Key words: Murray Cod, protection, enhancement, anabranches, Native Fish Strategy.
The decline in connectivity of lowland rivers to their floodplain habitats (among a range of other factors) has contributed substantially to the decline of their native fish populations. Murray Cod (Maccullochella peelii) are a fish species that are known to use both mainstem and anabranch habitats in the Murray-Darling Basin, but prior to this project, the importance of these anabranch habitats to this species were unresolved. For example, very little was known about use of the Murray mainstem by anabranch Murray Cod populations. There was also little known about the contribution these anabranch populations have to the mainstem population (or vice versa). The need was identified to understand the relevance of these unique anabranch systems to Murray-Darling Murray Cod populations to assist the appropriate management of these habitats.
Objectives of this project were:
- Compare anabranch and mainstem Murray Cod populations
- Determine the degree of movement of Murray Cod between these populations
- Provide a fishway operation guide to river managers of Broken Creek
Methods: Five anabranch systems and corresponding stretches of the Murray River were sampled. Populations of main channel and anabranch Murray Cod were compared by capturing by boat electrofishing. The movements of Murray Cod were assessed using radio-telemetry and mark re-capture methods.
In-situ data loggers were used for collection and remote transmission of information from radio-transmitters. Water temperature dataloggers were also deployed to monitor variation in water temperature between the Murray River mainstem and anabranch systems to determine whether variations between these two habitat types influenced variations in Murray Cod movement between the two systems.
Rough estimates of Structural Woody Habitat (SWH) or “snag” density was also calculated from aerial photographs to enable comparison between sites.
Findings: Numbers of Murray Cod were found to be higher in the Mullaroo Creek anabranch compared to the main Murray River channel. The Mullaroo Creek anabranch population comprised a higher proportion of larger individuals compared to the Murray River main channel population. Mature sized Murray Cod that inhabited the main river channel generally moved into this anabranch prior to and during the spawning period. In general, larger Murray Cod were found to make upstream migrations prior to and during the spawning season, with some of these movements occurring several months before the spawning season. Most migrating individuals returned to their home site after the spawning season was over. Juvenile Murray Cod (< 500 mm) were found to only make small localised movements during both spawning and non-spawning times.
Lessons learned and future directions: Anabranches were identified as an important year-round habitat and were likely to be important spawning habitat for Murray Cod. Passage to and from these off-channel habitats is important for Murray Cod populations.
This study also suggested that Murray Cod at the minimum legal size at the time the study was undertaken (500 mm) were unlikely to have spawned, with important implications for populations of Murray Cod in heavily fished areas. Since this study, the minimum legal size of Murray Cod in New South Wales and Victoria has been increased to 600 mm.
Stakeholders and Funding bodies: This project was funded through the Murray-Darling Basin Authority’s Native Fish Strategy and undertaken by a project team from the Arthur Rylah Institute (ARI) for Environmental Research.
Contact: Arthur Rylah Institute, 23 Brown St, Heidelberg, Victoria, Australia, +61 3 9450 8600.