Cold-water pollution impacts on two ‘warm-water’ riverine fish: interactions of dam size and life-history requirements

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

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

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.

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.

Determining factors responsible for increases in the mortality of freshwater fish larvae are important for the conservation of recruitment processes and for the long-term sustainability of freshwater fish populations. To assess the impact of one such process, Murray cod (Maccullochella peelii peelii Mitchell) and golden perch (Macquaria ambigua Richardson) larvae were arranged into treatment and control groups and passed through different configurations (overshot and undershot) of a low-level weir. Passage through an undershot weir resulted in the death of 95 ± 1% golden perch and 52 ± 13% Murray cod. By comparison, mortality was significantly lower in the overshot treatment and both controls. The relatively large number of undershot weirs within the known distribution of these species could impact upon recruitment over a large scale. It is therefore recommended that water management authorities consider the potential threats of operating undershot gated weirs on the survival of larval fish until further research determines appropriate mitigatory measures for these and other species.

Four species of Australian warmwater fishes were treated for ichthyophthiriasis with 5 g common salt/L. Juvenile silver perch (Bidyanus bidyanus), golden perch (Macquaria ambigua), Murray cod (Maccullochella peeli), and catfish (Tandanus tandanus) recovered from infestations of Ichthyophthirius multifiliis within 2 weeks. All fish in the control treatments died. The treatment eliminated the theronts and trophonts from all host fish by day 7 within the temperature range 19–26°C and by day 14 within the temperature range 11–18°C.

SummaryDegradation of instream habitats in the northern Murray–Darling Basin has occurred through numerous stressors, including siltation, clearing of bankside vegetation, intrusion of livestock and impacts of pest species. A better understanding of habitat preferences of native fish species could help guide future instream habitat restoration actions. The habitat choices of seven native fish species, juvenile Murray Cod (Maccullochella peelii), juvenile Golden Perch (Macquaria ambigua ambigua), juvenile Silver Perch (Bidyanus bidyanus), adult Murray–Darling Rainbowfish (Melanotaenia fluviatilis), adult Olive Perchlet (Ambassis agassizii), adult Un‐specked Hardyhead (Craterocephalus stercusmuscarum fulvus) and adult carp gudgeons (Hypseleotris spp.) were tested in preference troughs to help inform potential habitat restoration actions in the Condamine catchment. Each species was given a choice between pair combinations of open sandy habitat, submerged macrophytes, emergent plants and rocky rubble. Habitat preferences varied between species. Murray Cod, Golden Perch, carp gudgeons and Olive Perchlets preferred structure over open sandy habitat, whilst juvenile Silver Perch, Un‐specked Hardyhead and Murray–Darling Rainbowfish did not avoid open sandy habitats. Juvenile Murray Cod preferred rocky rubble habitat over all other habitat choices. Use of complex rock piles to provide nursery habitat for Murray Cod populations is a potential restoration option. Introduction of rock could also benefit Golden Perch and carp gudgeons. Use of emergent plants, submerged macrophytes and rocky rubble for habitat restoration all appear to have merit for one or more species of small‐bodied fishes or juvenile stages of larger sized fishes. Rocky rubble or floating attached macrophytes could be viable restoration options in areas too turbid to establish submerged macrophytes. These habitat interventions would complement existing actions such as re‐snagging and provision of fish passage to assist with sustainable management of native fish populations.

Short‐term (≤4 days) post‐release mortalities of two large, culturally and ecologically important Australian freshwater teleosts golden perch Macquaria ambigua (Richardson) and Murray cod Maccullochella peelii (Mitchell) were investigated. There was no angler‐induced mortality among golden perch that were immediately released in winter and spring, but 24% of this species and 15% of Murray cod died after delayed release in summer. Significant predictors of mortality were limited to times caught and total length for golden perch, and restraint methods and recovery times for Murray cod, but other parameters were also implicated as cumulative influences. The estimated mortalities may be sufficient to produce population‐level impacts for these two long‐lived species but could be considerably reduced through revisions of tournament regulations.

