Murray Cod Research Articles

Abstract Variation in river flow is a strong behavioural determinant for the movement of many freshwater fish species and often is linked to key aspects of their life cycle. The alteration of natural flow regimes to meet human water demands can result in changes to this variability, and cause declines in water‐dependent biota. Environmental flows are used as a remediation tool in some regulated rivers with the intention of restoring aspects of the natural flow regime to benefit riverine species, although empirical data are required to inform the efficacy of these interventions. Using acoustic telemetry, we quantified the movement responses of two large‐bodied native fish species (freshwater catfish, Tandanus tandanus and Murray cod, Maccullochella peelii) to variations in river flow over 4 years in two intermittent regulated rivers in the northern Murray–Darling Basin, Australia. Both rivers received periodic environmental flow releases and also there were several large natural flow events during the study period. Both species displayed a range of intra‐specific movement behaviours. Analysis of individuals' movements revealed five distinct functional groups, which were represented in both species. We found that periods of environmental flow delivery played an important role in the movement behaviour of both species. Murray cod were more likely to move during periods of environmental flow releases, whereas freshwater catfish were less likely to move on an environmental release following higher antecedent flows. No large‐scale philopatric movements were observed, yet Murray cod were more likely to move during the breeding period, indicative of nest site selection. We also found that the likelihood of movement in both species was higher in the smaller of the two rivers for a given magnitude of flow. Our results suggest that environmental flows may benefit certain fish species by facilitating, rather than cueing breeding behaviours, allowing individuals improved access to, and provision of, higher‐quality nesting habitats. These findings will aid water managers in creating economical and targeted environmental flow releases, timing larger flow pulses for the species with flow‐cued reproductive strategies, and providing a continuity of smaller baseflows for species whose reproduction is not dependent on flow cues but may be enhanced by greater connectivity.

Poor utility of environmental DNA for estimating the biomass of a threatened freshwater teleost; but clear direction for future candidate assessments

Abstract While there is a considerable amount of interest in information and communication technologies for development (ICT4D) in the Indigenous communities, it remains limited to those who can afford it and have the skills and knowledge to implement the technology and access appropriate digital tools. Hence, Indigenous communities are continually stigmatized as marginalized, leading to a cultural misrepresentation of histories that affects the continuing information disparity between Indigenous and Western knowledge systems, particularly the insufficient technology infrastructure designed for traditional users. In this article, ICT4D was conceptualized as a digital platform to support Senior Ngarrindjeri Elder Aunty Ellen Trevorrow in continuing her practice of weaving and storytelling throughout the pandemic. In this context, the community‐based participatory research (CBPR) principles within the structure of video ethnography were qualitatively designed to implement the ICT4D project culturally and ethically. Video recordings, image data, transcriptions, and the Ngarrindjeri ICT4D Pondi (Murray Cod) framework were embedded to justify the findings and the aim of illustrating Aunty Ellen's knowledge‐sharing process to online learners. Likewise, the results demonstrate the positive and negative impact of COVID‐19 on the continuity and orality of Aunty Ellen's cultural stories and practices. The future continuity of Aunty Ellen's knowledge ought to consider the inconsistency of technological infrastructure in regional areas, her waning health, and the interconnectedness of oral expertise, which often pose challenges. This study is a small step toward a better understanding of the value of oral knowledge; emphasizing the creation of e‐learning weaving instructional videos is valuable for future digital management of Indigenous knowledge relevant to LIS.

