Destocking waterways: Evidence that stocked Murray Cod (Maccullochella peelii) were extracted at pumped irrigation diversions within 24 hours of release

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.

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

Researchers have hypothesised that influxes of pelagic zooplankton to river channels after floods and high flows are necessary for strong recruitment of some native fish species, including Murray cod (Maccullochella peelii peelii) (Mitchell), in the Murray–Darling river system, Australia. This study investigated the composition of the diet and gut fullness of drifting Murray cod larvae weekly during two spawning seasons with contrasting flows, to determine if pelagic zooplankton comprised a greater proportion of the gut contents and guts were fuller in a high flow (2000) than in a low flow (2001) year. Gut fullness and yolk levels of 267 larvae were ranked, and prey identified to family level. Approximately 40 and 70% of individuals had been feeding in 2000 and 2001, respectively. Gut fullness increased with declining yolk reserves. Larvae in both the years had an almost exclusively benthic diet, irrespective of the flow conditions at the time. Substantial inundation of dry ground in 2000, albeit restricted to in-channel benches, anastomosing channels and oxbow lakes, did not lead to an influx of pelagic, floodplain-derived zooplankton subsequently exploited by Murray cod larvae. These results have the implications for the management of regulated temperate lowland rivers: high flows cannot automatically be assumed to be beneficial for the fish larvae of all species and their food resources, and caution should be exercised with the timing of flow releases.

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations, restocking of organisms and habitat modifications; thus, determining whether hybridization is occuring after reintroducing extirpated congeneric species is commensurately important for conservation. Restocking programs are sometimes criticized because of the genetic consequences of hatchery-bred fish breeding with wild populations. These concerns are important to conservation restocking programs, including those from the Australian freshwater fish family, Percichthyidae. Two of the better known Australian Percichthyidae are the Murray Cod, Maccullochella peelii and Trout Cod, Maccullochella macquariensis which were formerly widespread over the Murray Darling Basin. In much of the Murrumbidgee River, Trout Cod and Murray Cod were sympatric until the late 1970s when Trout Cod were extirpated. Here we use genetic single nucleotide polymorphism (SNP) data together with mitochondrial sequences to examine hybridization and introgression between Murray Cod and Trout Cod in the upper Murrumbidgee River and consider implications for restocking programs. We have confirmed restocked riverine Trout Cod reproducing, but only as inter-specific matings, in the wild. We detected hybrid Trout Cod–Murray Cod in the Upper Murrumbidgee, recording the first hybrid larvae in the wild. Although hybrid larvae, juveniles and adults have been recorded in hatcheries and impoundments, and hybrid adults have been recorded in rivers previously, this is the first time fertile F1 have been recorded in a wild riverine population. The F1 backcrosses with Murray cod have also been found to be fertile. All backcrosses noted were with pure Murray Cod. Such introgression has not been recorded previously in these two species, and the imbalance in hybridization direction may have important implications for restocking programs.

Rates of hybridization and introgression are increasing dramatically worldwide because of translocations, restocking of organisms and habitat modifications (Allendorf et al., 2001) thus determining whether hybridization is beneficial or detrimental for the species involved is commensurately important for conservation. Restocking programs are sometimes criticized because of the genetic consequences of hatchery-bred fish breeding with wild populations. These concerns are important to conservation restocking programs, including Percichthyidae. Two of the better known Australian Percichthyidae are the Murray Cod (Maccullochella peelii) and Trout Cod (Maccullochella macquariensis) which were formerly widespread over the Murray Darling Basin. In much of the Murrumbidgee River Trout Cod and Murray Cod were sympatric until the late 1970s when Trout Cod were extirpated. Here we use genetic SNP data to examine hybridization and introgression between Murray Cod and Trout Cod in the upper Murrumbidgee River and consider implications for restocking programs. For the first time we have confirmed restocked riverine Trout Cod as reproducing in the wild. We detected hybrid Trout Cod-Murray Cod in the Upper Murrumbidgee, recording the first hybrid larvae in the wild. Although hybrid larvae, juveniles and adults have been recorded in hatcheries and impoundments, and hybrid adults have been recorded in rivers previously (Douglas, Gooley & Ingram, 1994a; Douglas et al., 1995) , this is the first time fertile F1 have been recorded in the wild. The F1 backcrosses with Murray cod have also been found to be fertile. All backcrosses noted were with pure Murray Cod. Such introgression has not been recorded previously in these two species, and the imbalance in hybridization direction may have important implications for restocking programs.

