Multi-decadal trends in large-bodied fish populations in the New South Wales Murray–Darling Basin, Australia

Megalocytiviruses cause high mortality diseases that have seriously impacted aquaculture, with the most frequent outbreaks occurring in East and South-East Asia. The international trade of juvenile fish for food and ornamental aquaculture has aided the spread of these viruses, which have spread to Europe and Australia and other regions. Australian freshwater fishes were examined for susceptibility to infection with the exotic megalocytivirus, dwarf gourami iridovirus (DGIV), which belongs to a group with the type species, Infectious spleen and kidney necrosis virus (ISKNV). Fish were held at 23±1°C and challenged by intraperitoneal (IP) injection or by cohabitation with Murray cod, Maccullochella peelii (Mitchell) infected with DGIV. A species was deemed to be susceptible to DGIV based on evidence of viral replication, as determined by qPCR, and megalocytic inclusion bodies observed histologically. Horizontal transmission occurred between infected Murray cod and golden perch, Macquaria ambigua (Richardson), Macquarie perch, Macquaria australasica (Cuvier) and Murray cod. This indicated that DGIV shed from infected fish held at 23°C can survive in fresh water and subsequently infect these naïve fish. Further, DGIV administered IP was highly pathogenic to golden perch, Macquarie perch and Murray cod. Compared to these species, the susceptibility of southern pygmy perch, Nannoperca australis (Gunther) was lower. Freshwater catfish (dewfish), Tandanus tandanus (Mitchell), were not susceptible under the experimental conditions based on the absence of clinical disease, mortality and virus replication. This study showed the potential risks associated with naïve and DGIV-infected fish sharing a common water source.

Regionalisations based on species assemblages are a useful framework for characterising ecological communities and revealing patterns in the environment. In the present study, multivariate analyses are used to discern large-scale patterns in fish assemblages in the Murray–Darling Basin, based on information from the Murray–Darling Basin Authority’s first Sustainable Rivers Audit (SRA), conducted in 2004–2007. The Basin is classified into nine regions with similar historical fish assemblages (i.e. without major human intervention), using data that combine expert opinion, museum collections and historical records. These regions are (1) Darling Basin Plains, (2) Northern Uplands, (3) Murray Basin Plains, (4) Northern Alps, (5) Central East, (6) Avoca Lowland, (7) Southern Slopes, (8) Southern Alps and (9) South-Western Slopes. Associations between assemblages and physical variables (catchment area, elevation, hydrology, precipitation, temperature) are identified and used to reinforce the definitions of regions. Sustainable Rivers Audit data are compared with the historical assemblages, highlighting species whose range and abundance have changed since the early 19th century. Notable changes include declines in native species such as silver perch, river blackfish, mountain galaxias, Macquarie perch, trout cod and freshwater catfish, and the advent of alien species including common carp, eastern gambusia, goldfish, redfin perch, brown trout and rainbow trout. Less significant declines are evident for native carp gudgeons, golden perch, two-spined blackfish, bony herring and flathead gudgeon. Changes are evident even in regions where habitats have been little disturbed in the past 200 years.

