Murray Cod Research Articles (Page 1)

RETRACTION: Nonlethal Age Estimation of Three Threatened Fish Species Using DNA Methylation: Australian Lungfish, Murray Cod and Mary River Cod.

ABSTRACTFrom fertilisation to hatching, we investigated the embryonic development of hatchery‐reared Murray cod eggs incubated at 20°C and determined the developmental stages. Twenty‐eight readily identifiable stages with distinct morphological features were described and illustrated. Meanwhile, a simple and effective method for evaluating developmental competence at 1/2 epiboly was developed, which does not require the exhausting removal of egg chorion. Acute mortality was observed within 28 h post‐fertilisation, indicating that critical stages may occur during this period. The presented results may provide vital information to improve hatchery practices.

Purpose Biological specimens in collections can play a crucial role in supporting research in systematics, taxonomy and biogeography. Species specific collections in natural history museums are commonly small, with restricted availability for invasive sampling. Biological collections material (e.g. fishing trophies) held in private hand is commonly discounted as informal, although it represents a distributed and uncatalogued collection of potentially considerable extent. Using a case study of Murray Cod, an apex predator in the Murray-Darling River system of Australia, this paper aims to examine the access and usage of such specimens for research as well as their availability for non-invasive and invasive sampling. Design/methodology/approach Crowd sourced via a distributed social media approach, primarily Facebook (O’Connell et al., 2025), stewards (a generic term to encompass venue managers, custodians and owners) of taxidermy Murray Cod were interviewed to examine access to and usage of their taxidermy specimens for morphometric and biogeographical research in general as well as their willingness to loan specimens for non-invasive investigation (x-ray, CT-scanning) and/or to permit invasive sample taking of otoliths (for ageing studies). Findings The paper reviews access to biological collections in both formal (e.g. museums) and informal (e.g. private collections) settings, emphasising that informal collections often exceed formal ones in quantity. A case study shows private collectors are open to sharing their specimens for research, including morphometric, biogeographic and non-invasive studies. Many collectors are even willing to allow tissue sampling, provided the specimen’s appearance is preserved. This presents an important opportunity for research in systematics, taxonomy and biogeography, making informal collections a valuable but underutilised resource. Originality/value To the best of the authors’ knowledge, this study is the first of its kind that examines access to biological collections material held in both formal and informal collections and the limitations imposed on its use is non-invasive and invasive (sample-taking) scientific enquiry. It demonstrates that informal collections in private hand can supply a data set that far exceeds formal (museum) collections both numerically and with regard to access and research utility.

Modification of river flows is a major cause of freshwater fish population declines in many parts of the world. Identifying the precise mechanisms of these declines represents a significant challenge, as a range of stressors can simultaneously impact various components of fish health, fitness and population dynamics. Here we investigate the role of river flows and other biophysical factors on spatio‐temporal variation in freshwater fish body condition in Australia's highly modified Murray‐Darling Basin using three widely distributed native (Murray cod Maccullochella peelii, golden perch Macquaria ambigua and bony herring Nematalosa erebi) and one introduced (common carp Cyprinus carpio) species. Our aim was to uncover drivers of spatio‐temporal variation in fish condition at two spatial extents: at the basin scale, utilising a flow regime disturbance index, and at the river‐valley scale, employing individual flow gauge data to assess responses in fish condition to multiple measures of antecedent (365 day) flow. Linear mixed effects modelling revealed that at the basin scale, M. peelii and M. ambigua were in better condition in rivers with lower flow regime disturbance, and temporal trends in the condition of N. erebi, C. carpio and M. peelii reflected boom and bust dynamics related to wet and dry climate periods. At the river‐valley scale, mean antecedent daily flow magnitude was significantly positively related to the condition of M. peelii, M. ambigua and C. carpio, whereas the number of high‐flow days was negatively related to condition of N. erebi. Our study demonstrates that a simple body condition index calculated from routinely collected length–weight data is sensitive to multiple measures of hydrological disturbance in river systems that experience substantial temporal and spatial variability. We emphasise that studies considering multiple spatial scales are important for understanding complex scale‐dependent mechanisms influencing fish condition.