The release of cold-water from hypolimnetic zones of impoundments sharply reduces downstream riverine water temperature. This cold-water pollution (CWP) can extend for hundreds of kilometres, severely challenging the physiological ability of aquatic fauna, particularly ectotherms such as fish, to maintain essential processes such as metabolism, development and growth and survival. The impact of CWP on native fish, especially early life stages, is poorly known. We investigated the effect of a 24-hour exposure to a range of environmentally-related water temperatures (8, 10, 12, 14, 16, 18 and 20°C) on three age-classes (<24-hour-old, 7-day and 14-day-old larvae) of two Australian native fish species: Murray cod (Maccullochella peelii) and Macquarie perch (Macquaria australasica). Overall, larvae of M. peelii were more sensitive to lower water temperatures and hence CWP than M. australasica, indicated by higher rates of equilibrium loss. Larvae of M. peelii were most sensitive to exposure at seven days old whereas M. australasica larvae were most sensitive at <24-h-old. Using our results, we modelled pre- and post-impoundment temperature scenarios and estimated the downstream CWP footprint for both species in an Australian river reach. Larvae of M. peelii were predicted to be absent from the first 26 km of river downstream of the impoundment compared with no impact on the distribution of M. australasica. Managing riverine water temperature below impoundments is fundamental to promoting positive outcomes for endemic fish on not only a local, but global basis. This study emphasises the differential impact of CWP among the critical early life stages and fish species and highlights the urgent need to better manage hypolimnetic water releases to improve downstream river ecosystems.

Combining bio-telemetry and underwater imagery to elucidate the reproductive behaviour of a large, long-lived Australian freshwater teleost

Abstract Spawning patterns in fish that use temperature as a spawning cue are responding to long‐term changes in the climate. The threatened Murray Cod, Maccullochella peelii (Mitchell), is an important apex species in inland south‐eastern Australia. Information from 1461 spawnings collected from hatcheries over 40 years was combined with environmental, meteorological, and astronomical data to (1) identify factors affecting spawning and (2) detect changes in spawning associated with climate change. The spawning season extended over four months, from late‐August at low latitudes to mid‐January at high latitudes, but locally lasted 1–2 months. Spawning events were preceded by an increase in temperature in the days leading up to spawning, whereas a decline in temperature inhibited spawning. In the Murray‐Darling River basin, the spawning season of Murray Cod began earlier and shortened in duration inversely with the mean spring temperature anomaly. Spawning is predicted to occur up to 4 weeks earlier by 2090 in response to future temperature projections in the Murray‐Darling River basin. Since climate change is affecting aquatic environments globally, findings from this study have implications to the long‐term management of temperate freshwater species that are the subject of regulations aimed to protect fish during the spawning season.

Abstract Analysis of environmental (e)DNA can facilitate an understanding of the presence and distribution of aquatic species. However, eDNA detection using quantitative PCR requires validated and standardized species‐specific assay designs. This study presents two eDNA assays to detect Murray cod, Maccullochella peelii, and mulloway, Argyrosomus japonicus (two ecologically vulnerable Australian species), based on small fragments of the mitochondrial 12S ribosomal RNA gene. A comprehensive description of species‐specific assay development, from assay design to testing in silico, in vitro and in situ, has been included to guide effective assay design and validation in future studies. The results indicate that the assay was species specific for M. peelii within its natural distribution. However, the assay also amplified genomic DNA from two allopatric and endangered congeners (Maccullochella ikei and Maccullochella mariensis), thus potentially facilitating their eDNA detection elsewhere. In contrast, the A. japonicus assay was highly species specific with no amplification among close relatives. Both target‐species assays are highly sensitive to as few as four and 10 copies per PCR reaction, respectively. This study has demonstrated that the assays assessed are effective tools for detecting the targeted species in situ from environmental DNA samples, which will assist efforts to conserve and manage their stocks.