The Murray cod (Maccullochella peelii peelii) is a large fish species keenly sought by anglers. However, this species has declined in distribution and abundance and is now listed nationally as vulnerable. This study was undertaken in the Ovens and Murray rivers, to collect larvae and age-0 Murray cod and determine the distribution of larval Murray cod around the mid-Murray River irrigation storage of Lake Mulwala. Murray cod larvae were collected from 17 of 18 sites: main channels and flowing anabranch channels of regulated and unregulated rivers, sites upstream and downstream of the lake, in the upper and lower reaches of the lake, and in the outflowing Yarrawonga irrigation channel. Larval Murray cod were collected only by methods that sampled drift in flowing waters. Age-0 Murray cod were collected by electrofishing in the main river, but not in off-channel waters, suggesting that cod are likely to settle into habitats in the main channel at a post-larval stage. The widespread occurrence of drifting larvae suggests that this species may be subject to previously unrecognised threats as they pass through hydro-electric power stations or become stranded in anabranch and irrigation channels. Results of this study are likely to be applicable to other species with drifting larval stages, and are relevant to other locations in the Murray–Darling Basin.

There are few places in the world where concerns over the decline of freshwater ecosystems and their fishes are more apparent than in the Murray-Darling Basin (MDB), south-eastern Australia. The rivers of the MDB are in poor condition, and fish populations were estimated to be at 10% of pre-European levels (Murray-Darling Basin Commission 2004), with many species considered of conservation concern (Lintermans 2007). Issues regarding fish in the MDB are not new; concerns for popular commercial species such as Murray Cod were first documented over 100 years ago (Dannevig 1903), and by the late 1970s, threats to MDB fishes had been clearly identified (Cadwallader 1978). While early concerns were largely regarding commercial fisheries, the great importance of angling (Henry & Lyle 2003) and the associated socio-economic values and benefits to the recreational fishery of the MDB ($1.35 Billion direct expenditure per annum: Ernst & Young 2011), along with serious conservation concerns, are now paramount. By 2000, there was a growing sense of urgency to rehabilitate fish populations before it was too late, and this culminated in the development of the Native Fish Strategy (NFS) (Murray-Darling Basin Commission 2004), with an emphasis on rehabilitation rather than just sustainable management. Such efforts were needed to protect and restore all fishes, including those key species targeted by recreational fishers. In its first decade (2003–2013), the NFS fundamentally changed fish management in the MDB (Koehn & Lintermans 2012). This included major advances in research and management practices and advocating an overarching and achievable restoration goal to: rehabilitate native fish communities of the Basin back to 60% of their estimated pre-European levels after 50 years of implementation. This was reinforced by engagement of the community and recognition that this would take several decades to deliver. Some key achievements included the following: 1 Development, testing and implementation of one of the world’s largest fish passage programmes from Lake Hume to the Sea.

<em>Abstract</em> .—The collection and use of data to manage the freshwater fisheries of Australia’s Murray–Darling basin (MDB) has a poor history of success. While there was limited assessment data for early subsistence and commercial fisheries, even after more robust data became available during the 1950s its quality varied across jurisdictions and was often poorly collated, assessments were not completed, and the data were underutilized by management. The fishery for Murray Cod <em>Maccullochella peelii </em> is given as an example, where the fishery declined to the point of closure and then the decline continued to the extent that Murray Cod was listed as a threatened species and all harvest now only occurs through the recreational fishery. Lessons from such poor population assessments have not been fully learned, however, as there remains a paucity of harvest data for this recreational fishery. Without a proper assessment, a true economic valuation of this fishery has not been made. As the MDB is Australia’s food bowl, there are competing demands for water use by agriculture, and without a proper assessment of the worth of the fishery, it is difficult for Murray Cod to be truly considered in either economic or sociopolitical discussions. The poor state of MDB rivers and their fish populations (including Murray Cod) has, however, resulted in political pressure for the development of the sustainable rivers audit, a common assessment method for riverine environmental condition monitoring. This audit undertakes standardized sampling for fish and a range of other variables at a number of fixed and randomly selected sites on a 3-year rotating basis. While the sustainable rivers audit has provided a range of data indicating that the condition of rivers is generally very poor, these data have yet to be fully utilized to determine the potential state of the fisheries (such as Murray Cod) or to set targets for rehabilitation, such as for environmental flows. While, to date, data analyses have been somewhat restricted by fiscal constraints, more comprehensive use of data, together with full fishery valuations, should be seen as the way forward for improved management.