Abstract This study investigated whether trophy taxidermy specimens of Australia’s largest freshwater fish, Murray cod (Maccullochella peelii), can provide accurate records of historical body size. Taxidermy mounts came mostly from informal collections in hotels from across the Murray–Darling Basin, south-eastern Australia, comprising 20% whole-body and 80% head forms. We compared the morphology of mounts to live Murray cod, collected from the mid–Murray River in 2018, and identified the head morphometrics that most accurately described length and weight of whole mounts and live fish. Eight morphological characters were analysed for 60 whole mounts, 172 head mounts and 51 live fish. We found that inter-orbital distance, inter-nare width and upper jaw length were relatively robust to taxidermy processes and were reliable features for predicting fish total length and total weight. Shrinkage in head morphometrics due to taxidermy was evident, however, and we recommend that this be considered when reconstructing length and weight measures. We discuss how estimated body length and weight from head morphometrics of trophy fish, coupled with analysis of the accompanying remaining tissue and hard parts, opens up opportunities to explore patterns in genetics, life history, movement and trophic ecology of historical fish populations and of past environments.

Context Dryland rivers are unique ecosystems, where drought and flood play an important role in shaping the ecosystem. River regulation has altered the natural flow regime in many of these systems, affecting migration cues and connectivity for many species. Aims To quantify the discharge-related movements of Murray cod and golden perch within the Condamine–Balonne River subject to differing levels of river regulation. Methods We quantified flow regime variability, river regulation and fish movement to develop generalised additive mixed models to predict movement probability for Murray cod and golden perch. Results Both species showed strong positive relationships between discharge and movement. Murray cod did not show any association with river regulation; however, medium-sized individuals were significantly more likely to move than were smaller or larger fish. Golden perch movements varied among levels of regulation, were more likely to move as body weight increased and showed seasonality of movement, moving less during winter. Conclusions This study presents the largely unobserved movement behaviours of fish across a gradient of river regulation and environmental conditions in the northern Murray–Darling Basin. Implications This information is valuable for informing policy and management decisions that may affect species’ life-history requirements in analogous river systems.

SummaryMillions of native fish are entrained into irrigation pumps in Australian rivers every year. It is often assumed these fish are wild, but stocked fish may also be affected. During fish entrainment surveys at two pump intakes on the Macquarie River, New South Wales, a noticeable increase of entrained juvenile Murray Cod (Maccullochella peelii) was observed. DNA parentage analysis confirmed that a large proportion of these fish were linked to nearby fish restocking events. At both pump intakes, genetic analysis confirmed that at least 70% and 17% of the individuals sampled were stocked fish. This equated to up to 3% of the fish that were stocked – most of which were entrained less than 24 h after their release. Given the large number of unscreened irrigation pumps in this reach of river, and more broadly throughout the Murray–Darling Basin, fish losses at pump intakes have the potential to remove large numbers of stocked fish from the river where they are released to support native fish recovery and boost recreational fishing opportunities. The use of fish‐protection screens at pump intakes may be a suitable solution to reduce the number of fish entrained and thus increase the survival of recently stocked fish in the rivers.

Context Entrainment and removal of fish from aquatic ecosystems can occur at water pump offtakes. Exclusion screens that reduce these impacts are recognised as an important conservation measure. Aims Evaluate the effectiveness of the Australian screen design guidelines in protecting larvae and young-of-year age class of a native fish species, Murray cod Maccullochella peelii. Methods Entrainment and impingement of postflexion larvae and young-of-year were assessed in a controlled laboratory environment. Tests were conducted under a range of approach velocities (AV) and impingement durations for two screen materials. Key results Fish screens reduced larval entrainment by ≤84%. Screens had no significant effect on reducing larval entrainment at AV ≥0.125 m s−1. Impingement of young-of-year was positively associated with AV and mortality increased with impingement duration, irrespective of screen type. Conclusions To protect early life-stage Murray cod, it is recommended that water pump offtakes be fitted with 2-mm vertical wedge-wire stainless steel screens and AV be limited to ≤0.1 m s−1. Implications This study represents the first assessment of the effectiveness of the Australian screen design guidelines in protecting larvae, providing knowledge to further refine specifications for screen design and support the recovery of native fish populations.

