Native Fish
Over the past 10 years we have been monitoring and evaluating how environmental water is being used to support native fish so that they can breed, grow and move freely.
Image: Releasing a tagged Murray cod back into the river. Photo credit: Arthur Rylah Institute
Introduction
Restoring native fish populations is crucial in restoring the overall health of the Murray–Darling Basin system. Fish play a vital role in maintaining healthy ecosystems, have cultural significance, and are important for communities and recreational economies. Fish populations are a key indicator for the overall health of river systems.
Native fish populations have declined substantially in the last 100 years, with one of the main causes within the Murray–Darling Basin being the alteration of natural hydrology and hydraulic regimes. Together, these are known as the ‘flow regime’. Commonwealth environmental water plays a crucial role in creating a flow regime that supports many of the life-history processes for fish (pg. 1).
Zeb Tonkin, our Fish Theme leader, explains what our work has revealed so far in this introductory video.
The long-term dataset from 2014–15 to 2023–24 shows that environmental water contributed to increased fish spawning, recruitment, population growth, frequency of occurrence, body condition, and improved community composition in some Selected Areas for several native species (pg. iv). Environmental water reduced the number of low flow days and increased daily average flows and changes in flow variability. Furthermore, flows have improved habitat and resource availability, and longitudinal and lateral connectivity. All these elements are important for improving native fish populations.
Note: The contents on this page includes summarised text from the following report: Basin-scale evaluation of 2023–24 Commonwealth environmental water: Fish. Page number references have been noted throughout the content below for anyone using the full report.
Our approach
We investigate how water for the environment can be used to enhance specific parts or processes of the fish life cycle for species like the Murray cod, golden perch and bony herring. We have incorporated these findings into our fish population models to help environmental water managers more accurately plan when, where and how to deliver flows for fish so that they breed, spawn and, hopefully, successfully add to their numbers.
In collaboration with Selected Areas around the Murray–Darling Basin, we use our long-term monitoring to test our predictions and measure responses to management actions taken by water managers. This enables us to assess what did and didn’t work, so that these actions can be adapted and refined for the next time around.
We use a counterfactual modelling approach to determine the contribution of Commonwealth Environmental Water to fish populations for the Basin-scale evaluation from 2014-24. In this approach, models were used to predict fish responses under a range of flow condition scenarios from the 10 years of monitoring data in 6 of the 7 Selected Areas, and including a flow scenario without Commonwealth Environmental Water (the counterfactual scenario). This approach separates the impact of Commonwealth Environmental Water from natural changes in river flow and shows what might have happened without Commonwealth Environmental Water (pg.7).
Electrofishing in the Wakool River. Photo Credit: Robyn Watts, Charles Sturt University
Schematic of the evaluation approach
What have we learned?
How can water for the environment benefit native fish?
Supports life-history processes
Our assessment of long-term monitoring data on spawning and population size has indicated that environmental water has provided a range of benefits to native fish populations and supported critical life-history processes, such as breeding success, body condition and population growth (pg. 22). For example, environmental water has contributed to increased spawning rates and improved body condition of golden perch. These results provide evidence to inform our understanding of how environmental water is helping to maintain, protect and restore native fish populations in the Murray–Darling Basin.
Enables a diversity of fish species
It’s not just the big fish that are benefiting, we’ve also seen improvements in some of the Selected Areas for breeding, recruitment and expanded distributions of smaller species such as Murray-Darling rainbowfish and Australian smelt. These fish are not often thought about, but they are vital for the functioning of a healthy river ecosystem, providing food resources for larger fish, water birds and turtles. It is also important that our rivers have a fish community composition that is mixed with large, medium and small-bodied fish (pg. 33).
