass (Macquaria novemaculeata). Juvenile bass were exposed to one of four treatment conditions: 1) baseline temperature and salinity, 2) elevated temperature, 3) elevated salinity, and 4) elevated temperature and salinity. Elevated salinity increased rates of aggression, and elevated temperature decreased rates of growth, although the effects of both factors were mediated by the body size of individual bass. These results therefore highlight complex and variable effects of abiotic stressors and body size, emphasising the importance of considering individual-level attributes when evaluating the impacts of climate change on estuarine fishes.

Comparison of ELISA formats for detection of antibodies specific for nervous necrosis virus (Betanodavirus) in the serum of immunized barramundi Lates calcarifer and Australian bass Macquaria novemaculeata

Addressing the need for validated tests for the study of Nervous Necrosis Virus (NNV, Betanodavirus) immunity, ELISA protocols were developed, optimized and compared for detection of serum antibodies against the virus in vaccinated fish. Competitive and indirect ELISA were performed using several combinations of immunoreagents, pursuing the highest specific reactivity. Reference sera were obtained by immunizing and longitudinally sampling Australian bass Macquaria novemaculeata (n=10) and barramundi Lates calcarifer (n=5) with native viral or recombinant coat protein antigens. Optimal performance was achieved with an indirect sandwich ELISA using sheep anti-NNV antibodies and non-purified live NNV from cell culture supernatant as the capture system, and rabbit anti-fish immunoglobulin antibodies for the detection system. This optimized format had greater analytical sensitivity than competitive ELISA. An increase in reactivity of 3.5–8.7 times was detected post-immunization (P<0.001), and titres persisted for at least 28weeks. The assay discriminated non-specific reactions and was considered practical for routine use as it does not require purified virus or recombinant NNV capsid protein (rCP) as antigen. Moreover, the format was versatile as it could be adapted to a wider range of fish species. We believe that this optimized assay is suitable for the study of NNV vaccinal responses and has the potential to be used for NNV surveillance in subclinically infected fish populations after further validation.Statement of Relevance: The manuscript describes two new ELISA methods for the detection of anti-NNV antibodies in Australian bass and barramundi. NNV has been a limiting disease for the smooth operation of the aquaculture industry in Australia with Barramundi and Australian bass being two of the most relevant species to the disease. The methods are expected to provide screening tools to evaluate vaccination status in fish of 40g or more and evaluate the exposure status of broodstock with the potential use for vertical transmission control.

A multi-scale GIS and hydrodynamic modelling approach to fish passage assessment: Clarence and Shoalhaven Rivers, NSW Australia

Natural barriers such as waterfalls, cascades, rapids and riffles limit the dispersal and in-stream range of migratory fish, yet little is known of the interplay between these gradient dependent landforms, their hydraulic characteristics and flow rates that facilitate fish passage. The resurgence of dam construction in numerous river basins world-wide provides impetus to the development of robust techniques for assessment of the effects of downstream flow regime changes on natural fish passage barriers and associated consequences as to the length of rivers available to migratory species. This paper outlines a multi-scale technique for quantifying the relative magnitude of natural fish passage barriers in river systems and flow rates that facilitate passage by fish. First, a GIS-based approach is used to quantify channel gradients for the length of river or reach under investigation from a high resolution DEM, setting the magnitude of identified passage barriers in a longer context (tens to hundreds of km). Second, LiDAR, topographic and bathymetric survey-based hydrodynamic modelling is used to assess flow rates that can be regarded as facilitating passage across specific barriers identified by the river to reach scale gradient analysis. Examples of multi-scale approaches to fish passage assessment for flood-flow and low-flow passage issues are provided from the Clarence and Shoalhaven Rivers, NSW, Australia. In these river systems, passive acoustic telemetry data on actual movements and migrations by Australian bass (Macquaria novemaculeata) provide a means of validating modelled assessments of flow rates associated with successful fish passage across natural barriers. Analysis of actual fish movements across passage barriers in these river systems indicates that two dimensional hydraulic modelling can usefully quantify flow rates associated with the facilitation of fish passage across natural barriers by a majority of individual fishes for use in management decisions regarding environmental or instream flows.