Abstract Environmental DNA (eDNA) has been widely used for species surveillance. However, the lack of adequate quality control in many eDNA research projects and applications can lead to false‐negative results, greatly affecting biosecurity surveillance and conservation efforts. Exogenous DNA is routinely added to eDNA samples and used as a positive control, typically after DNA extraction. However, this type of positive control is only able to identify false negatives due to errors at the amplification stage. Therefore, errors in upstream processes, such as sample collection will not be identified by an exogenous control. We designed two independent sets of generic quality control qPCR assays (QCqPCR) targeting abundant endogenous DNA that is obtained during sample collection. Our QCqPCR assays target the chloroplast 16S and 23S ribosomal RNA sequences. In silico analyses indicated these regions were highly conserved among plants, algae and bacteria commonly found in freshwater, marine, or terrestrial environments. These QCqPCR assays were purposely mismatched against the human genome to avoid false positives resulting from human DNA contamination. Both assays remained highly efficient and sensitive under annealing temperatures between 58 and 62°C, allowing them to be multiplexed with most qPCR analyses. We validated our assays by multiplexing with a species‐specific Murray cod (Maccullochella peelii) assay on field‐collected environmental water samples. Potential false‐negative reactions can be identified by the failed or suppressed QCqPCR assay and the negative species‐specific assay. We recommend incorporating either one of the QCqPCR assays in qPCR‐based eDNA analysis to identify potential false negatives and improve the reliability of eDNA surveys.

Abstract Regulation and water extraction have reduced river flow worldwide. In some rivers, this has led to reductions in the frequency and duration of floodplain inundation and, in turn, declining condition of flood‐dependent vegetation. One management response has been the construction and modification of infrastructure, or ‘regulators’, to engineer floodplain inundation at discharges that do not otherwise cause overbank flooding. Such actions have the potential to benefit floodplain vegetation, yet the atypical hydrodynamics of engineered floods may threaten riverine ecological processes and biota and increase the risk of adverse water quality. We investigated the influence of engineered flooding, and associated reductions to in‐channel hydraulics, on key ecological processes. Specifically, the transport and retention of larvae of an iconic fish species (Murray Cod, Maccullochella peelii), and a common microinvertebrate Trichocerca. Analysis of different flow management scenarios for floodplain inundation indicates that maintaining in‐channel velocities greater than 0.2 m s−1 produces higher Trichocerca densities, while velocities greater than 0.3 m s−1 creates suitable habitat for Murray cod larvae, thereby providing targets for avoiding detrimental effects of engineered floods on in‐channel biota. To evaluate responses to regulator operation, velocity was represented by a hydrological model using pre‐computed results from detailed hydrodynamic models. These results were used to relate upstream discharge and downstream water level to the proportion of each reach in the hydrological model meeting each velocity criterion. The model also represents changes in discharge, inundated area and a water quality parameter, dissolved oxygen. Model scenarios for engineered floodplain inundation were used to demonstrate the potential benefits, impacts and trade‐offs between the different metrics identified. The model framework enables a more holistic evaluation of infrastructure operation, extending analysis beyond discharge and inundated area to risks and benefits to key indicators of the ecosystem. This refined integrated approach to the management of regulated river systems may become critical in the future where water resources are projected to further decline under a changing climate.

Abstract Each year, millions of fish are extracted from Australian waterways by the pumping and diversion of water into irrigation systems. Fish protection screens can help reduce these losses but are largely untested in Australian rivers. In this study, a large, gravity‐fed irrigation offtake on Gunbower Creek, Victoria, Australia, was investigated for fish and debris entrainment. Experiments were performed under screened and unscreened conditions across various river flows. Mark–release–recapture experiments were undertaken with fingerlings of two recreationally significant fish species, Murray cod (Maccullochella peelii) and golden perch (Macquaria ambigua), together with wild fish community assessments, to determine how effective the screen was at reducing fish entrainment into the irrigation channel. The mean percentage of recaptured fingerlings was significantly lower when the irrigation channel offtake was screened compared with unscreened. Entrainment of released fish into the irrigation channel was reduced by >98%. Similarly, wild fish entrainment was significantly lower when the irrigation channel was screened. When screened, fewer wild species dominated the sampled fish community, and entrained fish were generally <40 mm in length. Debris loads decreased significantly in the irrigation channel owing to the screen, indicating the economic benefits of fish screens. Implementing screens could aid in native fish conservation efforts in riverine environments.