Helps fish move
Movement is critical for fish to complete their life cycle, being linked to key life-history processes such as spawning migrations, dispersal from breeding areas, or accessing feeding or nursery habitats. We investigated the scale over which these species move and how we can use environmental water to enhance this (pg. 55). Golden perch movements can occur at large spatial scales, moving hundreds and even thousands of kilometres during their lifetime. These movements to and from breeding and feeding areas have a major influence on populations in a specific reach or river. Murray cod do not move as much, typically staying within a specific reach or river. For both species, we found a positive association between river flow and these movements, with these results enabling us to quantify how we can influence populations via movement using water for the environment.
Population models developed to inform future management
Population models have been constructed for Murray cod, golden perch and bony herring. These models enable us to estimate how a population is tracking through time and to forecast what they might look like many years into the future. They do this by integrating the most up-to-date research on how key life-history processes such as spawning, survival and movement respond to flow management (pg. 3). We can now sit down with water managers and use the models to test a range of management scenarios to help prioritise the best way to deliver water for the environment aimed at achieving long term outcomes for native fish populations.
Volume of Commonwealth water for the environment delivered to Murray-Darling Basin regions
In 2023-24, Commonwealth environmental water volume contributions and number of watering actions for fish were the highest since the monitoring program began. The main objectives for enhancing native fish populations were to improve habitat, enhance connectivity or provide cues for movement, and to a lesser extent, support various targeted species outcomes. It is also important to note that many of these expected outcomes for fish are intrinsically linked. For example, freshes can provide cues for movement as well as spawning, while improved habitat can enhance fish survival which can subsequently benefit species or spawning outcomes (pg. 16).
How water for the environment supports fish life history processes
This information is drawn from the Flow-MER Fish Theme Report as part of the 2023-24 Basin-scale evaluation of Commonwealth environmental water. These reports bring together monitoring, evaluation and research findings from long-term data sets and current research. Counterfactual modelling was performed across the entire 2014-15 to 2023-24 dataset, further demonstrated that Commonwealth environemntal water contributed to the increased fish spawning, recruitment, population growth, frequency of occurrence and body condition and improved body composition in some Selected Areas for several species of fish.
Spawning rates
Water for the environment can contribute to spawning by influencing velocity, habitat condition and connectivity between and within rivers and wetlands (pg. 19). Modelling has shown that spawning for golden perch increased in some years for all Selected Areas (note, lower confidence in Edward/Kolety–Wakool and Lachlan areas), driven by increased flows in the days leading up to sampling (pg.iii).
Recruitment
Water for the environment can contribute to the recruitment of native fish by reducing the number of low flow days, which maintains food and habitat sources (pg. 19). Modelling has shown that recruitment of Australian smelt increased as a result of environmental water delivery in some years at several Selected Areas (pg. v)
Population growth
Environmental flows create flow variability and reduce low-flow days, which can increase population growth (pg. 22). Modelling shows adult Murray cod population growth rates increased with environmental water use in most areas. Golden perch population rates increased in most years in most areas (pg. v).
Distribution
Water for the environment can improve native fish species distribution by supporting hydrological connectivity within and between rivers, wetlands and floodplains, thereby increasing available habitat and food resources (pg. 55). Although this evaluation was not designed to assess movement, other metrics of spawning and frequency of occurrence have strong links to movement, and so positive links between environmental water use and movement can be inferred. The importance of flow for golden perch and Murray cod movement are a focus of Flow-MER research.
Body condition
Improved body condition can be driven by the reduction in low flow days and an increase in average daily flows. Flow increases can promote fish body condition through increased productivity, availability of food resources and habitat, and connectivity (pg. 43). Individual body condition for Murray cod, golden perch and bony herring increased as a result of environmental water, in some Selected Areas (pg. iv).
Habitat
Environmental flows improve and increase fish habitat by inundating more aquatic areas (such as floodplains and wetlands) and improving the conditions of current available habitat (pg. 16). Over-wintering flows are crucial in preventing cease-to flow events, which constrict habitat availability and connectivity. For example, many freshes were delivered to provide cues for movement and to improve fish habitat for multiple species.
