The
spatial distribution of fish in unregulated rivers is controlled by natural barriers. While channel morphological features (such as
marked changes in stream gradient or substrate character) define natural
barriers, their
impacts are both flow and species dependent.
The flow dependence of barrier impacts means that river regulation has
the potential to seriously affect the distribution of natural barriers along a
river. Most studies relating to natural
barriers in streams are from North America where fish have different
characteristics from Australian fish. Few Australian studies have investigated
the effects of natural barriers upon riverine fish. Murray
cod Maccullochella
peelii peelii are
‘keystone’ native Australian fish that are known to migrate hundreds
of kilometres to spawn in lowland rivers.
It is not known if similar behaviour occurs in upland rivers, but it is
likely to be affected by the presence of natural barriers. The
aim of this study was to characterise natural barriers to Murray cod migration along
the Murrumbidgee River in the ACT, identify how those barriers respond to flow
changes and make predictions about their impacts upon Murray cod movement. Detailed 3D maps of 30 potential barriers
were produced from survey data collected during low flow conditions. Velocity data was also collected for each
potential barrier. Physical
characteristics of the barriers were highly variable and it is likely that
sequences of barriers may prove insurmountable for Murray cod. Additional extraction of water from the river
may have implications for Murray cod passage along the Murrumbidgee River in
the ACT.