Context As social–ecological systems, recreational fisheries often vary temporally in response to environmental changes affecting ecological processes and human behaviour. Monitoring such variability in this ecosystem service can guide adaptive management measures for sustainability. Aims This novel research for Australian, sought to quantify interannual changes in the freshwater recreational fisheries of five key (i.e. commonly caught) finfish species (Murray cod, Maccullochella peelii; golden perch, Macquaria ambigua; Australian bass, Percalates novemaculeata; brown trout, Salmo trutta; and rainbow trout, Oncorhynchus mykiss) in relation to a series of extreme climate-related events and the COVID-19 pandemic. Methods Annual estimates during 2013–14, 2017–18 and 2019–20 of freshwater fishing effort and catch across New South Wales, Australia, were derived from off-site surveys and compared in relation to a severe drought period, the ‘Black Summer’ bushfires, widespread flooding and the COVID-19 pandemic, all of which affected fish productivity or human mobility. Key results There were significant declines in fishing effort between 2013–14, the year preceding the extreme environmental events and the pandemic, and 2017–18 and 2019–20. Catch across the five species was also significantly lower in 2019–20. Catch of species such as golden perch and rainbow trout declined from 2013–14 to 2019–20. Conclusions and implications This study can inform adaptive measures against societal and climate-related changes in weather by enabling scientists and managers to identify problematic trends.

Bold behaviour of non-native species is hypothesized to facilitate invasion success, yet extreme boldness in wild and domesticated animals can be maladaptive. The purpose of this study was to compare individual behaviour among Australian native hatchery-reared (n = 33) and wild (n = 38) Murray cod (Maccullochella peelii) with invasive common carp (Cyprinus carpio; n = 30). Three laboratory tests measured individual behaviour: (1) emergence from a shelter, (2) exploration of a novel environment, and (3) approaching a predator. Wild invasive carp and hatchery-reared cod were generally faster and more likely to emerge and explore novel environments when compared with wild Murray cod. The 'bold-type' behaviours of hatchery-reared native cod were more like invasive carp than they were to 'shy-type' wild conspecifics, yet an important difference was that hatchery-reared cod spent substantially more time near a large predator while carp rapidly escaped. We suggest that these results are consistent with a bold-type invasion syndrome in invasive carp and learned boldness of hatchery-reared Murray cod. The propensity of invasive carp to rapidly explore and enter new environments, along with a fast predator escape response may have been important to their invasion success, while extreme risk-taking and predator naivety of hatchery-reared Murray cod may exacerbate post-release mortality rates in fisheries and conservation stocking programmes.

During bouts of prolonged drought and little to no river flow, waterholes are a haven for freshwater fishes. However, as humans divert more water for their own use and as global temperatures rise, these small bodies of water are increasingly vulnerable to climbing temperatures and low oxygen levels. Fortunately, some fishes are able to change quickly to cope with high temperatures, and occasionally these changes also help them do better with low oxygen levels. But size matters when it comes to dealing with these stressors. Big fish are thought to deal with low oxygen better than small fish, and small fish are thought to deal better with high temperatures, but all are likely to be vulnerable when both stressful circumstances occur simultaneously. To investigate how body size influences a fish's ability to tolerate both high heat and low oxygen, Darren McPhee, with researchers from the University of Queensland and the Queensland Department of Regional Development, Australia, turned to massive Murray cod – which can grow to the size of a giant panda (over 100 kg) – to find out how they deal with the combined threat.Working with fish ranging from 0.2 g to 3 kg, the team transferred the animals to water at temperatures mimicking a hot summer (28°C). After 4 weeks, all of the cod, regardless of size, increased the temperature at which they lose their balance (known as the upper thermal limit), indicating that the cod are able to deal with persistent high temperatures. But when the team tested the fish's abilities to remain upright at high temperatures as the water oxygen levels decreased, their ability to tolerate heat declined. Surprisingly, the fish's body sizes had an unusual impact on their ability to withstand high temperatures when their oxygen supply was restricted. As expected, when the fish had access to well-oxygenated water (100% and 50% oxygen saturation), the smaller animals coped better with high temperatures than the big fish. But this pattern flipped when the oxygen levels in the water were low (30% and 16% oxygen saturation), where the largest fish tolerated the high temperatures better than the smaller fish. Intriguingly, this didn't mean that the big fish were thermal tolerance champions. At the lowest oxygen level (16% oxygen saturation), the mid-sized fish coped best with the high temperatures.The scientists also tested how short-term and persistent exposure to different temperatures affected the cod's ability to survive low oxygen in several different ways. In the short term (less than 24 h), fish exposed to high temperatures breathed faster and lost their balance at an oxygen level twice as high as what they would normally handle. But when exposed to a high heat for 4 weeks, the responses to low oxygen weren't quite as drastic. Again, the largest cod were the least bothered by decreasing oxygen – not changing their breathing frequency and taking longer to lose their balance (unlike the little fish).Although Murray cod of all sizes demonstrated an amazing ability to cope with high temperatures in the lab, McPhee and colleagues also went into the wild to check the water quality of three Queensland waterholes that are much loved by the cod. The researchers found that the fish are living dangerously close to the edge – at the limit of the high temperatures and low oxygen levels that they can endure – especially during the summer. And, as the smallest cod are most vulnerable to the combined stress of high temperature and low oxygen, this slow-growing species is at genuine risk from catastrophic loss if their littlest ones succumb in the face of the deadly duo.