Community composition
Community composition is the measurement of the proportion of a catch that is native. Environmental water contributed to improved community composition in the Lower Murray, Goulburn, Murrumbidgee Rivers and Edward/Kolety–Wakool river system (pg. 22).
Water quality
Environmental water can maintain water quality for fish by ensuring adequate flow, oxygen levels, and nutrient balance. Poor water quality can lead to hypoxic conditions which may cause mortality or emigration. Commonwealth environmental water plays a crucial role (particularly in drier years) to maintain water quality as the Basin enters the next arc of low flows and reduced water availability, which will be further exacerbated by climate change. For example, in 2023–24, Commonwealth environmental water was used to ameliorate poor water quality to help prevent the likelihood of hypoxic events and associated fish deaths in several rivers across the Basin (pg. 66).
Notes for water managers and adaptive management
In some systems which are impacted by extreme low flows (e.g. Broken River, Campaspe River, Goulburn River, northern basin rivers), environmental water deliveries may be best saved and delivered in drier times to reduce the number of low flow days, though this may require changes in environmental water delivery policies. Understanding the fish response to natural high flow events provides critical information about the delivery of additional environmental water when it is available and what this could achieve for fish populations.
The spatial scale of water delivery needs to match the life history of fish, with monitoring increasingly revealing the large spatial scale over which some processes (e.g. movement) can operate. For some species, such as silver perch and golden perch, hundreds of kilometres of continuous lotic (flowing) habitat, which may require inter-regional connected flow events, are needed for fish to complete life-history processes. For other species, relatively small spatial scales (10s of kilometres) are required, such as for Murray cod.
Each annual evaluation contributes to our understanding of how the ecosystems of the Basin respond to delivery of Commonwealth environmental water in the short term and long term. In summary Commonwealth environmental water should:
- Reduce the number of low flow days and increase average daily flows in Selected Areas, to provide a range of benefits including supporting critical life-history processes (pg. 57).
- Provide flow over winter to prevent cease-to-flow conditions, which is important to avoid in some southern basin systems to improve fish recruitment.
To improve our approach to Basin-scale evaluation:
- Evaluation of Commonwealth environmental water requires better alignment of recovery frameworks and clearer objectives. Altered flow regimes are only one of several threats that native fish species face. We strongly recommend to integrate Flow-MER into the broader recovery framework for the Basin, such as the Native Fish Recovery Strategy (pg. 58).
- Realistic outcomes for the hydrological regimes, fish species and Selected Areas need to be included into the Flow-MER monitoring program. The monitoring program would benefit from clear and realistic expectations for anticipated ecological responses (i.e. during a drought or a wet year), especially if the objective is to disentangle environmental water from the wider flow regime (pg. 59).
How water for the environment benefits different fish species
This evaluation describes outcomes for fish across the Basin from the use of Commonwealth environmental water for the water year 2023–24 as well as cumulative outcomes since monitoring began in 2014–15. This information is used to answer the following evaluation question: What did Commonwealth environmental water contribute to sustaining native fish at the Basin scale?
Murray cod
Golden perch
Murray-darling rainbowfish
Australian smelt
Silver perch
Bony herring
Unspecked hardyhead, Murray hardyhead, flathead gudgeon and carp gudgeon
Exotic species
Over the past 10 years, Commonwealth environmental water has had significant benefits to Murray cod by restoring base flows and spring freshes. These results demonstrate the importance of using water for the environment to fill the gaps in flow hydrographs altered by river regulation.
Key findings 2023-24
- There were low levels of recruitment of Murray cod in the Lower Murray River, and recruitment was either very low or absent in other Selected Areas. Possible reasons for poor recruitment include unfavourable flow conditions during nesting and early life stages, insufficient prey availability in summer, and populations still recovering from the previous year’s flooding disturbance impacts (e.g. hypoxic conditions leading to fish emigration or mortality) (pg. 15).
- Quantitative analyses found no positive associations between Commonwealth environmental water and recruitment of Murray cod (pg. 15).