Recreational fishery management aims to prevent species decline and provide sustainable fisheries. Overfishing has been frequently suggested as a cause of historic fishery declines within the Murray–Darling Basin (MDB), Australia, but there have been few quantitative surveys for providing fishery-dependent data to gauge status. The Murray Cod Maccullochella peelii and the Golden Perch Macquaria ambigua are important species targeted by recreational fishers across the MDB. The fisheries are controlled by size and bag limits and gear restrictions (both species) as well as a closed season (Murray Cod only). A complemented fisher survey design was used to assess the recreational fishery for both species in a 76-km reach of the Murrumbidgee River in 2012–2013. Progressive counts were used to quantify boat- and shore-based fishing effort. Catch and harvest rate information was obtained from shore-based fishers via roving surveys and from boat-based fishers via bus route surveys. Murray Cod catch rates (fish/angler-hour) were 0.228 ± 0.047 (mean ± SE; boat based) and 0.092 ± 0.023 (shore based), and harvest rates (fish/angler-hour) were 0.013 ± 0.006 (boat based) and 0.003 ± 0.001 (shore based). Golden Perch catch rates were 0.018 ± 0.009 (shore based) and 0.002 ± 0.001 (boat based), and harvest rates were 0.006 ± 0.002 (shore based) and 0.001 ± <0.001 (boat based). The Murray Cod fishery had maximal catch and harvest during the 5-month period after the closed season ended. The closed season aims to protect spawning Murray Cod, but this strategy's effectiveness may have been influenced by high fishing effort and deliberate bycatch during the closure period. To sustain and improve these MDB fisheries, we suggest quantification of catch-and-release impacts on spawning Murray Cod, provision of fish passage, re-stocking of Golden Perch, and education on fishing techniques that minimize Murray Cod bycatch during the closed season. Received October 22, 2014; accepted March 6, 2015

Results of a fish tagging study carried out in the Murray River, South Australia from 1974 to 1978, are presented. Of 14 333 fish tagged, 1276 were recaptured and the records of these recaptures yield evidence on the movements of several species of both commercial and recreational interest. Some golden perch (Macquaria ambigua) migrated extensively upstream, many moving more than 1000 km. The migration followed, and appeared to be triggered by, a rise in water level at the onset of major flooding. The upstream migrations, made only by mature fish, appear to be a reproduction strategy to ensure that the eggs, which are buoyant, are distributed downstream. In contrast, common carp (Cyprinus carpio), which lay adhesive, demersal eggs, did not migrate but made only random, short- distance movements. Although data on other species were fewer due to their smaller populations, it appears that Murray cod (Maccullochella peeli) and catfish (Tandanus tandanus), both of which have demersal eggs, show similar movement patterns to carp whereas silver perch (Bidyanus bidyanus), which lays buoyant eggs, is similar to golden perch. The implications of these results, with regard to the establishment and maintenance of populations of each of the species, are discussed.

Temperature is essential to the maintenance of optimal physiological functioning in aquatic organisms. Fish can manage natural fluctuations in temperature; however, in freshwater ecosystems acute and rapid temperature changes can originate from sources such as large dams and industrial effluents. These rapid temperature changes may induce several physiological and behavioural responses that can result in lethal and sub-lethal consequences. The present study assessed immediate sub-lethal and short-term (10days) lethal responses of three species of Australian freshwater fish larvae and early-stage juveniles to a range of different 'field-relevant' cold shocks (-4, -6, -8 and -10°C). Murray cod (Maccullochella peelii), silver perch (Bidyanus bidyanus) and golden perch (Macquaria ambigua) were tested at two age groups to elucidate the interaction between ontogeny and sensitivity to cold shock. Cold shock caused mortality and reductions in swimming ability (time to exhaustion and lines crossed) in all species of fish at both age groups. Sensitivity was correlated to the magnitude of cold shock; a 10°C drop in temperature caused the highest mortalities. Ontogeny interacted with the severity of cold shock; the younger fish experienced higher mortalities and greater impairment to swimming ability. This study demonstrates the potential lethal and sub-lethal impacts of cold shock on freshwater fish at a critical life-history stage. Understanding the impacts of cold shock will aid management of freshwater ecosystems for the benefit of fish populations, with the current study identifying critical life stages to be considered in remediation and guiding thresholds necessary to reduce the impact of cold shock on native fish populations.