IntroductionHumans have substantially altered landscapes across the globe, generating novel ecosystems with varying states of modification. The principles of reconciliation ecology emphasise that such novel ecosystems must also be considered for conservation outcomes. This requires an understanding of how anthropogenic habitat alterations in a novel ecosystem may disrupt life cycle processes of key biota, thereby enabling the development of management strategies that may bypass or ameliorate potential bottlenecks in life history stages. We explore this by assessing Murray cod (Maccullochella peelii), a native fish species of considerable cultural, recreational and conservation significance in a severely altered river reach in south-eastern Australia. This species was considered to have disappeared in the reach following alterations to the river’s thermal profile (cold water pollution - CWP), but instead has persisted.MethodsWe examined the life cycle of Murray cod and use multiple lines of evidence to assess the impacts of CWP pollution on key processes such as adult migration, recruitment and juvenile survival. We also evaluated the potential for recovery and persistence of the species in this novel system using mitigation measures such as flow management and stocking and opportunistically tested their effectiveness.ResultsWe found the key impact pathway of CWP on the Murray cod population is via its effects on the key processes influencing natural recruitment. Conversely, the effects of CWP on emigration and survival of larger juveniles and adults did not appear to be a major bottleneck for the population. Using a stochastic population model, we showed that the population could be sustained by stocking and opportunistic water management while still meeting irrigation demands. Monitoring to test our recommendations showed natural recruitment and improved survival of stocked fish during years with limited flow releases, thereby corroborating our predictions.DiscussionOur use of multiple lines of evidence to assess a species’ responses to impacts of habitat alteration in a novel ecosystem enabled the formulation of recommendations for management that could then be tested. The success of such actions add to the growing body of literature that shows species of conservation significance can be maintained in novel ecosystems which should be specifically considered within the context of conservation planning across the landscape.

Occurrence and characterisation of Eustrongylides species in Australian native birds and fish

Context Following recent major fish kill events, there is increasing interest in stocking Murray cod fingerlings to help fish stocks recover. Strategies that can increase post-release survival of stocked fingerlings are important for conservation stocking of Murray cod and may also have application to other species. Aims The aim of this work was to evaluate whether pre-release predator recognition and avoidance training conferred a survival advantage on stocked Murray cod (Maccullochella peelii) fingerlings. Predator-free release enclosures were also evaluated as a method to improve survival. Methods Fingerlings were trained by exposure to predators and skin extract. Fingerlings were marked with visual implant elastomer (VIE) tags to denote training status and release strategy (trained, untrained, soft release, standard release). Fingerlings were released at three sites in the northern Murray–Darling Basin. Marked fish were recaptured by electrofishing 24 h post-release and quarterly up to 15 months post-release. Key results Trained Murray cod had higher relative survival than did untrained fish. The mean recapture rate of trained fish was twice that of untrained fish. Fingerlings released directly into the waterbody had significantly higher recapture rates than did fingerlings acclimated for 90 min in predator-free enclosures. Conclusions Predator conditioning training provides a survival advantage to stocked Murray cod fingerlings. However, predator-free release enclosures conferred a disadvantage. Implications Pre-release training of Murray cod fingerlings could be beneficial to conservation and recreational-fish stocking programs. We recommend stocking trained cod fingerlings directly into the receiving environment, rather than into soft-release enclosures, so as to maximise their chance of survival.