Facilitating upstream passage of small-bodied fishes: linking the thermal dependence of swimming ability to culvert design

Fish passage through road culverts is poorly understood, particularly for small-bodied fishes, despite this information being integral to the restoration of waterway connectivity. We assessed the prolonged swimming performance of a small-bodied fish, empire gudgeon (Hypseleotris compressa; 3.2–7.7 cm total length, TL), and juvenile Australian bass (Percalates novemaculeata; 3.5–7.8 cm TL). Swimming trials were conducted in a hydraulic flume across a range of fixed and increasing velocities in response to acute and long-term thermal treatments. A new statistical approach (Tobit analysis) was used to relate the thermal dependence of swimming endurance to hydraulic characteristics of culverts, providing estimates of maximum water velocity allowing upstream fish passage. Reductions in water temperature of 10°C, similar to those caused by cold-water releases from dams, significantly impaired critical swimming speeds of both species. Traversable water-velocity models identified H. compressa as a weak swimmer, requiring very low water velocities (≤0.10 m s–1 or 2.86 body lengths (BL) s–1) for unrestricted passage, whereas P. novemaculeata was predicted to traverse water velocities of ≤0.39 m s–1 or 12.12 BL s–1. Culvert designs can be improved by limiting water velocities to accommodate weak-swimming fishes and by accounting for the thermal sensitivity of swimming performance.

Effects of carp, gambusia, and Australian bass on water quality in a subtropical freshwater reservoir

Freshwater systems are frequently compromised by excessive macrophyte biomass and cyanobacterial blooms associated with eutrophication and possibly exotic fish. The effects of piscivorous Australian bass (Macquaria novemaculeata), planktivorous gambusia (Gambusia holbrooki [Girard]), and benthivorous carp (Cyprinus carpio L) on water quality in Emigrant Creek Dam (ECD), Australia, were studied in 20 experimental enclosures (depth 1 m, volume 3.2 m3), stocked at a density of 1875 kg/ha. Fish additions had immediate effects on water quality, except for bass, which merely recycled preexisting water column nutrients. Water turbidity, total suspended solids (TSS), total nitrogen (TN) and total phosphorus (TP) concentrations, and chlorophyll a (Chl-a), were all significantly higher in gambusia, but more so in carp enclosures. Assuming a linear relationship, mean daily Chl-a increases in carp enclosures were 4.6 μg/L/100 kg carp/ha. Moreover, Chl-a (>90 μg/L) was positively correlated with TP (r2 = 0.91), TN (r2 = 0.89), TFe (r2 = 0.96), and TMn (r2 = 0.83) in carp enclosures; however, Fe and Mn resuspension from sediments does not seem to be a principal mechanism affecting total phytoplankton biomass. Macrophyte, epiphyte, and periphyton biomass were also significantly lower in carp enclosures, a consequence of sediment and nutrient resuspension, dietary excretion, and macrophyte damage. In the absence of carp, epiphytes, periphyton, and macrophytes competed with phytoplankton for available nutrients, providing a critical stabilizing role. Exotic fish removal may prove to be an effective management tool in subtropical systems to maintain high water quality.

Thermal acclimation of interactions: differential responses to temperature change alter predator–prey relationship

Different species respond differently to environmental change so that species interactions cannot be predicted from single-species performance curves. We tested the hypothesis that interspecific difference in the capacity for thermal acclimation modulates predator-prey interactions. Acclimation of locomotor performance in a predator (Australian bass, Macquaria novemaculeata) was qualitatively different to that of its prey (eastern mosquitofish, Gambusia holbrooki). Warm (25°C) acclimated bass made more attacks than cold (15°C) acclimated fish regardless of acute test temperatures (10-30°C), and greater frequency of attacks was associated with increased prey capture success. However, the number of attacks declined at the highest test temperature (30°C). Interestingly, escape speeds of mosquitofish during predation trials were greater than burst speeds measured in a swimming arena, whereas attack speeds of bass were lower than burst speeds. As a result, escape speeds of mosquitofish were greater at warm temperatures (25°C and 30°C) than attack speeds of bass. The decline in the number of attacks and the increase in escape speed of prey means that predation pressure decreases at high temperatures. We show that differential thermal responses affect species interactions even at temperatures that are within thermal tolerance ranges. This thermal sensitivity of predator-prey interactions can be a mechanism by which global warming affects ecological communities.

