Murray cod

Over the past 9 years, Commonwealth environmental water has had significant benefits to the 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. In 2022–23, recruitment for Murray Cod was limited in most Selected Areas due to the extreme high-flow conditions of this year and the preceding year. Consecutive years of wet conditions has resulted in high unregulated flows during key reproductive periods. In some areas, these high flow conditions may have also caused hypoxic (low oxygen) conditions leading to mortality or movement of adult fish away from monitoring sites, thus leading to low recruitment. The other important factor to consider is that populations of Murray cod are still continuing to recover from the hypoxic events in 2016–17 (page vi), with impacts of these past events still persisting in populations.

Outcomes 2022–23

• There was evidence of recruitment of Murray cod in the Lower Murray River (page 14)
• Recruitment was limited in other Selected Areas likely due to unfavourable flow conditions (such as high velocities) or hypoxic conditions during key spawning stages (page 14)
• Murray cod populations have continued to show a steady recovery from these hypoxic events in recent years (Counterfactual modelling showed that Commonwealth environmental water contributed to population growth in some years in some Selected Areas). (page iii)

Outcomes 2014–23

• Murray cod was detected in all years in all Selected Areas
• Commonwealth environmental water contributed to increased spawning, recruitment, population growth, frequency of occurrence and body condition, and improved community composition in some Selected Areas for Murray cod. These responses were driven by reductions in the number of low flow days and increase in average daily flows and changes in flow variability (page iv).
• Recruitment of Murray Cod increased as a result of Commonwealth environmental water delivery in some years in Goulburn River and Lower Murray River.
• The adult Murray cod population growth rate increased with the delivery of Commonwealth environmental water (fewer number of low flow days, and changes in flow variability) in most Selected Areas.
• Strong responses of individual body condition for Murray cod were evident in the Gwydir River System, Lower Murray River and Murrumbidgee River System (page iv).
• The contribution of Commonwealth environmental water to Murray cod and Murray–Darling rainbowfish recruitment was predicted to be greatest in the southern Basin.

Golden perch

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 2022–23

• Recruitment of golden perch was evident in the Lower Murray River, with little evidence of recruitment in other Selected Areas, possibly due to high flows reducing detectability of larvae, eggs and new recruits or limited spawning/recruitment due to competition or predation from the substantial recruitment of common carp (assuming food limitation) (Page 3).
• Golden perch spawning occurred in the Goulburn River, and golden or silver perch eggs were detected in the Lower Murray River (eggs not differentiated between species). Spawning was largely associated with unregulated flows (Page 3).
• Commonwealth environmental water did not have any positive associations with golden or silver perch spawning in 2022–23, likely due to relatively minor contributions of Commonwealth environmental water to high spring flows in 2022–23 (Page 14).

Outcomes 2014–23

• 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 golden perch. These responses to Commonwealth environmental water were driven by reductions in the number of low flow days and, to a lesser extent, increased average daily flows and changes in flow variability (both increases and decreases in flow variability depending on Selected Area) (Page iv).
• Averaged over all survey years, the likelihood of spawning for golden perch in the Lower Murray River increased as a result of Commonwealth environmental water delivery (Figure 5.1 page 24)
• The provision of Commonwealth environmental water contributed to increased individual body condition of golden perch in the Goulburn River (page 21).
• Averaged over all survey years, there were negative responses in the Gwydir River System to Commonwealth environmental water delivery for recruitment and frequency of occurrence of golden perch population growth rates and frequency of occurrence (Appendix H, Table H.3. page 109) 

Murray-Darling rainbowfish

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 (page v).

Outcomes 2022–23

• Murray-Darling rainbowfish had lower abundance/recruitment than in previous years at most Selected Areas, with high flows negatively impacting populations of the species, which prefers low-to-moderate stable flows during nesting periods (pages 14 and 19)
• Quantitative analysis found positive associations between Commonwealth environmental water and Murray-Darling rainbowfish in the Goulburn River. This suggests the poor recruitment of Murray Darling rainbowfish may have been worse without Commonwealth environmental water contributions (page 14,15 and 19).

