Murray Cod Research Articles

The Australian native marine fish species, silver sweep Scorpis lineolata, is susceptible to the megalocytivirus Infectious spleen and kidney necrosis virus (strain DGIV-10) obtained from a freshwater ornamental fish, dwarf gourami Trichogaster lalius. This was demonstrated by direct inoculation and through cohabitation. Transmission by cohabitation was also demonstrated from inoculated freshwater Murray cod Maccullochella peelii to euryhaline Australian bass Macquaria novemaculeata and to marine silver sweep. The virus was also transmitted from infected marine silver sweep to euryhaline Australian bass and then to freshwater Murray cod. This study is the first to demonstrate the virulence of a megalocytivirus derived from ornamental fish in an Australian marine species and the first to show a feasible pathway for the exchange of megalocytiviruses between freshwater and marine finfish hosts. These results demonstrate that megalocytiviruses from freshwater ornamental fish have the potential to spread to diverse aquatic environments.

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.

Globally, river degradation has decimated freshwater fish populations. To help reverse this trend in a southeastern Australia river, we used multiple restoration actions, including reintroduction of instream woody habitat, riparian revegetation, removal of a weir hindering fish movement, fencing out livestock, and controlling riparian weeds. We monitored the responses of native fish at the segment scale (20 km) and reach scale (0.3 km) over 7 years to assess the effectiveness of the different restoration strategies. Two closely related species, Murray cod Maccullochella peeli and trout cod Maccullochella macquariensis, increased at the restored segment compared with the control segment. However, inherent differences between river segments and low sample size hampered assessment of the mechanisms responsible for segment‐scale changes in fish abundance. In contrast, at the reach scale, only M. peeli abundance significantly increased in reaches supplemented with wood. These differential responses by 2 closely related fish species likely reflect species‐specific responses to increased habitat availability and enhanced longitudinal connectivity when the weir improved passage around a fishway. Changes in M. peeli abundance in segments supplemented with and without wood suggest an increase in carrying capacity and not simply a redistribution of individuals within the segment, facilitated the observed expansion. Our findings confirm the need to consider individual fish species' habitat preferences carefully when designing restoration interventions. Further, species‐specific responses to restoration actions provide waterway managers with precise strategies to target fish species for recovery and the potential to predict fish outcomes based on ecological preferences.

Effects of PUFA-enriched Artemia on the early growth and fatty acid composition of Murray cod larvae

Estimating the size selectivity of fishery users and sampling methods can be difficult to achieve due to data limitations. However, these limitations can be moderated by borrowed information from other sources such as other systems, times, and species. Here we develop a model that integrates an externally sourced boat electrofishing length–vulnerability model with internally sourced boat electrofishing and angling catch data to estimate length-dependent vulnerability of fish to angling in a data-limited situation. We apply the model to Murray cod (Maccullochella peelii) as an example and show that angling for Murray cod selectively captures a narrow range of sizes that includes medium to large size fish. Although boat electrofishing also followed a similar pattern, the range of fish sizes vulnerable to capture was much broader, including a more uniform vulnerability of all size classes evaluated. Understanding the length selectivity to capture has key implications for effective determination of fisheries regulations, as well as interpreting monitoring data. Thus, we see this modelling approach as a good option when more informative data are not available to support the estimation process.

Abstract Translocation of individuals is a widely used tool in the conservation of threatened species. The movement behaviours of translocated individuals in their new environment are a key factor that can influence translocation success (i.e. survival and reproduction). In this study, eel‐tailed catfish (Tandanus tandanus) and Murray cod (Maccullochella peelii) movements were monitored using fine‐scale acoustic telemetry over a 5‐month period in two lowland rivers in eastern Australia. Fine‐scale movement and habitat selection were compared among translocated lacustrine and resident riverine T. tandanus and between species. We found no difference in fine‐scale movement behaviours between translocated and resident T. tandanus; however, there was some variation in movement response to environmental variation between species. In contrast, habitat selection varied not only between the two species, but also between resident and translocated T. tandanus individuals. Notably, activity was significantly lower in the first day after release compared to the remainder of the study period. Although T. tandanus translocated from a reservoir had never experienced the environmental fluctuations of a riverine system, individuals still responded in the same way as resident riverine fish, suggesting an innate behavioural response. However, this was not the case for habitat selection, where translocated individuals preferred habitats more common in their source population's lacustrine environment. The findings of this study are important in guiding future conservation efforts involving the translocation of wild‐caught fish, primarily the importance of the suitable habitat at the release site to ensure the success and persistence of translocated populations.