Cold water pollution (CWP) is caused by releases of unseasonably cold water from large, thermally stratified dams. Rapid and prolonged decreases in water temperature can have depressive effects on the metabolism, growth and swimming performance of fish. However, it is unknown if reducing the rate of temperature decrease could mitigate these negative effects by allowing thermal acclimation/acclimatization to occur. This study investigated the rate of temperature decrease as a potential CWP mitigation strategy in juvenile Murray cod Maccullochella peelii. M. peelii were exposed to a gradual, intermediate or rapid temperature decrease from 24 to 14°C. Energetic costs, locomotor performance, growth and survival were measured to determine if the initial thermal regime affected the thermal acclimation capacity of M. peelii. Cold exposure had significant acute and lasting depressive effects regardless of the rate of temperature decrease, although M. peelii showed varying degrees of thermal compensation in swimming performance and metabolism after 8 weeks of exposure to low temperatures. The short‐term effects of CWP‐like reductions in temperature are significant, but over time M. peelii can offset some of the depressive effects of CWP through thermal plasticity. This study highlights the importance of understanding physiological responses of fish to inform management and conservation. We conclude that rate of water temperature decline cannot be used to mitigate the sublethal effects of CWP on juvenile M. peelii but may still be useful for managing the negative effects in other native Australian fish species.

Murray cod (Maccullochella peelii) is a freshwater percichthyid fish that has a high market value and potential for culture in recirculating aquaculture systems (RASs). Illumination is an important environmental factor that affects the growth and physiological condition of fishes. A comprehensive understanding of the relationship between the light factors and the growth, nutrient composition and stress response of juvenile Murray cod in RAS is important to achieve satisfactory theoretical and practical aquaculture performance. Juvenile Murray cod were randomly assigned to nine RAS tanks with a volume of 1.5 m3, with each tank containing 120 fishes (3.5 ± 0.5g). The fish were cultured for 120 days under different light intensities (1200, 2400, and 3600 lx) and photoperiods (12L: 12D, 18L: 6D and 24L: 0D). The results showed that the final weight and feed conversion ratio of Murray cod under a light intensity of 1200 lx were significantly better than those under 3600 lx (P < 0.05). The serum total protein and globulin were higher at the light intensity of 1200lx, compared to other light intensities. The blood urea nitrogen level improved with increasing light intensity at LD12:12 and LD18:6 of photoperiod, as well as the superoxide dismutase level improved significantly from LD18:6 to LD24:0 of photoperiod. The findings indicated that the optimal light condition to enhance growth at juvenile stage is 1200 lx of the light intensity and LD18:6 of the photoperiod.

A disease outbreak occurred in Murray cod Maccullochella peelii peelii in a recirculating aquaculture farm in Tianjin city, China, in 2019. Strain MRX-2019 was isolated and considered to be the etiological pathogen; it was identified as Flavobacterium columnare based on a 16S rDNA gene sequence analysis and physiological and biochemical tests. The effect of salinity on the growth of MRX-2019 was investigated in vitro. Salinity >4‰ (i.e. 6‰) inhibited MRX-2019 growth, whereas 8 and 10‰ salinity killed it. The effect of 4‰ salinity on F. columnare was not significant (p > 0.05). When MRX-2019-infected Murray cod were treated with 4, 6, or 8‰ salinity, the mortality rate was reduced by 8.9, 67.76, or 75.56%, respectively, compared with that of the control. However, the mortality rate increased by 7.77% at 10‰ salinity. In this study, we found that maintaining the fish in freshwater with 6-8‰ salinity effectively reduced the mortality of these fish when infected with F. columnare. The findings provide an environmentally friendly control strategy for columnaris disease in Murray cod.

Context The Murray–Darling Basin – Australia’s largest river system – is heavily dominated by alien fish. Native fish species have suffered numerous localised extinctions and ∼47% are listed on federal and/or state threatened species lists. Aims This paper explores the hypothesis that alien fish and alien fish stockings can be the primary cause of decline and localised extinction of large-bodied native fish species, as opposed to habitat degradation and river regulation. The Lower Goodradigbee River, which is unregulated, in excellent instream health over the great majority of its course, and replete with high quality habitat, is utilised as a case study. Methods I investigated the hypothesis by synthesising historical records with contemporary scientific research and recent field observations. The role of alien fish species, particularly alien trout species (Oncorhynchus mykiss and Salmo trutta) and constant stockings of them, were closely examined. Results Data support the hypothesis that domination by alien trout species and their continual stocking have lead to historical declines and localised extinctions of large-bodied native fish species. Continued alien trout stockings, along with more recent invasions of alien carp (Cyprinus carpio) and alien redfin perch (Perca fluviatilis), are inhibiting native fish recovery. A suspected field sighting of the alien fish pathogen atypical Aeromonas salmonicida is reported, and the status of the declining native crayfish Murray cray (Euastacus armatus), and potential alien fish impacts upon them, are examined. Conclusions The impacts of alien fish and alien fish stocking in Australia require major re-evaluation and dedicated research. Implications It is strongly recommended that stocking of alien trout into the Lower Goodradigbee River for angling cease in order to conserve surviving native fish and Murray cray populations. Conservation stockings to effect a Murray cod (Maccullochella peelii) recovery in the Lower Goodradigbee River are warranted.

