Following a long-term fish research and monitoring project, recent changes to environmental flow management have supported Murray cod spawning along the Gunbower Creek. Located between the townships of Torrumbarry and Koondrook in Victoria, the Gunbower Creek, off the Murray River, is primarily an irrigation supply channel. Since 2005, annual fish surveys funded by the Murray Darling Basin Authority’s Living Murray Program have found that while small-bodied fish populations are generally strong, populations of large-bodied fish such as the Murray cod are in decline. A number of factors may have contributed this decline, including poor water quality (such as the blackwater following the 2010 floods), inappropriate flows for fish migration and spawning, insufficient food, lack of appropriate habitat and lack of connection between habitats due to water regulating structures.
In an effort to encourage Murray cod spawning and juvenile development, the North Central Catchment Management Authority (CMA), working closely with Goulburn Murray Water managed a changed environmental flow along the Gunbower Creeks to create a more stable flow rate at Cohuna Weir. This change supported spawning activity, enabling Murray cod to protect nests and to enhance larval dispersal into productive areas amongst aquatic vegetation on the edges of the creek. Results from monitoring have confirmed that during October – December 2013, Murray cod larvae have been found in three age cohorts, indicating there has been at least three successful spawning events throughout the managed environmental flow event.
Monitoring throughout December 2013 continued to find cohorts of Murray cod larvae, rapidly advancing in size, having absorbed the remainder of their yolk sack and beginning to feed on very small prey. Dr Clayton Sharpe, who leads the project team and has been conducting the annual monitoring on Gunbower Creek since 2005, is confident that Murray cod spawning events have been enhanced by the managed environmental flows, as he explains:
Gunbower Creek already has the habitat qualities to provide suitable spawning and nursery habitat for Murray cod. With appropriate flow management the Murray cod population in Gunbower Creek has high potential for recovery. A similar type of flow regime targeting spawning of large bodied fish is also being delivered in other areas of the Murray-Darling River system this year and together, through this research, we are forming a body of knowledge to improve the management of these iconic fish species for future generations.
Murray Cod can produce thousands of larvae from one successful spawning event, and although there is a high mortality of young fish, reserachers believe that those that survive their first summer and winter have a good chance of surviving through to maturity.
It is recommended that, based on these results, environmental flows continue to be delivered through Gunbower Creek in a pattern that targets within-channel flows, without major changes to normal irrigation flow delivery, to maximise Murray cod spawning and larval access to food resources and shelter from predation.