Outcomes 2014–24
- Murray cod were detected in all years in all Selected Areas.
- Counterfactual modelling showed that Commonwealth environmental water contributed to increased fish spawning, recruitment, population growth, frequency of occurrence and body condition, and improved community composition in some Selected Areas for the Murray cod (pg. iv).
- The provision of Commonwealth environmental water contributed to increased individual body condition of Murray cod in some Selected Areas.
- Population growth rates of Murray cod decreased in response to the time since most-recent fish-death event (pg. 39)
- The contribution of to recruitment of Murray cod was predicted to be greatest in the southern Basin, including in the Broken River, Goulburn River, Loddon River, Lower Murray River and Campaspe River (pg. 48)
The Commonwealth environmental water (CEW) effect map (left) shows the relative effect size of CEW delivery on Murray cod recruitment for each flow gauge. Larger circles and lighter blues indicate larger effects. Smaller maps (right) show differences between actual flows and modelled flows without CEW for average daily flows (flow median) and low flow days (flow low) – 2 key flow metrics for Murray cod. For average daily flows, larger circles/lighter blues indicate increased flows due to CEW. For low flow days, larger circles and lighter blues indicate the number of low flow days that decreased with CEW (i.e. declining low flow days indicate fewer number of low flow days and, in theory, better flow environment). A subset of flow gauges was used across the Basin (n = 97) for predictions, and several gauges were filtered out of the prediction process due to incomplete flow data. Green line denotes Gwydir River System, aqua line denotes Lachlan River System, pink line denotes Murrumbidgee River System, orange line denotes Edward/Kolety–Wakool river systems, yellow line denotes Goulburn River, and light blue line denotes Lower Murray River.
Using water for the environment to provide high flows during spawning and restoring hydrological connectivity has resulted in significant benefits for golden perch. These results demonstrate the importance of using water for the environment to fill the gaps in flow hydrographs altered by river regulation.
Outcomes 2023-24
- Golden perch spawned in the Goulburn River and in the Lower Murray River
- There were low levels of recruitment of golden perch in the Lower Murray River and limited evidence of recruitment in the other Selected Areas. It is possible that high instream flows may have reduced the detectability of larvae, eggs and new recruits in some Selected Areas (pg. iii)
- Eggs and or larvae of golden perch (indicator of successful spawning) were detected in Goulburn and Lower Murray River (pg. vii).
Outcomes 2014–24
- Golden perch were detected in all years across all Selected Areas.
- Counterfactual modelling showed that Commonwealth environmental water contributed to increased fish spawning, recruitment, population growth, frequency of occurrence and body condition in some Selected Areas for the golden perch (pg. iv)
- The likelihood of spawning of golden perch (i.e. presence of larval fish and eggs) increased as a result of Commonwealth environmental water delivery in some years across all Selected Areas (pg. v)
- Adult population growth rate increased with Commonwealth environmental water delivery for golden perch in most years in the Goulburn River, Lachlan River System, Lower Murray River and Murrumbidgee River System (due to Commonwealth environmental water reducing the number of low flow days) (pg. 5)
- Individual body condition for golden perch increased with Commonwealth environmental water delivery
- Golden perch spawning was detected in the Goulburn River, Lachlan River System, Lower Murray River and Murrumbidgee River System; however, only low numbers of recruits were detected in most Selected Areas (pg. vii)
Using water for the environment to deliver flows from spring to autumn has contributed to Murray-Darling rainbow fish recruitment and distribution in previous years. These results demonstrate the importance of restoring specific flow components such as having fewer low flow days and flow variability.
Outcomes 2023-24
- Murray-Darling rainbowfish were in lower abundance (representative of poor recruitment) for the last several years in some Selected Areas, likely due to high flows negatively impacting on the populations. Quantitative analysis did not find any positive associations with Commonwealth environmental water and this species (pg. 16).