Abstract Movement is a key driver of the distribution of animals and the structure of populations, communities, and ecosystems. Habitat loss and fragmentation can compromise movement and contribute to population declines. However, there is often insufficient knowledge about when, why, and where animals move, particularly in highly modified environments. We present results from an 8‐year study on the movement behavior of Murray cod Maccullochella peelii, an Australian freshwater fish species that has undergone major declines due in part to river flow regulations. We studied movement within and between different habitat types in a highly modified floodplain ecosystem in the lower Murray River to (1) identify the key environmental conditions associated with movement, (2) examine how a new regulating structure can be managed to influence movement behavior, and (3) explore movement mediated recovery following a hypoxic event. Movement within and between an anabranch and main river channel habitats increased during the core spawning period and during elevated discharge. The likelihood of Murray cod moving to an anabranch system from the Murray River declined substantially following construction of a new flow regulating structure (a weir and vertical slot fishway). Managed flows delivered through the anabranch after regulator construction in accordance with targeted recommendations (time‐of‐year and magnitude of discharge) increased the movement of adult fish within and between habitats. Finally, a hypoxic event caused not only high mortality but also resulted in a high proportion of fish migrating outside of the study reach, before returning to the system over several years. These results demonstrate how flow management can help a keystone species access habitats required to complete critical life history requirements including recovery from disturbance events. Importantly, the work provides an example of how timely and robust applied research has informed a major intervention program aimed at enhancing ecological outcomes.

In December 2019, a mass mortality among cultured Murray cod (Maccullochellapeelii peelii) fry occurred on a freshwater farm located at Foshan city of Guangdong province, China. The cumulative mortality was up to 45% within 15days. The diseased fish showed clinical signs, including abnormal swimming behaviour, loss of appetite and dark body colouration before mass mortality. Samples of brain and retina tissues were collected from affected fish and subjected to reverse transcriptase polymerase chain reaction detection and virus isolation in cell culture. Approximately 430bp product was detected from the brain and retina tissues and culture supernatant of betanodavirus-infected SSN-1 cells. The typical cytopathic effect of betanodavirus infection, which is characterized by vacuolation, was observed in SSN-1 cells at three days after inoculating with the tissue filtrate of diseased Murry cod fry, and the TCID50 of the infected SSN-1 cell supernatant was 107.8 . Histopathological examinations revealed vacuolation and necrosis in the brain and retina of naturally and experimentally infected Murray cod fry. Electron microscopic observation also showed the aggregation of numerous spherical, non-enveloped viral particles measuring 22-28nm in diameter in the cytoplasm of betanodavirus-infected SSN-1 cells. Sequence and phylogenetic analysis based on RdRp and Cp genes further indicated that the betanodavirus isolated from Murray cod belonged to the RGNNV genotype. Much higher mortality was obtained in challenged Murray cod fry compared with the controls through immersion challenge. This study is the first report of the natural infection of betanodavirus in freshwater fish in China.