Multiple-scale assessments of fish-habitat associations are limited despite the fact that riverine fish assemblages are influenced by factors operating over a range of spatial scales. A method for assessing fishhabitat assemblages at multiple scales is proposed and tested in a large Australian dryland river, the Barwon–Darling River. Six discrete mesohabitat types (large wood, smooth bank, irregular bank, matted bank, mid-channel and deep pool) nested within 10 km long river reaches were sampled. Individual reaches were, in turn, nested within four larger geomorphological zones, previously identified along the river. Fish assemblages varied significantly between mesohabitat types and at different spatial scales. Golden perch (Macquaria ambigua), Murray cod (Maccullochella peelii peelii) and common carp (Cyprinus carpio) were strongly associated with large wood, but golden perch and Murray cod exhibited higher habitat specificity than carp. Bony herring (Nematalosa erebi) were more common in shallow edgewater habitats. At the riverscale, regional differences in the fish assemblage occurred at scales closely corresponding to geomorphological zones and these differences were associated with changes in the relative abundance of species rather than the addition or replacement of species. The proposed hierarchical framework improves the efficiency of fish surveys in large rivers by viewing meso-scale fish-habitat associations in the context of larger-scale geomorphological processes.

The present radio-tracking study compared adult daytime microhabitat use by three large Australian native freshwater fishes (Murray cod, Maccullochella peelii, trout cod, M. macquariensis, golden perch, Maquaria ambigua) and introduced carp, Cyprinus carpio, in the Murray River, south-eastern Australia. The paper describes habitat patches used by all species and quantifies differences among species. All species were strongly associated with structural woody habitat (>68% cover), deeper (>2.4 m), slower water (<0.2 m s–1) closer to the river bank, with variations in substrate. Murray cod and trout cod used deeper habitats (2.8 m and 2.9 m, respectively), with higher surface water velocities (0.37 m s–1 and 0.49 m s–1, respectively) and further from the bank than the habitats of golden perch (2.6 m; 0.31 m s–1) or carp (2.4 m; 0.20 m s–1), the latter species using wood higher in the water column than did cod species. Trout cod used habitats furthest from the bank and carp those closest. These data provide support and direction for reintroduction of structural woody habitat patches for rehabilitation which, in general, should have >70% cover, be >1.5 m high, located <15% of the river channel (width) closest to the bank, with surface water velocities of 0.3–0.6 m s–1.

Localized catastrophic events can dramatically affect fish populations. Management interventions, such as stocking, are often undertaken to re‐establish populations that have experienced such events. Evaluations of the effectiveness of these interventions are required to inform future management actions. Multiple hypoxic blackwater events in 2010–2011 substantially reduced fish communities in the Edward‐Wakool river system in the southern Murray‐Darling Basin, New South Wales, Australia. These events led to extensive fish kills across large sections of the entire system following a period of prolonged drought. To expedite recovery efforts, 119 661 golden perch Macquaria ambigua and 59 088 Murray cod Maccullochella peelii fingerlings were stocked at five locations over 3 years. All fish stocked were chemically marked with calcein to enable retrospective evaluation of wild or hatchery origin. Targeted collections were undertaken 3 years post‐stocking to investigate the relative contribution of stocking efforts and recovery via natural recruitment in the system. Of the golden perch retained for annual ageing (n = 93) only nine were of an age that could have coincided with stocking activities. Of those, six were stocked. The dominant year‐class of golden perch were spawned in 2009; before the stocking programme began and prior to blackwater events. All Murray cod retained (n = 136) were of an age that coincided with stocking activities, although only eight were stocked. Among the Murray cod captured, the dominant year‐class was spawned in 2011, after the blackwater events occurred. The results from this study provide first evidence that natural spawning and recruitment, and possibly immigration, were the main drivers of golden perch and Murray cod recovery following catastrophic fish kills. Interpreted in the context of other recent examples, the collective results indicate limited benefit of stocking to existing connected populations already naturally recruiting in riverine systems.

– To determine if the Murray cod Maccullochella peelii peelii is an apex predator in the lowland rivers of the Murray‐Darling Basin, its feeding ecology was compared with that of the sympatric top predator, golden perch Macquaria ambiguaambigua based on samples supplied by recreational anglers. Diet and prey size were reconstructed from alimentary tract contents of Murray cod (N = 39) and golden perch (N = 52), and their feeding morphology was assessed and included calculation of length–gape relationships and relative gut index. Both species fed principally on fish and decapods although Murray cod was the more piscivorous (frequency of occurrence 44% versus 6%, total number 50% versus 2%, total weight 90.4% versus 16.0%). Based on reconstructions of prey sizes, fishes up to 1 kg in weight were preyed on by Murray cod, distinguishing it from other top predatory fishes in the Murray‐Darling Basin and supporting its classification as an apex predator.

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.

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.