Estimating the stocking potential of fish in impoundments by modelling supply and steady-state demand

SUMMARY 1. Fish stocking is an increasingly common management tool for freshwater and marine environments and is often used to create and maintain fisheries in closed waters. The densities at which fish are stocked can have a large impact on a stocking programme’s success and sustainability. Stocking densities in impoundment sport-fisheries, for example, are often based on social or practical factors, and ecologically based stocking models are needed to assist the selection of stocking densities that are appropriate for the environment. 2. In this study, stocking density is calculated with a numerical model that balances the supply of prey production with the energetic demand of stocked fish. The model aims to deliver outcomes over a range of potential management objectives, by providing specific consumption scenarios that represent the trade-off between population abundance and individual body size in stocked fisheries. 3. The model uses a steady-state population approach to calculate stocking density, which optimises population consumption by maintaining a consistent biomass distribution and encourages sustainable stocking by considering the energetic needs of all cohorts. Carrying capacity is represented by the steady-state stocking density under a minimum consumption scenario (when fish meet only their minimum energetic needs). The comparison between a desired consumption rate and the existing level of production is used to assess the status or ‘health’ of the existing population and is used to determine whether stocking can occur and whether stocking densities can be sustainably increased. The probability of incorrectly assuming populations are achieving a given consumption level is also estimated, which is an ideal approach for interpreting multiple probability distributions. 4. A Monte-Carlo analysis of uncertainty was used to provide a probability distribution of stocking density of Australian bass (Macquaria novemaculeata) in three Australian impoundments under various seasonal and consumption scenarios. The likely consumption rates of the existing populations were determined using historical stocking densities, which showed that the three populations were of reasonable health, although one impoundment may be overstocked. The steady-state stocking densities depended on the desired consumption rate, and there was an eightfold difference in the stocking density aimed at providing large ‘trophy’ fish and the density required to reach carrying capacity. 5. Model outputs of existing abundance and biomass density agreed with empirical estimates of abundance and relative density in these impoundments, which provides support to the model’s

Critical Thermal Minima of age‐0 Australian bass, Macquaria novemaculeata, fingerlings: implications for stocking programmes

Abstract Fishes are often stocked outside natural distribution ranges with inadequate information on target streams, particularly thermal regimes. Australian bass, Macquaria novemaculeata (Steindachner), is a catadromous species that is regularly stocked into upland reaches of rivers and impoundments in south‐eastern Australia. Critical Thermal Minima (CTMin) were determined for age‐0 Australian bass fingerlings with a mean fork length of 64.4 ± 0.4 mm and weighing 3.8 ± 0.8 g. Four treatments were used, including three replicate aquaria for each treatment. Fingerlings were acclimated at either 8 or 15 °C at densities of 15 fish in 56‐L glass aquaria. Water temperatures were then decreased at either 1 °C day−1 or 1 °C h−1 until loss of equilibrium (LOE), which occurred between 3 and 7 °C. Mean CTMin among treatments was 3.22–4.64 °C and was influenced by acclimation temperature and rate of temperature decline. Fingerlings acclimated at 8 °C subjected to a temperature decline of 1 °C h−1 experienced highest LOE temperature. Post‐LOE mortality among treatments was highest at 100% in the 8 °C acclimation with a 1 °C day−1 temperature decline. Mortalities following LOE occurred within 5 days. The results suggest that stocking age‐0 Australian bass is unlikely to be successful in areas where winter temperatures fall below 6 °C.

MAGNITUDE, FREQUENCY AND DURATION OF INSTREAM FLOWS TO STIMULATE AND FACILITATE CATADROMOUS FISH MIGRATIONS: AUSTRALIAN BASS (MACQUARIA NOVEMACULEATA PERCIFORMES, PERCICHTHYIDAE)

ABSTRACTThe 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 &amp; Sons, Ltd.

A hybrid zone and bidirectional introgression between two catadromous species: Australian bassMacquaria novemaculeataand estuary perchMacquaria colonorum

The presence and distribution of hybrid individuals and the existence of a hybrid zone between the catadromous Australian bass Macquaria novemaculeata and estuary perch Macquaria colonorum were investigated throughout the range of both species in Australia. Bayesian analyses and genotypic simulations identified 140 putative hybrids (11·5% of the total sample) with varying levels of introgression. Most hybrids were observed in an area extending from the Snowy River to the Albert River suggesting a hybrid zone in the eastern Bass Strait region. Sixteen hybrids, however, were found outside this zone, possibly reflecting the movement of hybrid offspring between estuaries or their inadvertent release during fish stocking programmes. Biparental backcrossing was found to occur suggesting that hybrids were fertile. These results have implications for the management of the extensive stocking programme in M. novemaculeata and for understanding the potential role of habitat degradation and reduced water flow in facilitating hybridization in species with migratory life histories.