- The small-bodied Murray-Darling rainbowfish was lower in abundance and there was little evidence of recruitment for the past several years in some Selected Areas. High flows may have negatively impacted populations of this species, as it prefers low-to-moderate stable flows during its extended spawning and recruitment period (late spring to early autumn) (pg. iv).
- Our quantitative analyses found no positive associations between Commonwealth environmental water and Murray-Darling rainbowfish recruitment (pg. iv).
Outcomes 2014–24
- Commonwealth environmental water contributed to an increase in fish spawning, recruitment, population growth, frequency of occurrence and body condition, and improved community composition in some Selected Areas for Murray-Darling rainbowfish (pg. iv).
- There were a few positive effects of Commonwealth environmental water on recruitment of Murray-Darling rainbowfish (pg. v).
- The contribution of Commonwealth environmental water to the recruitment of Murray cod and Murray-Darling rainbowfish was predicted to be greatest in the southern Basin (pg. v).
Using water for the environment to deliver flows from spring to autumn contributed to Australian smelt recruitment and distribution. This is due to their protracted spawning season, which means that flows delivered from spring to autumn likely meet their life history requirements.
Outcomes 2023–24
- Australian smelt may have benefited from the hydrological conditions. High unregulated in-stream flows likely facilitated hydrological connectivity and increased habitat and food resources (pg. 16).
- Our quantitative analysis found positive associations between Commonwealth environmental water and Australian smelt recruitment in the Goulburn River, Lachlan River System and Lower Murray River (pg. iv).
- Quantitative analyses found positive associations between Commonwealth environmental water and Australian smelt in the Goulburn River (pg. 16).
Outcomes 2014–24
- Delivery of CEW contributed to Australian smelt being detected at more sites within most Selected Areas.
- Recruitment of Australian smelt increased as a result of Commonwealth environmental water delivery (fewer low flow days and changes in flow variability) in some years in several Selected Areas. This species is classified as opportunistic, being small-bodied and having a short life span, prolonged spawning seasons and frequent reproduction (pg. 63).
- There was evidence that Commonwealth environmental water delivery (fewer low flow days) contributed to increased probability of detection of Australian smelt at more sites within Selected Areas in most years (pg. v).
- Commonwealth environmental water contributed to detections of Australian smelt at more sites within the:
- Edward/Kolety–Wakool River systems (5 out of 10 years)
- Goulburn River (10 out of 10 years)
- Lachlan River System (5 out of 10 years)
- Lower Murray River (9 out of 10 years)
- Murrumbidgee River System (7 out of 10 years) (pg. 35).
Outcomes 2023–24
- Silver perch spawned in the Lower Murray River and in the Murrumbidgee River System. Spawning in the Goulburn River coincided with a large natural overbank flow and a Commonwealth environmental water delivery of a fresh. In the Lower Murray River, silver perch spawning coincided with large in-stream flow pulses which included contributions of Commonwealth environmental water (pg. iii).
- Drifting silver perch eggs were detected in the Murrumbidgee River System.
- Quantitative analysis did not show a positive association in spawning responses with Commonwealth environmental water in the Lower Murray or Murrumbidgee Selected Areas (pg. 15)
Outcomes 2014–2024
- Detection of silver perch varied in detection across all Selected Areas and all years.
- Silver perch spawning increased in response to the day of year (with strong evidence) and change in daily flow in the 7 days prior to sampling.
- There were no positive associations between CEW delivery and silver perch spawning at the Selected Areas across the monitoring period (pg. 26)
Outcomes 2014–2024
- Modelling showed that Commonwealth environmental water contributed to increased fish spawning, recruitment, population growth, frequency of occurrence and body condition, and improved community composition in some Selected Areas for several native species, including bony herring (pg. iv).
- There was no evidence of recruitment of bony herring with delivery of CEW. One potential explanation is that spawning and recruitment of bony herring can occur with and without large flows and flooding; therefore, our flow metrics may not be sensitive enough to ascertain this species’ response (pg. 65).