Common names: Macquarie perch, white-eye, silver-eye, mountain perch, bream. Conservation status: Endangered (ANZECC 1999). Identification: D VIII–XII, I + 11–14, A III + 8–11, P 14–17, V I + 5, LL 42–60, gill rakers 17–21, vertebrae 28–31. Moderate-sized percoid fish (max. 460 mm, 3.5 kg), almost black or dark silvery grey to light or off-white silvery grey in colour, eye large, jaws equal (Harris & Rowland 1996). The species status is unresolved since investigations indicate that there are putatively two species (‘eastern’ and ‘western’) of Macquarie perch separated by the Great Dividing Range and possibly two sub-species within the ‘eastern’ species. Distribution: Formally widespread throughout the more southerly tributaries of the Murray-Darling River system of south-east Australia, as well as the Hawkesbury and Shoalhaven River systems on the eastern seaboard (Cadwallader 1981, Harris & Rowland 1996). Current distribution is fragmented and only small discrete populations remain. ‘Western’ populations are restricted to the upper reaches of some tributaries of the Murray-Darling River system, in particular, the Mitta Mitta River system above Darmouth Dam, the Goulburn, Murrumbidgee and Lachlan river systems, and one translocated population in the Yarra River. ‘Eastern’ populations are confined to the Shoalhaven and Hawkesbury river systems (Ingram et al. 1990, Harris & Rowland 1996). Abundance: No reliable population estimates exist for either the ‘eastern’ or ‘western’ forms. Habitat and ecology: Macquarie perch occupy a range of habitats from riffle and boulder areas in relatively small upland streams to larger deeper rivers with sand and clay substrates and submerged timber and snags as well as impounded waters. Their diet includes aquatic invertebrates, especially insects and crustaceans. Reproduction: Spawning, which occurs in shallow upland streams, is apparently triggered by increasing daylength and water temperatures and occurs in October to late December when temperatures range between 14◦C and 18◦C. Spawning Macquarie perch (‘western’ form) are oviparous and the demersal and slightly adhesive eggs (2.0–4.0 mm in diameter) appear to be shed into the water upstream of riffle areas. Hatching commences approximately five days after fertilisation and continues for up to six days (at water temperatures of 18–20◦C). Newly-hatched embryos are approximately 4.5–6.5 mm in length. Feeding commences on zooplankton about 3–5 days after hatching is completed (Gooley 1986, Ingram et al. 1994). Threats: A general degradation of the riverine environment caused by a number of contributing factors including modifications to rivers for hydro-electric, flood mitigation and irrigation schemes, increased pollution from domestic, agricultural and industrial sources, sedimentation, over-fishing, and impacts of introduced species, such as competition, predation and spread of disease (Cadwallader 1978, Ingram et al. 1990). Conservation action: Total bans on the capture of Macquarie perch exist in the Australian Capital Territory (ACT) and New South Wales (NSW). The capture, keeping and trading of Macquarie perch is controlled by legislation in Victoria. Captive breeding programs have been undertaken at government fish hatcheries in NSW and Victoria to produce and release juveniles of the ‘western’ form into selected sites within its former range to re-establish natural populations (Gooley 1986, Ingram et al. 1990). However, these programs have ceased due to difficulties associated with inducing spawning in both captive broodfish and mature fish in spawning condition captured from the wild. Between 1986 and 1997 approximately 456 000 juveniles were released into nine sites throughout the Murray-Darling basin. Early results indicate survival at some of these sites only. Attempts to re-establish populations of the ‘western’ form by translocation of mature fish have occurred in Victoria, New South Wales and the Australian Capital Territory (ACT Government 1999) with mixed success (Lintermans 2000). Conservation recommendations: Improve state and federal legislation to further protect Macquarie perch and preserve and rehabilitate riverine habitats where both existing and re-introduced (stocked) populations occur. Much basic biological information still needs to be collected for both ‘eastern’ and ‘western’ forms including resolution of species status, habitat requirements, life history patterns in the wild and surveys of existing populations to determine size, distribution and breeding status. Remarks: Since the status of Macquarie perch was upgraded to endangered, fisheries regulations have increased the level of protection from recreational fishing in Victoria, and a three year project has been instigated to investigate the Lake Dartmouth population.

