The decline in connectivity of lowland rivers to their floodplain habitats (amongst 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 the importance of these habitats to this species remain unresolved. The objectives of this project were:
- Compare anabranch and mainstem Murray cod populations
- Determine the degree of movement of Murray cod between mainstem/anabranch populations
- Provide a fishway operation guide to river managers of the Broken Creek
Populations of main channel and anabranch Murray cod were compared by capturing Murray cod by boat electrofishing. The movements of Murray cod was assessed using radio-telemetry and mark re-capture methods.
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, though some of these movements occurred 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.
Anabranches are an important year-round habitat and a likely important spawning habitat for Murray cod. Passage to and from these off-channel habitats is important for Murray cod populations.
This study suggests that Murray cod at the legal size for taking (500 mm) are unlikely to have spawned. This has important implications for populations of Murray cod in heavily fished areas. Since this study, the legal size of Murray cod has been increased to 600 mm.