- Bony herring body condition increased with delivery of Commonwealth environmental water (changes in flow variability) in some years across all Selected Areas
- Population growth rates of bony herring (both with strong evidence) increased with increasing maximum antecedent flows (pg. 39)
- Commonwealth environmental water contributed to detections of bony herring at more sites in 2 out of 10 years within the Goulburn River and Lower Murray River and in 1 out of 10 years within the Edward/Kolety–Wakool river systems (pg. 35).
Outcomes 2014–2024
- Small-bodied native species are generally less dependent on flows for spawning and recruitment compared to large-bodied species. However, in 2023 – 24, there is evidence that the flow regime in the Goulburn River, Lachlan River System and Lower Murray River favoured recruitment for some small-bodied native fish species, likely through the provision of increased hydrological connectivity, habitat and food resources (pg. iv).
- High unregulated in-stream flows likely facilitated hydrological connectivity and increased habitat and food resources for small-bodied native fish (pg. 20).
- Flows were delivered to Lower Murray wetlands to support and provide breeding opportunities for the nationally Endangered Murray hardyhead (EPBC Act) and to support small-bodied fish conservation actions including reintroduction and surrogacy for the threatened Murray hardyhead, southern purple-spotted gudgeon, southern pygmy perch and Yarra pygmy perch (EPBC Act, South Australian Fisheries Management Act 2007). Murrumbidgee wetlands were watered to support habitat for the NSW Endangered southern pygmy perch (New South Wales Fisheries Management Act 1994) (pg. 18).
- Murray–Darling rainbowfish and unspecked hardyhead (Craterocephalus fulvus), 2 small-bodied native species, had lower abundance and recruitment for the last couple of years (2022–23, 2023–24) in the Edward/Kolety–Wakool river systems, Lower Murray River and Murrumbidgee River System (pg. 20)
Outcomes 2014–2024
- 5 exotic species were detected during the 10-year monitoring program that conducted in-stream river sampling of adult fish communities in the Selected Areas (pg. 21).
- Exotic species (common carp and goldfish) declined markedly in abundance this year after the initial boom response to the 2022–23 wet conditions when widespread flooding and high flows were experienced across Selected Areas (pg. iv).
- Enhanced recruitment of exotic common carp occurred with the delivery of CEW in the Edward/Kolety–Wakool river systems, Gwydir River System, Lachlan River System, Lower Murray and Murrumbidgee River System. Responses of common carp to CEW were primarily driven by positive associations between increased average daily flows and recruitment (pg. 64)
- The frequency of occurrence of eastern gambusia decreased in response to Commonwealth environmental water in several Selected Areas
Fish outcomes at different river systems
There are 7 Selected Areas where Flow-MER work is undertaken. The findings below are drawn from work in these Areas. A summary of key findings across Selected Areas for 2023-24 can be found in Appendix C of the report (page 75- 85), with key findings shared below:
- Spawning of Murray cod, bony herring, Australian smelt, carp gudgeon, flathead gudgeon and river black fish was detected. There was no evidence of golden or silver perch spawning
- There was no Murray cod young-of-year recruits detected despite evidence of widespread spawning. This failure to recruit this year may be due to insufficient prey availability in the summer. Absence of Murray cod aged 1 year confirmed the species also failed to recruit in the 2022-23 year
- Adult Murray cod relative abundances have shown a continual decline since 2020–21 sampling.