A Gram-negative, strictly aerobic, motile, rod-shaped bacterium with monopolar flagellum, designated as F51T, was isolated from the skin ulcer of farmed Murray cod sampled from Zhejiang Province, China. Strain F51T grew at 4-37°C (optimal temperature, 28°C), pH 5.0-8.5 (optimal pH, 7.5) and NaCl concentration of 0-6.0% (w/v) (optimal concentration, 2.0%). Phylogenetic analysis based on average nucleotide identity (76.2-78.4%) and in silico DNA-DNA hybridization (22.3-23.2%) values revealed that strain F51T forms a distinct lineage in the clade of genus Pseudomonas with less than 98.9% 16S rRNA gene sequence similarity to type strains of the genus and represents a novel species related most closely to Pseudomonas floridensis LMG 30013T. Three housekeeping genes (rpoB, rpoD and gyrB) of strain F51T were analysed to further confirm that the isolate is distinctly delineated from related Pseudomonas species. Chemotaxonomic analysis indicated that the sole respiratory quinone of strain F51T is Q-9; its predominant cellular fatty acids are C16:0, summed feature 3 (iso-C15:0 2-OH and/or C16:1ω7c), summed feature 8 (C18:1ω7c and/or C18:1ω6c) and C10:0 3-OH; and its major polar lipids consist of diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol, two unidentified glycolipids, three unidentified phospholipids and an unidentified aminophosphoglycolipid. This composition is typical of the chemotaxonomic attributes of Pseudomonas. Based on its phenotypic, chemotaxonomic and phylogenetic features, strain F51T is considered to represent a novel species for which the name Pseudomonas ovata sp. nov. is proposed. The type strain is F51T (= KCTC 62133T = MCCC 1K03458T).

Most assessments of the effectiveness of river restoration are done at small spatial scales (<10 km) over short time frames (less than three years), potentially failing to capture large‐scale mechanisms such as completion of life‐history processes, changes to system productivity, or time lags of ecosystem responses. To test the hypothesis that populations of two species of large‐bodied, piscivorous, native fishes would increase in response to large‐scale structural habitat restoration (reintroduction of 4,450 pieces of coarse woody habitat into a 110‐km reach of the Murray River, southeastern Australia), we collected annual catch, effort, length, and tagging data over seven years for Murray cod (Maccullochella peelii) and golden perch (Macquaria ambigua) in a restored “intervention” reach and three neighboring “control” reaches. We supplemented mark–recapture data with telemetry and angler phone‐in data to assess the potentially confounding influences of movement among sampled populations, heterogeneous detection rates, and population vital rates. We applied a Bayesian hierarchical model to estimate changes in population parameters including immigration, emigration, and mortality rates. For Murray cod, we observed a threefold increase in abundance in the population within the intervention reach, while populations declined or fluctuated within the control reaches. Golden perch densities also increased twofold in the intervention reach. Our results indicate that restoring habitat heterogeneity by adding coarse woody habitats can increase the abundance of fish at a population scale in a large, lowland river. Successful restoration of poor‐quality “sink” habitats for target species relies on connectivity with high‐quality “source” habitats. We recommend that the analysis of restoration success across appropriately large spatial and temporal scales can help identify mechanisms and success rates of other restoration strategies such as restoring fish passage or delivering water for environmental outcomes.