Increasing drought frequency and duration pose a significant threat to fish species in dryland river systems. As ectotherms, fish thermal and hypoxia tolerances directly determine the capacity of species to persist in these environments during low flow periods when water temperatures are high and waterbodies become highly stratified. Chronic thermal stress can compound the impacts of acute hypoxic events on fish resulting in significant fish mortality; however, it is not known if all size classes are equally susceptible, or if the allometric scaling of physiological processes means some size classes are disproportionately affected. We investigated the physiological responses of Murray cod (Maccullochella peelii) over a four-fold body size range (0.2-3000g) to acute changes in water temperature and oxygen concentration following 4 weeks of acclimation to representative spring (20°C) and summer (28°C) water temperatures. We recorded maximum thermal tolerance (CT max), oxygen limited thermal tolerance (PCTmax ), lowest tolerable oxygen level (as the oxygen level at which lose equilibrium; O2,LOE), gill ventilation rates and aerial surface respiration threshold, blood oxygen transport capacity and lactate accumulation. Acclimation to elevated water temperatures improved thermal and hypoxia tolerance metrics across all size classes. However, body size significantly affected thermal and hypoxia responses. Small M. peelii were significantly less hypoxia tolerant than larger individuals, while larger fish were significantly less thermal tolerant than smaller fish. Hypoxia constrained thermal tolerance in M. peelii, with both small and large fish disproportionally compromised relative to mid-sized fish. Our findings indicate that both very small/young (larvae, fry, fingerlings) and very large/older M. peelii in dryland rivers are at significant risk from the combined impacts of a warming and drying climate and water extraction. These data will inform policy decisions that serve to balance competing demands on precious freshwater resources.

Context Temperature regulates most ecological processes in freshwater ecosystems. Anthropogenic changes to natural thermal regimes, especially cold water released from stratified impoundments (cold-water pollution, CWP), is a widespread and major threat to fish populations globally. Aims Because mitigation options for CWP are often expensive, we aimed to provide robust ecological evidence to inform expenditure. Methods We modelled population responses to CWP remediation for two warm-water fish species (Murray cod and golden perch) downstream of two impoundments (Copeton and Pindari dams) in the semi-arid northern Murray–Darling Basin, Australia. Key results Predicted populations of both species were severely affected by CWP at both sites; however, impacts were species- and site-specific. Effects were greater on golden perch than Murray cod because of their higher temperature requirements for spawning. Predicted spawning opportunities decreased by 77–100% for golden perch and by 38–92% for Murray cod. The larger Copeton Dam had greater impacts than did the smaller Pindari Dam. Conclusions Remediation of CWP can help restore populations and meeting the needs of golden perch may benefit a range of other species. Implications Globally, CWP is a serious threat to warm-water fishes that may compromise and even nullify other restoration efforts. However, restoration may be impeded by current low abundances, angler harvest and restrictions to fish passage.

Context Native fish populations in Australia’s Murray–Darling Basin (MDB) have experienced severe declines since European settlement. Information on their status is needed to guide management and recovery. Aims To quantify trends in MDB fish populations in New South Wales (NSW) from 1994 to 2022. Methods Relative abundance, biomass, and size structure were examined using generalised additive mixed models at NSW MDB and river catchment (valley) scales for five native species (Murray cod, Maccullochella peelii; golden perch, Macquaria ambigua; silver perch, Bidyanus bidyanus; Macquarie perch, Macquaria australasica; freshwater catfish, Tandanus tandanus) and one alien species (common carp, Cyprinus carpio). Key results There was strong inter-annual variation in relative abundance, biomass and population structure for all species. At the Basin scale, relative abundance of Murray cod, golden perch and common carp increased across the time series, with no clear trends for silver perch, Macquarie perch or freshwater catfish. Patterns in relative abundance, biomass, and population structure were variable among valleys for most species. Conclusions and implications Although native fish populations in the MDB remain degraded and face escalating threats, recent increases in the abundance of some native species are an encouraging sign that integrated restoration efforts can improve the outlook for native fish.