Murray cod, Maccullochella peelii (Mitchell) is an iconic Australian species endemic to the Murray‐Darling Basin (MDB) of inland south‐eastern Australia. Murray cod has been a valuable food source and supported a large commercial fishery throughout much of the 20th century. Over‐fishing and habitat destruction have resulted in significant declines in Murray cod populations throughout much of its range. Since the early 1980s, large numbers of Murray cod have been stocked into waterways to support both recreational fishing and conservation efforts. In this study, the likely impacts of past and current stocking practices on genetic diversity of Murray cod were modelled and new strategies to maximise genetic diversity in stocked populations are explored. The results suggest that a large, well‐managed breeding and stocking programme could help maintain genetic diversity of Murray cod across the MDB. In catchments within the MDB where the effective population size is very small, a well‐designed stocking programme, following strict guidelines for numbers of families reared and number of individuals maintained per family, could increase genetic diversity in a few generations.

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.

– Native Murray cod (Maccullochella peelii peelii) are listed as a nationally vulnerable species, whereas non‐native common carp (Cyprinus carpio) are widespread and abundant. Understanding key aspects of life history, such as movement patterns and habitat selection by juvenile Murray cod and common carp, might be useful for conserving Murray cod populations and controlling common carp numbers. We used radio‐telemetry to track eight juvenile Murray cod and seven juvenile common carp in the Murray River, Australia, between March and July 2001. Common carp occupied a significantly greater total linear range (mean ± SD: 1721 ± 1118 m) than Murray cod (mean ± SD: 318 ± 345 m) and the average daily movement was significantly greater for common carp (mean ± SD: 147 ± 238 m) than for Murray cod (mean ± SD: 15 ± 55 m). All Murray cod and five of the seven common carp displayed site fidelity or residency to one, two or three locations. Murray cod were found only in the mainstream Murray River among submerged woody habitats, whereas common carp occurred equally in mainstream and offstream areas, and among submerged wood and aquatic vegetation. Murray cod were found in deeper (mean ± SD: 2.3 ± 0.78 m) and faster waters (mean ± SD: 0.56 ± 0.25 m·s−1) compared with common carp (mean ± SD: 1 ± 0.54 m; 0.08 ± 0.09 m·s−1) respectively. The presence of juvenile Murray cod only amongst submerged wood is an indication that these habitats are important and should be preserved. Conversely, juvenile common carp were equally present among all habitats sampled, suggesting that habitat selection is less specific, possibly contributing to their widespread success.

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

ABSTRACTThe Murray‐Darling Basin (MDB) is one of the most regulated and fragmented river systems in Australia. The basin's extensive river infrastructure development has supported growing water demands for irrigation, industry, domestic, and electricity production, but increased use of pumps for water diversion threatens aquatic species and biodiversity conservation. Of particular concern is the entrainment of fish from natural waterways into water intake infrastructure, which causes the loss of millions of fish every year in the MDB. Fish protection screens have been installed to prevent fish entrainment, but few studies have empirically evaluated their effectiveness, and none in Australian rivers. We investigated the effectiveness of fish screens in reducing the entrainment of multiple aquatic species and debris at differently sized discharge pump diversions in Australia. Screening pumps reduced fish entrainment by 93%–100% and debris entrainment by over 99%. When appropriately designed and operated, fish screens reduced fish, crustacean, and debris entrainment by small and large water pumps in multiple riverine environments. Impingement of organisms at screen installations requires further investigation. We recommend fish protection screens as a complementary tool for supporting native fish recovery and river management programmes, while still enabling an effective water supply for human needs.

Conservation of Murray cod (Maccullochella peelii), a large endangered fish species of Australia’s Murray–Darling Basin, relies on a detailed understanding of life history, including movement patterns and habitat use. We used radio-tracking to investigate the movement of 36 Murray cod in main channel and anabranch habitats of the lower River Murray during a flood and associated hypoxic blackwater event. During a flood peak of ~93 000 ML day–1, dissolved oxygen decreased to 1.2 mg L–1. Four movement types were observed: (1) localised small-scale movement, (2) broad-scale movement within anabranch habitats, (3) movement between anabranch and main channel habitats, and (4) large-scale riverine movement. Murray cod exhibited high fidelity to anabranch habitats but also moved extensively between anabranches and the main channel. Fish were consistently located in the main channel or permanent anabranches, suggesting that use of ephemeral floodplain habitats is limited, and highlighting the importance of connectivity between off-channel and main channel habitats. Mortality of radio-tagged fish was considerable (25%) in association with low dissolved oxygen concentrations, indicating that hypoxic blackwater may have had a substantial impact on Murray cod populations in the lower River Murray.

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.