Stability of Infectious spleen and kidney necrosis virus and susceptibility to physical and chemical disinfectants

Megalocytiviruses, particularly red seabream iridovirus, infect a broad range of fish including both freshwater and marine species. Although a limited number of infectious spleen and kidney necrosis virus (ISKNV) strains have been reported in association with mortality events in marine aquaculture species, the potential host range for ISKNV strains, particularly of those that have been detected in ornamental fish, has not been well characterised. There have also been few reports on the susceptibility of euryhaline fish species that could potentially transmit megalocytiviruses between freshwater and marine environments. We found that the euryhaline Australian native percichthyid fish, Australian bass Macquaria novemaculeata, is susceptible experimentally to ISKNV (strain DGIV-10), obtained from a freshwater ornamental fish, dwarf gourami Trichogaster lalius. Australian bass developed clinical disease following direct inoculation and also following cohabitation with infected fish, and were able to transmit DGIV-10 to naïve Murray cod Maccullochella peelii. This study demonstrated the potential for a euryhaline species to become infected with, and transmit, the megalocytivirus ISKNV between fish populations.

Abstract Because of the high costs of collecting field data, many species recovery and management plans do not include a monitoring feedback component to measure the success of interventions and refine management strategies. Here, we demonstrate how leveraging existing monitoring data can provide broad-scale, cost-effective information about a threatened fish species, the Murray Cod Maccullochella peelii, which is of cultural and recreational importance in Australia. We applied a Bayesian hierarchical model of abundance to Murray Cod catch data collected as part of broad-scale, general condition monitoring in the Murray–Darling Basin. The model uses replicated sampling at spatially independent sites to disentangle the confounding effects of detection probability and abundance on catch data. We demonstrate the reliability of the analysis for determining trends in abundance with a simulation study, and we show that basinwide abundance of Murray Cod declined by over 50% between 2010 and 2013. We found that detection probability of Murray Cod can vary substantially across space and through time, suggesting that accounting for variable detection will be important in any future evaluation of Murray Cod populations. This study highlights variable detection as an issue in monitoring regimes and demonstrates a method for the cost-effective use of existing monitoring data to evaluate species abundance trends.

Worldwide, riverine fish are the target of environmental water because populations have declined in lotic river habitats following river regulation. Murray cod is an endangered Australian riverine fish with remaining populations associated with lotic river reaches with instream habitat, including some creeks operated as part of irrigation systems. Our objectives were to develop a life history model, apply the building block method of environmental flows to enhance the abundance of juvenile Murray cod and promote population recovery. From 2008 to 2018 we evaluated changes to Murray cod juvenile abundance before and after implementation of a perennial environmental flow regime that began in 2013. During the first year of the environmental flow, larvae were collected as evidence of spawning. Murray cod abundance can be enhanced with environmental flows that target: (1) an annual spring spawning or recruitment flow with no rapid water level drops; (2) maximising hydrodynamic complexity (i.e. flowing habitats that are longitudinally continuous and hydrodynamically complex); and (3) an annual base winter connection flow. Recognition that incorporating hydraulics (water level and velocity) at fine and coarse time scales, over spatial scales that reflect life histories, provides broader opportunities to expand the scope of environmental flows to help restore imperilled fish species in regulated ecosystems.

Persistent hypoxic or low-oxygen conditions in aquatic systems are becoming more frequent worldwide, causing large-scale mortalities to aquatic fauna. It is poorly understood, however, whether species can acclimate to long-term hypoxic conditions. In two experiments, we exposed juvenile freshwater fish (Murray cod, Maccullochella peelii) to low-oxygen conditions and investigated acclimation effects. Experiment 1 determined how responses could be modified by exposure to different temperatures (20, 24 and 28°C) and oxygen conditions (control 6-8 mgO2 L-1 and low-oxygen 3-4 mgO2 L-1) over 30days. Experiment 2 determined the acclimation ability of fish exposed to two temperatures (20 and 28°C) and low-oxygen conditions (3-4 mgO2 L-1) for three different acclimation periods (7, 14 and 30days). Responses were measured by determining critical oxygen tension (P crit), loss of equilibrium and aerobic capacity using resting respirometry. In experiment 1, resting oxygen requirements were negatively affected by long-term low-oxygen exposure except at the highest temperature (28°C). However, long-term acclimation in low-oxygen improved tolerance as measured by loss of equilibrium but not P crit. In experiment 2, fish could tolerate lower oxygen levels before reaching loss of equilibrium after 7days acclimation, but this declined overtime. Murray cod were most tolerant to low-oxygen at the lowest temperature (20°C) and shortest exposure time (7days). Extended low-oxygen exposure resulted in reduced aerobic capacity of fish particularly at the lowest temperature. While prior exposure to low-oxygen may allow fish to cope with hypoxic conditions better in the long-term, acclimation time was inversely related to tolerance, suggesting that resistance to hypoxia might decrease as a function of exposure time. Our study fills a much-needed gap in our understanding of how freshwater species acclimate to hypoxia, and in particular, how exposure to prolonged periods of low-oxygen and elevated temperatures affect organisms physiologically.