The Murray Cod Maccullochella peelii and Golden Perch Macquaria ambigua are important recreational species in Australia's Murray–Darling Basin (MDB); both species have declined substantially, but recovery is evident in some areas. Minimum length limits (MLLs)—implemented to ensure fish could spawn at least once prior to harvest eligibility—have increased three times in the past decade. We quantified variation in length at 50% maturity (LM50), age at 50% maturity (AM50), and von Bertalanffy growth parameters (k = Brody growth coefficient; L∞ = asymptotic length; t0 = theoretical age at zero length) of these species within two rivers and two reservoirs of the MDB; to investigate whether fish length is a suitable surrogate for AM50 in setting MLLs. Between 2006 and 2013, we collected 1,118 Murray Cod and 1,742 Golden Perch by electrofishing and gillnetting. Values of k and L∞ were greater for reservoir fish than for riverine fish. For both species, AM50 was generally greater in rivers than in reservoirs; for Murray Cod, LM50 was greater in reservoirs than in rivers. A yield-per-recruit model demonstrated that smaller Murray Cod MLLs would be required for rivers and that an MLL at or below 600 mm (the existing MLL) across all populations could lead to overfishing in some systems. The differences in growth rate and the onset of reproductive maturation between riverine and reservoir populations suggest that system-specific regulations would be more effective at reducing the overfishing risk and meeting fishing quality objectives. Received August 18, 2014; accepted September 10, 2015

Investigations into the feeding of the early stages of fishes can provide insights into processes influencing recruitment. In this study, we examined ontogenetic changes in morphology and feeding behaviour of two native Australian freshwater species, Murray cod, Maccullochella peelii peelii, and golden perch, Macquaria ambigua, and the alien species, common carp, Cyprinus carpio. Murray cod free embryos are large and well developed at the onset of feeding, whereas the other two species begin exogenous feeding much younger and are smaller and less-developed. Carp commence exogenous feeding 3 days earlier than golden perch, and show more advanced development of the eyes and ingestive apparatus. We conducted feeding experiments, presenting larvae of the three species with a standardised prey mix (comprising equal numbers of small calanoid copepods, large calanoid copepods, small Daphnia, and large Daphnia). Larvae of most tested ages and species showed a preference for mid-sized prey (300–500 µm wide). This was true even when their gapes substantially exceeded the largest prey offered. Daphnia were consumed more than similar-sized copepods. The results of this study suggest that survival through their larval period will be threatened in all three species if catchable prey <500 µm in width are not available throughout such time. They also suggest that interspecific competition for prey may occur, especially when larvae are very young. The precocious development of structures involved in feeding and the extended transition from endogenous to exogenous feeding of early carp larvae are likely to have contributed to the success of this species since its introduction to Australia.

Hypoxic or oxygen-free zones are linked to large-scale mortalities of fauna in aquatic environments. Studies investigating the hypoxia tolerance of fish are limited and focused on marine species and short-term exposure. However, there has been minimal effort to understand the implications of long-term exposure on fish and their ability to acclimate. To test the effects of long-term exposure (months) of fish to hypoxia we devised a novel method to control the level of available oxygen. Juvenile golden perch (Macquaria ambigua ambigua), and silver perch (Bidyanus bidyanus), two key native species found within the Murray Darling Basin, Australia, were exposed to different temperatures (20, 24 and 28°C) combined with normoxic (6-8mgO2L-1 or 12-14kPa) and hypoxic (3-4mgO2L-1 or 7-9kPa) conditions. After 10months, fish were placed in individual respirometry chambers to measure standard and maximum metabolic rate (SMR and MMR), absolute aerobic scope (AAS) and hypoxia tolerance. Golden perch had a much higher tolerance to hypoxia exposure than silver perch, as most silver perch died after only 1month exposure. Golden perch acclimated to hypoxia had reduced MMR at 20 and 28°C, but there was no change to SMR. Long-term exposure to hypoxia improved the tolerance of golden perch to hypoxia, compared to individuals held under normoxic conditions suggesting that golden perch can acclimate to levels around 3mgO2 L-1 (kPa~7) and lower. The contrasting tolerance of two sympatric fish species to hypoxia highlights our lack of understanding of how hypoxia effects fish after long-term exposure.