- An adult river blackfish was detected in the mid-Wakool River in adult fish population surveys which is the second consecutive year of detection for this species
- River blackfish was detected in the mid-Wakool River for the first time in adult fish population surveys
- Silver perch relative abundances remain low but stable throughout the years
- Golden perch abundances were similar to previous years dominated by mature fish
- Adult Murray–Darling rainbowfish relative abundances have remained low which has been the case since 2021–22
- Unspecked hardyhead was not detected in the 2024 surveys, despite being present in surveys in all previous years
- Adult common carp populations and other exotic species declined markedly in abundance and biomass this year after the initial boom response to the 2022–23 flooding in the system
- Fish surveys shown that the fish community has declined in health since the 2022–23 floods
- Golden perch spawning coincided with a large overbank flow in October 2023 and during an environmental flow fresh in November 2023. Spawning of other native fish detected included Murray cod, cod (Maccullochella spp.), Australian smelt, gudgeon (Philypnodon grandiceps or Hypseleotris spp.), carp gudgeon and trout cod. Murray cod larva was the most abundant species detected which was markedly higher than last year where only 1 Murray cod larva was detected
- One young-of-year golden perch was recorded (though are rarely collected in the reach, as most likely early life stages drift into the Murray River), and the population consisted of larger sized, possibly older, fish, similar to previous years.
- No young-of-year silver perch were detected with only one adult collected. Only 2 young-of-year Murray cod were detected, which is similar to the previous year. However, these low numbers contrast prior to 2022–23, when young-of-year comprised larger proportions of the catch
- Relative abundances of adult Murray cod which decreased markedly in 2022–23 were again low in numbers. This may be due to Murray cod populations needing several years or longer to recover from the previous year disturbance (Theim et al. 2017). In contrast, relative abundances of adult carp gudgeons and Murray–Darling rainbowfish increased markedly compared to the previous year, likely due to these short-lived species maturing quickly and producing large numbers of offspring with favourable environmental conditions
- Adult bony herring which has been detected only occasionally throughout the surveys was again detected for the second consecutive year in similar numbers to last year.
- The nationally threatened trout cod was collected again in electrofishing surveys for the fifth consecutive year. Trout cod are known from other sites in the Goulburn River.
- Relative abundances of adult common carp and goldfish markedly decreased from the previous year
- Rainfall was at, or above, average during November 2023, April and May 2024, other months it was below average and after significant flooding in 2021–22 and 2022–23, inundation in the Gwydir catchment was greatly reduced in 2023–24. Delivery of CEW improved connectivity and water quality.
- Murray cod spawning was detected with low numbers of larval fish captured in the Gwydir and Mehi channels.
- Decreases in relative abundance from the previous year were evident for carp gudgeon and spangled perch (Leiopotherapon unicolor) (lowest recorded since monitoring began) and small increases for Australian smelt.
- Murray cod and bony herring were detected in similar abundances to the previous year.
- Species traditionally known to occur in the system, including olive perchlet (Ambassis agassizii), silver perch, southern purple-spotted gudgeon and Darling River hardyhead (Craterocephalus amniculus), were not detected in the monitoring program.
- Monitoring detected the same suite of species compared with previous years, though total number and native individual species numbers declined in 2023–24. The native fish community appears to remain depauperate and in poor condition with low recruitment in the Gwydir River System, despite the wetter conditions over the last several years. Other measures (e.g. stocking) complementary to the delivery of environmental water are likely needed to improve fish populations
- Spawning of Murray cod, flathead gudgeon, Australian smelt and carp gudgeon was detected in the lower Lachlan River System. A single larval golden perch was detected which is only the third time since the monitoring program began that larvae of this species has been recorded. No golden perch recruitment was detected.
- Murray cod had very low larval abundances again this year similar to the past 3 spawning seasons. This may be due to falling water levels that occurred in late September to early October coinciding with Murray cod nesting behaviour and spawning windows leading to low larval abundances. A low number of young-of year were detected for the first time since 2020–21, though overall poor recruitment has been evident in the past few years.
- Adult Murray cod had similar relative abundances this year, compared with the previous year, though abundances were still markedly lower than in 2020–21. Golden perch abundances decreased compared with the previous year, and abundances remained low compared to surveys before 2021–22. Bony herring increased in relative abundance from last year, but still markedly lower compared with pre-2019–20. No silver perch was detected. One adult freshwater catfish was detected; this species hadn’t been detected in the surveys since 2014–15.