Mark–recapture of fish is often used to inform fisheries or conservation management. Given that variability in tagging efficiencies can affect estimates of population size, it is important that rates of tag rejection are quantified. We double tagged over 45000 large-bodied, long-lived fish in a turbid lowland river in south-eastern Australia. During yearly recapture fish surveys, data on tag rejection were collected. We found that the probability of tag rejection varied as a function of fish species, tag type and tagger experience. Floy tags were more likely to be rejected as dart tags in large-bodied golden perch Macquaria ambigua (6 and 3% respectively), silver perch (18 and 4% respectively) and trout cod (19 and 7% respectively), whereas rates of rejection in the large-bodied Murray cod Maccullochella peelii were similar for both tag types (12 and 10% respectively). More experienced taggers had lower rates of rejection, which varied between 1 and 48% for individual taggers. We trialled three morphological locations for passive integrated transponder (PIT) tagging of fish, with rejection rates lowest for tags implanted in the stomach cavity and highest for those implanted in the pectoral musculature. This study presents the first tag rejection rates for dart, floy and PIT tags for lowland Australian river fish, information that can help guide future monitoring programs.

Egg and larval fish drifting downstream are likely to encounter river infrastructure leading to mortality. Elevated fluid shear is one likely cause. To confirm this and determine tolerable strain rates resulting from fluid shear, egg and larvae of three Australian species were exposed to a high-velocity, submerged jet in a laboratory flume. Mortality was modelled over a broad range of strain rates, allowing critical thresholds to be estimated. Eggs were very susceptible to mortality at low strain rates and 100% of golden and silver perch died once strain rate exceeded 629 and 148s–1 respectively. Larvae were less vulnerable than eggs, but mortality increased at higher strain rates and at younger ages. Most ages of larvae will be protected if strain rate does not exceed 600s–1, although a lower guideline of less than 400s–1 may be needed in areas where very early stage Murray cod larvae drift. Golden perch and silver perch were not susceptible to shear once maturity reached ~25 days post-hatch (nearing juvenile metamorphosis). The thresholds described here will prove useful when refining design and operational guidelines for hydropower and irrigation infrastructure to improve fish survival.

To facilitate future research in freshwater fish recruitment response to environmental flow delivery, size-at-age and growth models are presented for eight fish species occurring in south-eastern Australia; three small-bodied species and the juvenile 0+ age classes of five large-bodied species. Otolith increments were used to estimate the daily age of golden perch Macquaria ambigua, bony bream Nematalosa erebi, common carp Cyprinus carpio; Murray cod Maccullochella peelii, freshwater eel-tailed catfish Tandanus tandanus, Australian smelt Retropinna semoni, un-specked hardyhead Craterocephalus stercusmuscarum fulvus and Murray–Darling rainbowfish Melanotaenia fluviatilis. Linear growth models provided the best fit for length-at-age data of juvenile 0+ age large-bodied species; whereas von Bertalanffy growth functions provided the best fit to length-at-age data of small-bodied species. The results provide novel baseline data for future research in this area.

Abstract Conservation strategies for endangered species often include protection from harvest by humans. Correct species identification is paramount for this form of management to be effective. Trout cod (Maccullochella macquariensis) is a threatened Australian freshwater fish, occupying habitats in the southern Murray–Darling basin. Trout cod, although protected from angling, morphologically resemble Murray cod (Maccullochella peelii), a species that is a key target of recreational anglers. During a long‐term mark–recapture study, angler return data were collected both for Murray cod and trout cod. Up to 40% of trout cod captured were identified by anglers as Murray cod, and the chance of misidentification increased with the increasing size of trout cod, implying that this species could be inadvertently retained by anglers. Moreover, unnecessary angling mortality of adult breeding individuals is likely to delay the time for recovery of this threatened species. As a large and vocal user group, anglers can play a key role in promoting the conservation of aquatic areas and fish species. There is a need for anglers and fishery managers to understand this problem and to work together on a solution, through the tighter enforcement of regulations where trout cod are present, and through an increased emphasis on education.