ABSTRACTFish population dynamics are influenced by intrinsic and environmental drivers across multiple spatial and temporal scales. A thorough understanding of these drivers is essential for maintaining fish recruitment in flow‐regulated rivers. In the Murray–Darling Basin (MDB) in Australia, golden perch (Macquaria ambigua) are an iconic species with a life history characterised by irregular, strong recruitment of year classes. In‐channel flow pulses and overbank flows are important for spawning and recruitment; however, the drivers of fluctuations in golden perch recruitment have not been sufficiently quantified to allow for full operationalisation into river and fishery management. We used long‐term standardised electrofishing data to model relationships between the relative abundance of young‐of‐the‐year (YOY) golden perch with large‐scale climate indices, local river hydrology and temperature, and river/fishery management actions. While consistent recruitment was observed in only five rivers, there were strong, positive associations between the abundance of YOY golden perch and two broadscale climatic drivers (Australian Monsoonal Index and total rainfall across the northern MDB). The driver of these relationships is likely to be the effects of climate on local river discharge and temperature. YOY abundance increased with temperature and generally increased with river discharge to an optimum before declining at a very high discharge. We also found positive but variable effects of stocking, suggesting that stocking of fish can augment natural populations but that outcomes are spatially and temporally inconsistent. Our results have the potential to enable proactive management targeted towards supporting the hydrological conditions necessary for self‐sustaining golden perch populations.

Karyotypic data from Australian native freshwater fishes are scarce, having been described from relatively few species. Golden perch (Macquaria ambigua) and Murray cod (Maccullochella peelii) are two large-bodied freshwater fish species native to Australia with significant indigenous, cultural, recreational and commercial value. The arid landscape over much of these fishes’ range, coupled with the boom and bust hydrology of their habitat, means that these species have potential to provide useful evolutionary insights, such as karyotypes and sex chromosome evolution in vertebrates. Here we applied standard and molecular cytogenetic techniques to characterise karyotypes for golden perch and Murray cod. Both species have a diploid chromosome number 2n = 48 and a male heterogametic sex chromosome system (XX/XY). While the karyotype of golden perch is composed exclusively of acrocentric chromosomes, the karyotype of Murray cod consists of two submetacentric and 46 subtelocentric/acrocentric chromosomes. We have identified variable accumulation of repetitive sequences (AAT)10 and (CGG)10 along with diverse methylation patterns, especially on the sex chromosomes in both species. Our study provides a baseline for future cytogenetic analyses of other Australian freshwater fishes, especially species from the family Percichthyidae, to better understand their genome and sex chromosome evolution.

Stock enhancement is a management tool used for fishery recovery worldwide, yet the success of many stocking programs remains unquantified. Murray cod (Maccullochella peelii) and golden perch (Macquaria ambigua) are important Australian recreational target species that have experienced widespread decline. Stocking of these species has been undertaken for decades, with limited assessment of effectiveness. A batch marking and recapture approach was applied to assess stocked Murray cod and golden perch survival, contributions to wild fisheries, and condition in rivers and impoundments. Stocked fish were marked with calcein. Marked fish were detected during surveys undertaken 3 years and 10 months from initial marking, and it is probable that marks will persist beyond this time. The proportion of calcein marked fish in the population sub-sample whose age was equal to, or less than, the number of years since release, varied by 7–94% for Murray cod, and 9–98% for golden perch. Higher proportions of marked fish were found in impoundments than rivers. Marked Murray cod had significantly steeper length–weight relationships (i.e. higher weight at a given length) to unmarked fish. Our results show that application of methods for discriminating stocked and wild fish provides critical information for the development of adaptive, location-specific stocking strategies.