Outcomes 2014–23

• Commonwealth environmental water contributed to an increased in fish spawning, recruitment, population growth, frequency of occurrence and body condition, and improved community composition in some Selected Areas for Murray-Darling rainbowfish (page iv). These responses to Commonwealth environmental water were driven by reductions in the number of low flow days and, to a lesser extent, increased average daily flows and changes in flow variability (both increases and decreases in flow variability depending on Selected Area) (page iv).
• The contribution of Commonwealth environmental water to Murray–Darling rainbowfish recruitment was predicted to be greatest in the southern Basin.
• Detection of Murray-Darling rainbowfish with Commonwealth environmental water delivery was more pronounced within the Goulburn River and Lower Murray River Selected Areas, although were more variable at other Selected Areas. These pronounced effects in the Goulburn River and Lower Murray River resulted from Commonwealth environmental water markedly reducing the number of low flow days in these Selected Areas (page v).
• Averaged over all survey years, there were negative responses in the Gwydir River System to Commonwealth environmental water delivery for recruitment and frequency of occurrence of Murray-Darling rainbowfish (Pages 21 and 64). The Gwydir River System fish community demonstrates a long-term decline with multiple threatening processes (e.g., exotic species, barriers to fish movement, thermal pollution and, for some species, numbers too low to allow successful breeding) (page 63). As such, native fish populations are very unlikely to be recovered via environmental water alone.

Australian smelt

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 (page 65).

Outcomes 2022–23

• Quantitative analyses found positive associations between Commonwealth environmental water and Australian smelt in the Goulburn River (page 15).
• Australian smelt in the Edward/Kolety–Wakool and Murrumbidgee river systems had the highest abundance/recruitment detected since monitoring began (page 19). Australian smelt had higher abundances (representative of recruitment for this short-lived species) than in previous years in the Edward/Kolety–Wakool and Murrumbidgee river systems, likely because greater hydrological connectivity increased favourable breeding habitats (within channel and connecting wetlands) and food resources for these species (page 14 and 19).

Outcomes 2014–23

• 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 Australian smelt (page iv). These fish responses to Commonwealth environmental water were driven by reductions in the number of low flow days and, to a lesser extent, increased average daily flows and changes in flow variability (both increases and decreases in flow variability depending on Selected Area) (page iv).
• Commonwealth environmental water contributed to increased recruitment for Australian smelt in the Edward/Kolety–Wakool, Goulburn, Lachlan, Lower Murray and Murrumbidgee River system Selected Areas (pages iv, 21 and Table 5.1 on page22).
• Although some species such as Muray cod, golden perch and carp gudgeon were detected in all Selected Areas, Australian smelt had a patchier spatial and temporal distribution (page iv).

Silver perch

Environmental water has contributed to spawning of the silver perch through enabling connectivity and flow velocity variation. However, analysis of silver perch spawning is complicated by the sparsity of data for this species, the fact that their major known spawning area in the mid-Murray fell outside Flow-MER Selected Area monitoring until recently (page 65).

Outcomes 2022–23

• Golden or silver perch eggs were detected in the Lower Murray River (eggs not differentiated between species) (page iii).
• Commonwealth environmental water did not have any positive associations with golden or silver perch spawning in 2022–23, likely due to relatively minor contributions of Commonwealth environmental water to high spring flows in 2022–23 (page 14).

Outcomes 2014–2023

• Silver perch spawning increased in response to the day of year (with strong evidence) and (with weaker evidence) change in daily flow in the 7 days prior to sampling (high uncertainty) (Appendix H, Page 107).
• Quantitative modelling indicated that Silver perch had no positive associations with Commonwealth environmental water delivery at Selected Areas. (Page 25)
• Although some species such as Muray cod, golden perch and carp gudgeon were detected in all Selected Areas, silver perch had a patchier spatial and temporal distribution (page iv)
• Analysis of silver perch spawning is complicated by the sparsity of data for this species, and the fact that their major known spawning area in the mid-Murray fell outside Flow-MER Selected Area monitoring until recently (page 65).

Bony herring

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, Responses to Commonwealth environmental water were driven by reductions in the number of low flow days and, to a lesser extent, increased average daily flows and changes in flow variability (page iv).