Seasonal residency and movement patterns of two co‐occurring catadromous percichthyids within a south‐eastern Australian river

Abstract – Understanding movement patterns and habitat utilisation is critical for the management of diadromous fishes. An acoustic telemetry array was used to monitor 33 estuary perch, Macquaria colonorum and 39 Australian bass, Macquaria novemaculeata in the freshwater and estuarine reaches on the Shoalhaven River, south‐east Australia. On average, tagged M. novemaculeata were detected for a considerably shorter period than M. colonorum, and evidence suggested that fishing pressure may have impacted on their survival. Macquaria colonorum displayed significant shifts in seasonal and size‐related habitat use, with fish predominantly residing in deep (&gt;5 m) areas within the middle (mesohaline) reaches of the estuary during the austral spring to autumn months. In winter, M. colonorum individuals made frequent downstream migrations, often to localised areas, within the lower estuary (LE). In contrast, M. novemaculeata were distributed in shallow (&lt;2 m) habitats throughout the year, within the upper (oligohaline) estuarine reaches of the river, as well as in fresh water. Like M. colonorum, M. novemaculeata made extensive downstream and upstream movements, often coincident with reproductive behaviour, water temperature and increased freshwater inflows. It is postulated that the high site fidelity and repetitive homing displayed by both species is influenced by ontogenetic behaviour and prey availability. Furthermore, the extent of instream distribution by both species, and the lack of observed annual spawning migrations by some M. novemaculeata individuals, indicates the once considered ‘catadromous’ life cycle of these fishes may not be obligatory. A management approach is recommended to ensure that both these species are not over‐exploited within a portion of their instream range, thus maintaining their full reproductive potential.

Effects of barotrauma and mitigation methods on released Australian bass Macquaria novemaculeata

The effects of barotrauma on the short-term mortality and physical condition of Australian bass Macquaria novemaculeata were investigated after being: (1) conventionally angled and released during two tournaments in deep impoundments and (2) released untreated or subjected to venting or recompression during a manipulative experiment. All fish were released into two 20 m deep bathy-cages and monitored for 3 days. Of 238 M. novemaculeata angled during the tournaments, 43 (18·1%) had clinical signs of barotrauma or were vented and five of these later died (11·6% mortality). Catch histories varied significantly between both barotrauma and non-barotrauma fish and tournaments, but only hook ingestion significantly influenced mortality (P < 0·05). During the manipulative experiment, venting significantly influenced mortality (13·3%) compared to no treatment or recompression (no deaths). Magnetic resonance images and dissections of barotrauma fish indicated large variation among clinical signs. On the basis of these results, wherever possible M. novemaculeata suffering barotrauma should be immediately released with no treatment. Fish that are unable to resubmerge should be recompressed, while those held in live wells and released in shallow water should be vented.

Generalist niche, specialist strategy: the diet of an Australian percichthyid

Dietary analysis revealed that an impoundment population of Australian bass Macquaria novemaculeata holds a generalist niche, but one arising from persistent individual specialization and interindividual variation. This 'individual specialist' strategy appeared adaptive, but the strength of individual specialization was largely independent of variation in diet composition, except during blooms of Daphnia sp. Diet composition and dietary overlap showed only moderate ontogenetic variation, and niche breadth showed no relationship with ontogeny. Macquaria novemaculeata showed an asymmetric predator and prey size distribution, consistent with many aquatic predators, with positive relationships between fish size and average, maximum and minimum prey size. There was no asymmetry in the relative size-based niche breadths of individuals, however, which indicates that the niche is a fixed 'window' of relative prey sizes. The difference in the dietary niche and prey-size relationships of M. novemaculeata at the population and individual levels highlights the necessity of assessing the niche at both these levels.

Optimising chemical marking techniques for Australian bass, Macquaria novemaculeata, fry and fingerlings prior to restocking

Many stocking programs are performed without any subsequent assessments to determine the survival of the stocked fish, often due to the difficulties in distinguishing stocked from wild fish. A series of optimisation trials were undertaken to determine the suitability of calcein (2,4-bis-[N,N′–di(carbomethyl)-aminomethyl]fluorescein) for chemically marking hatchery-reared Australian bass, Macquaria novemaculeata (Steindachner), fry and fingerlings. The technique was optimised by trialling immersion times for salt (2.5 or 5 min), calcein concentrations (0.5 or 1%) and calcein immersion times (5 or 10 min). A general-purpose modulated probe fluorometer was used as a non-lethal detection method to establish mark retention and distinguish between calcein-marked and unmarked fish. Five minutes of saline immersion, followed by 10 min of calcein immersion at a calcein concentration of 1% was the optimal marking method. Growth and survival of fish was not affected as a result of any of the calcein-marking methods. It is therefore effective to calcein-mark Australian bass fry and fingerlings before stocking and to non-lethally detect marked individuals using a field-portable meter to assist in the assessment of stocking programs. Findings from this study may also reflect the potential use of calcein to chemically mark fry and fingerlings of other fish species.