Large dryland and semi-arid rivers are among the world’s most heavily modified ecosystems, and the Darling–Baaka River of eastern Australia highlights the challenges in conserving such ecosystems. Since 2000, the hydrology at the downstream end of the system (the Lower Darling River, LDR) has been transformed from a naturally near-perennial flowing system to an intermittent one by increased water abstraction, prolonged drought and climate change. This hydrological change has placed immense pressure on the native fish populations, such as the imperilled Murray cod (Maccullochella peelii), as evidenced by the 2018–19 catastrophic fish kills. Here we outline an ecohydraulic conceptual model for designing environmental flows to support spawning and recruitment of Murray cod. An environmental flow based on this model was released in 2016–17, following 524 consecutive days of continuous zero flows. The LDR flow consisted of an increased discharge in late winter–spring to promote broad-scale lotic (i.e. &amp;gt;0.3 m s–1) conditions, hydraulic complexity and continuous base flows to maintain connectivity and water quality. Monitoring of Murray cod during and following the flow revealed successful spawning and recruitment. This finding is significant because it provides justification for altering current water management policies that are failing to protect this nationally significant ecosystem.

Understanding the impacts of extreme events is essential to effective fisheries management. During the summer of 2018–19 millions of native fish died in Lower Darling–Baaka River adjacent to Menindee, New South Wales, Australia. Hypoxia during a period of protracted low flow, triggered by climatic events, was responsible for the fish kills. From June 2019 to March 2020, further broader-scale fish kills occurred throughout ~600 km of the Darling–Baaka River as disconnected refuge pools contracted and water quality deteriorated. This study examined the status of the remnant fish populations, compared the fish assemblage of the Menindee fish death reach with an unaffected reach and monitored change of the fish community over 18 months after the initial fish kills. Significantly lower abundances of Murray cod (Maccullochella peelii), bony herring (Nematalosa erebi), carp gudgeon (Hypseleotris spp.) and freshwater prawn (Macrobrachium australiense) were captured within the Menindee fish death reach compared with the unaffected reach. Varied responses were observed in species abundances within the affected reach in the 18 months after the initial fish kills, attributed to the various life-history traits and reproductive strategies. The results presented highlight a fish community in continued stress. Continued monitoring will guide and track the effectiveness of recovery management interventions in the region.

Understanding how and why the size of populations varies is critical knowledge for conservation and management. While considerable work has explored how different demographic parameters affect population growth, less is known the drivers of variability in these parameters. Long-term time series tracking population size that are coupled with empirical data to examine the relative importance of different drivers are rare, especially in freshwater systems. Even rarer are studies that collect this information concurrently from multiple species with contrasting life history strategies in the same system to assess whether population size and the relative importance of drivers also vary. We studied changes in the abundance and size structure of four native freshwater fish species in the Murray River, southeastern Australia, over a continuous 19-yr period. Two species with traits typical of "equilibrium" species (Murray cod Maccullochella peelii and trout cod Maccullochella macquariensis) and two with traits of "periodic" species (golden perch Macquaria ambigua and silver perch Bidyanus bidyanus) were sampled annually and capture-mark-recapture modeling was used to ask (1) how did population size change during this period, (2) how were changes in population size related to variability in hydrology, and (3) how were changes in population size driven by different processes (local recruitment or migration events)? Populations of all four species varied throughout the study, and our results are consistent with the notion that local recruitment is an important driver of this variability for Murray cod and trout cod, whereas immigration is more important for the two other species. Increases in spring river discharge strongly influenced these responses for trout cod and golden perch. Our study provides fundamental insights into the population dynamics of these valued species, and how management strategies might differ based on their life histories. Management should focus on allowing connectivity for golden and silver perch, and on promoting local scale recruitment and survival for Murray cod and trout cod. More generally, our study highlights the importance of understanding the processes underpinning population persistence, how these processes may vary for different species, and ultimately how this knowledge can inform targeted management actions.