Outcomes 2022–23

• Detection of bony herring with Commonwealth environmental water delivery was more pronounced within the Goulburn River and Lower Murray River Selected Areas, whereas responses were more variable at other Selected Areas. These pronounced effects in the Goulburn River and Lower Murray River resulted from Commonwealth environmental water markedly reducing the number of low flow days in these Selected Areas (page iv).
• There were no effects of Commonwealth environmental water on recruitment bony herring in all Selected Areas (page iv). 

Outcomes 2014–23

• The provision of Commonwealth environmental water contributed to increased individual body condition of bony herring in the Edward/Kolety–Wakool, Lachlan and Murrumbidgee river systems (pages v, 22 and Table 5.1).
• Population growth rate increased with Commonwealth environmental water delivery for bony herring in some years in the Edward/Kolety–Wakool, Lower Murray and Murrumbidgee River systems due to fewer number of low flow days (page v).
• Averaged over all survey years, there were negative responses in the Gwydir River System to Commonwealth environmental water delivery for the frequency of occurrence of bony herring (Appendix H, page 109). The Gwydir River System fish community demonstrates a long-term decline with multiple threatening processes (e.g., exotic species, barriers to fish movement, thermal pollution and, for some species, numbers too low to allow successful breeding) (page 63). As such, native fish populations are very unlikely to be recovered via environmental water alone.

Unspecked hardyhead, Murray hardyhead, flathead gudgeon and carp gudgeon

• Native small-bodied species detected across all Selected Areas included Australian smelt, carp gudgeon and unspecked hardyhead, as well as the exotic eastern gambusia. The only small-bodied species detected in all years and Selected Areas was carp gudgeon. Two small-bodied native fish species only detected in one Selected Area each were the dwarf flathead gudgeon (Lower Murray River) and obscure galaxias (Goulburn River) (page 88).
• Flows were delivered to Murray wetlands to support and provide breeding opportunities for the nationally endangered Murray hardyhead (Environment Protection and Biodiversity Conservation Act 1999) and to Murrumbidgee wetlands to support habitat for the NSW endangered southern pygmy perch (Fisheries Management Act 1994, NSW) (page 17).
• In the Murrumbidgee River Selected Area, the relative abundances of Australian smelt, carp gudgeons and unspecked hardyhead were the highest detected since monitoring began (page 79).
• Unspecked hardyhead larvae were detected in the Edward/Kolety– Wakool river systems regularly, and less frequently in the Murrumbidgee River System (page 94).
• Species not detected in 2022-23 include obscure galaxias in the Goulburn River (last detected in 2022), flathead gudgeon in Edward/Kolety–Wakool river systems (last detected in 2022), and dwarf flathead gudgeon (last detected in 2021). Flathead gudgeon was generally detected less frequently in most Selected Areas (page 32).

Exotic species

Unregulated high flows and overbank flooding in 2022–23 provided favourable conditions
for exotic species, with very strong recruitment of common carp and goldfish (pages 15 and 19). Flooding has been shown to be a significant driver of carp (page 66). The use of water for the environment can both benefit and inhibit exotic species. Where there are clear benefits for native fish, perverse outcomes for exotic fish may still be acceptable (page 66).

Outcomes 2022–23

• Some exotic species (common carp, goldfish) displayed strong recruitment in 2022–23, which may be due to the unregulated high flows and overbank flooding providing favourable breeding conditions (pages 15 and 19).
• Quantitative analyses of exotic species identified few positive associations between Commonwealth environmental water and exotic fish species recruitment, except for common carp in the Edward/Kolety–Wakool river systems Selected Area (pages 15 and 19).

Outcomes 2014–23

• The frequency of occurrence of eastern gambusia, and population growth rates of common carp decreased in response to Commonwealth environmental water delivery in several Selected Areas (Appendix H page 109).
• Environmental water deliveries in regulated rivers can benefit exotic species. Despite some associations between environmental water delivery and carp recruitment, recent population modelling work has shown that large natural floods and prolonged droughts accounted for the greatest amplitude of projected carp population increases and declines, respectively. In-channel flow pulses and small floods, which are a primary focus of environmental flows, had little influence on long-term carp population trajectories. These insights are important because the spread of carp has fundamentally altered the Basin’s aquatic environments with devastating decreases in macrophytes, macroinvertebrates, and increases in turbidity. Furthermore, where there are clear benefits for native fish, perverse outcomes for exotic fish may still be acceptable (see full discussion and additional references on page 66).