Different populations of organisms can vary widely in their responses to environmental conditions and this variation is fundamental to the persistence of species. Using a common garden experiment, we examined temperature-specific growth and survival responses of larvae among populations of Murray cod (Maccullochella peelii) from four regions of the Murray–Darling Basin (MDB), Australia. Fish larvae from the four regions differed significantly in their growth and survival responses at high water temperatures ≥26°C. At 30°C, survival rates of larvae by Day 20 ranged from 0% in the Lachlan region to 82% in the southern region. Opposite to the geographical differences in survival, growth of larvae was highest in the Lachlan (14.8–15.4-mm standard length 95% CI) and lowest in the southern region (13.4–13.9-mm standard length 95% CI) at 26°C where sufficient numbers survived for comparison. Geographical differences in growth and survival responses did not follow a consistent latitudinal gradient as observed for other species, but were closely linked with previously described genetic structure. Our results suggest that the upper thermal limit of M. peelii larvae is near common river temperatures in the MDB and that maintaining functional response diversity and underlying genetic diversity will be important for ensuring the resilience of this apex predator under future climate change.

A yellow-pigmented, Gram-staining-negative, aerobic, non-motile and rod-shaped bacterium, designated strain F30T, was isolated from fresh water of a diseased farmed Murray cod with a profound ulceration pond in Zhejiang province, PR China. Growth was observed at NaCl concentrations of 0.5-3.5 % (w/v) (optimum, 1.5-2.0 %), temperatures of 10-35 °C (optimum, 28 °C) and pH 5.0-9.0 (optimum, 6.5). Phylogenetic analysis based on 16S rRNA gene sequences indicated that F30T represented a member of the genus Chryseobacterium, showing the highest similarity to Chryseobacterium jejuense DSM 19299T (99.0 %) and Chryseobacterium nakagawai NCTC 13529T (99.0 %), and less than 98.7 % similarity to other species of the genus Chryseobacterium with validly published names. The average nucleotide identity and in silico DNA-DNA hybridization values between F30T and the reference strains were 78.4-90.5 % and 2.6-42.5 %, respectively. The results of chemotaxonomic analysis indicated that the fatty acids, as well as the polar lipid profiles of F30T were similar to those of species of the genus Chryseobacterium, and the sole respiratory quinone was MK-6. On the basis of its phylogenetic, chemotaxonomic and phenotypic data, strain F30T represents a novel species, for which the name Chryseobacteriumaurantiacum sp. nov. is proposed. The type strain is F30T (=KCTC 62135T=MCCC 1K03457T).

The allocation of time and energy to different behaviours can impact survival and fitness, and ultimately influence population dynamics. Intrinsically, the rate at which animals expend energy is a key component in understanding how they interact with surrounding environments. Activity, derived through locomotion and basic metabolism, represents the principal energy cost for most animals, although it is rarely quantified in the field. We examined some abiotic drivers of variability in locomotor activity of a free-ranging freshwater predatory fish, Murray cod (Maccullochella peelii), for six months using tri-axial accelerometers. Murray cod (n = 20) occupied discrete river reaches and generally exhibited small-scale movements (<5 km). Activity was highest during crepuscular and nocturnal periods when water temperatures were warmest (19–30°C; January–March). As water temperatures cooled (9–21°C; April–June) Murray cod were active throughout the full diel cycle and dormant periods were rarely observed. Light level, water temperature and river discharge all had a significant, non-linear effect on activity. Activity peaked during low light levels, at water temperatures of ~20°C, and at discharge rates of ~400 ML d-1. The temporal changes observed in the behaviour of Murray cod likely reflect the complex interactions between physiological requirements and prey resource behaviour and availability in driving activity, and highlight the importance of empirical field data to inform bioenergetics models.