- Seven native species were detected in the lower Lachlan River System, which is similar to last year.
- Spawning of golden and silver perch was detected in Selected Area sites coincident with high in-stream flows in the Lower Murray River. For silver perch larvae the natal origin was mostly the mid-Murray and to a lesser extent the Lower Murray River.
- Low levels of recruitment were evident for golden perch with high in-stream flows which CEW contributed to and assisted with enhancing spring-summer flow pulses and maintaining elevated flows during peak spawning timing windows.
- Adult golden perch relative abundances declined from the previous year. These reduced abundances may be a result of upstream emigration or movement of adults and or delayed sampling in 2023–24. Adult silver perch relative abundance was relatively low similar to most years
- Spawning of Murray cod was detected; and there was minor recruitment of Murray cod with high in-stream spring–summer flows (which CEW contributed to). Adult relative abundances have declined the previous 2 years, and may be due to reduced electrofishing efficiency because of higher flows during sampling resulting in lower catch rates in 2022–23 and delayed sampling in 2023–24.
- The fish assemblage characterised the floodplain and flowing riverine environment, i.e. low abundances of small-bodied fish. Delayed sampling in 2023–24 limited the interpretations of the large-bodied fish community such as bony herring and common carp which changes in fish behaviour associated with colder water temperatures
- The fish community was dominated by common carp, and native species were in low abundances. The most abundant native species included Australian smelt, carp gudgeon, spangled perch and bony herring at Flow-MER monitoring sites. Condition metrics were the lowest recorded in the ten years of monitoring.
- Golden perch were not collected in the Darling/Baaka in 2023–24 and there was evidence of limited recruitment in the Upper Warrego–Warriku. The Lower Warrego–Warriku population was dominated by more mature individuals. This contrasts with previous years (prior to 2022) where the Lower Warrego–Warriku population was dominated by young-of-year. Recent recruitment was evident for bony herring in the Upper Warrego–Warriku and to a more limited extent in the Lower Warrego–Warriku and Darling/Baaka.
- No Murray cod, silver perch or freshwater catfish were detected at the Flow-MER monitoring sites, which suggests the species are extirpated at these sampling sites or only present in very low numbers.
- Species once common to the Area, such as freshwater catfish, Murray cod, olive perchlet, southern purple-spotted gudgeon and Darling river hardyhead, were rarely detected or were not detected at all and are now likely below self-sustaining levels. Recovery for all these species may need more than just flow management, and other strategies will be required to improve these species’ populations in the Warrego–Warriku and Darling/Baaka rivers (e.g. stocking, translocations, habitat improvement)
- Wetland fish communities were lower in abundance this year compared to the previous year (experienced wetter conditions) which is expected with the majority of wetlands in a drying phase. Wetland communities continued to be depauperate and dominated by generalist species including Australian smelt, carp gudgeon and bony herring. However, higher species richness was evident at the more persistent and regularly watered sites, including Bayil Creek, Wagourah and Yarradda Lagoon.
- The young-of-year of small-bodied native and exotic fish species were found at most wetlands.
- Native species detected spawning in-stream included Murray cod, bony herring, silver perch (drifting eggs detected), Australian smelt, flathead gudgeon and unspecked hardyhead. There was no evidence of golden perch spawning in-stream.
- Adult relative abundances of Murray cod, golden perch and bony herring increased compared to the previous year, and the highest abundances of golden perch were observed since monitoring began. The relative abundances of all 5 small-bodied native fishs decreased from the previous year (Australian smelt, carp gudgeon, flatheaded gudgeon, Murray–Darling rainbowfish and unspecked hardyhead). Murray–Darling rainbow fish declines is a trend that has continued over the past 2 years though recruitment did occur this year. For large-bodied species recruitment was detected for silver perch at one survey site and all sites for bony herring but no recruitment was detected for Murray cod or golden perch.