Five species of freshwater fish that are native to Australia and are currently used for aquaculture or are being considered for future development as cultured species – golden perch (Macquaria ambigua), silver perch (Bidyanus bidyanus), spangled perch (Leipotherapon unicolor), Murray cod (Maccullochella peeli), and dewfish (Tandanus tandanus) – were sampled for cellulose acetate gel electrophoresis. Baseline data on enzyme loci were produced to detect which ones have potential use as gene markers. Twenty-six enzymes were screened for each of the five species, and the specific electrophoretic running conditions resolved 40 enzyme loci. Three types of tissue that did not require destructive sampling were examined (fin tissue, white muscle, and body mucus) to assess whether the products of enzyme loci could be detected. Some of the enzyme loci described from liver, white muscle, and eye tissues were resolved from each of these three tissues that can be nondestructively sampled. Thus, these tissues c...

We conducted an acoustic telemetry study of native golden perch (Macquaria ambigua) to examine movement behaviour in areas affected by methane seeps and hypoxia in the intermittent Condamine River, Murray‐Darling Basin (MDB), Australia. Fish were collected during periods of no flow and hypoxia (dissolved oxygen [DO] &lt;1 mg/L). Despite these conditions, 38 of 43 fish tagged with acoustic transmitters were detected for &gt;3 months post‐tagging in the study reach and 27 fish were being detected after 14 months. During periods of elevated river flow and relatively high DO, 30 fish moved away from their original tagging locations, with three undertaking movements (&gt;7 km) outside the study reach and not returning. Generalised additive mixed models showed a significant increase in the probability of movement as soon as flow commenced and when water temperatures exceeded 19°C. As flows receded, most fish that had moved exhibited accurate homing behaviour to their original tagging location. The patterns of movement and site fidelity exhibited by golden perch correspond with previous studies of the species in intermittent rivers not affected by methane seeps and severe hypoxia, suggesting that the methane seeps and hypoxia did not inhibit fish movement nor render the affected habitats unsuitable for habitation. Golden perch can survive and remain active in water with much lower DO (&lt;1 mg/L) than previously described for large‐bodied native fishes in the MDB. However, fish condition in the study reach was slightly lower than other regions of the MDB, providing preliminary evidence that fish residing in habitats affected by chronic hypoxia and methane seepage may experience sub‐lethal stress. Our results demonstrate the importance of field‐based data on the behavioural and physiological responses of fish to chronic hypoxia and methane exposure to guide appropriate management responses.

Populations that are adaptively divergent but maintain high gene flow may have greater resilience to environmental change as gene flow allows the spread of alleles that have already been tested elsewhere. In addition, populations naturally subjected to ecological disturbance may already hold resilience to future environmental change. Confirming this necessitates ecological genomic studies of high dispersal, generalist species. Here we perform one such study on golden perch (Macquaria ambigua) in the Murray-Darling Basin (MDB), Australia, using a genome-wide SNP data set. The MDB spans across arid to wet and temperate to subtropical environments, with low to high ecological disturbance in the form of low to high hydrological variability. We found high gene flow across the basin and three populations with low neutral differentiation. Genotype-environment association analyses detected adaptive divergence predominantly linked to an arid region with highly variable riverine flow, and candidate loci included functions related to fat storage, stress and molecular or tissue repair. The high connectivity of golden perch in the MDB will likely allow locally adaptive traits in its most arid and hydrologically variable environment to spread and be selected in localities that are predicted to become arid and hydrologically variable in future climates. High connectivity in golden perch is likely due to their generalist life history and efforts of fisheries management. Our study adds to growing evidence of adaptation in the face of gene flow and highlights the importance of considering ecological disturbance and adaptive divergence in biodiversity management.