Abstract In areas where commercial fishing is prohibited, demographic metrics obtained from recreational fisheries and fishery‐independent methods can be integral to population ecology and management. For Australian bass Percalates novemaculeata (Steindachner) and Murray cod Maccullochella peelii peelii (Mitchell), two key finfish species native to the recreational‐only fresh waters of New South Wales (NSW), Australia, recreational fishing surveys were combined with electrofishing methods to capture information on spatial patterns of exploitation and variation in size. The species’ catch across NSW fresh waters (2013–2014) was estimated using a telephone‐diary longitudinal panel survey and examined for spatial effects using generalised linear mixed effects models. Length data, collected through electrofishing (2013–2015), were compared among geographic zones using Kolmogorov–Smirnov tests and kernel density estimate tests. Catch patterns for each species were mainly spatially static, but significant regional differences in the size composition of both species were detected. The findings are discussed in the context of current catch‐and‐release practices for Australian bass and the application of size limits for Murray cod to illustrate the value of quantitative spatial information from recreational surveys and fishery‐independent methods in developing evidence‐based management and conservation initiatives.

Abstract Fish assemblages in dryland rivers have life‐history strategies that have evolved in response to environmental conditions and triggers, particularly water temperatures and flow regimes. The regulation of rivers through the construction of dams, weirs and other water diversion structures has altered natural flow regimes and the associated ecological processes of river systems worldwide. Over a 3‐year period, using standardized fish sampling and daily otolith ageing, the recruitment of eight freshwater fish species was monitored in response to various abiotic drivers, including hydrology and water temperatures, throughout the Macquarie River, a large regulated river system of the Australian Murray‐Darling Basin. A data‐driven statistical classification system is provided that groups species into reproductive guilds, based on their recruitment response to hydrology and water temperature, specifically designed for use in environmental flow management. The eight species were grouped into three distinct reproductive guilds that showed similar recruitment responses to the abiotic drivers. Murray‐Darling rainbowfish, Murray cod, and eel‐tailed catfish were considered as a single guild, characterized by seasonal recruitment during a relatively narrow thermal window under low to moderate stable discharges. The second group included common carp, bony herring, and golden perch, which recruited primarily in association with larger flow events. Un‐specked hardyhead and Australian smelt formed a more differentiated guild, recruiting over a broad range of temperatures and discharges. Limitations associated with using a reproductive guild approach to simplify water management are discussed. This study highlighted important relationships among hydrology, water temperatures, and successful recruitment that can, in turn, be used to inform development of adaptive flow management plans and effective use of environmental water for the conservation management of native fish communities. Important considerations in the design of studies that aim to examine relationships between recruitment and abiotic drivers are also discussed.

Characterization of a Streptococcus species isolated from Siganus guttatus in South China

Age‐based demography is fundamental to management of wild fish populations. Age estimates for individuals can determine rates of change in key life‐history parameters such as length, maturity, mortality and fecundity. These age‐based characteristics are critical for population viability analysis in endangered species and for developing sustainable harvest strategies. For teleost fish, age has traditionally been determined by counting increments formed in calcified structures such as otoliths. However, the collection of otoliths is lethal and therefore undesirable for threatened species. At a molecular level, age can be predicted by measuring DNA methylation. Here, we use previously identified age‐associated sites of DNA methylation in zebrafish (Danio rerio) to develop two epigenetic clocks for three threatened freshwater fish species. One epigenetic clock was developed for the Australian lungfish (Neoceratodus forsteri) and the second for the Murray cod (Maccullochellapeelii) and Mary River cod (Maccullochellamariensis). Age estimation models were calibrated using either known‐age individuals, ages derived from otoliths or bomb radiocarbon dating of scales. We demonstrate a high Pearson's correlation between the chronological and predicted age in both the Lungfish clock (cor = .98) and Maccullochella clock (cor = .92). The median absolute error rate for both epigenetic clocks was also low (Lungfish = 0.86 years; Maccullochella = 0.34 years). This study demonstrates the transferability of DNA methylation sites for age prediction between highly phylogenetically divergent fish species. Given the method is nonlethal and suited to automation, age prediction by DNA methylation has the potential to improve fisheries and other wildlife management settings.