The Eastern Cod didn't do it!
20 July 2015
North Coast Local Land Services, in partnership with Office of Water and University of Wollongong are continuing to provide world class research through the Clarence River Fish Track project. Knowledge generated by the project continues to capture the attention of recreational fishers and the scientific community with the release of research findings recently published in the international science journal, Geomorphology.
The project is investigating the influence of river flows and barriers (e.g. waterfalls and weirs) on the seasonal movements of freshwater fish species, such as the Australian bass and the endangered eastern freshwater cod.
North Coast Local Land Services Team leader, Tony Broderick said, "The Clarence Fish Track project uses acoustic tag and receiver technology to track the movements of fish through the river system.
"Acoustic tags, surgically implanted into the gut cavity of the fish, emit a 'beep' that is detected and recorded by acoustic receivers. There are more than 80 receivers extending over 550 km of river from the lower estuary through to the upper freshwater reaches.
"Each acoustic tag has a signature 'beep' enabling the specific fish to be identified." he continued.
North Coast Local Land Services' long term project partners, DPI Fisheries, Clarence Valley Council, Coffs Harbour Water, Essential Energy and NSW Office of Water, were joined by University of Wollongong to determine the flow events required to enable Australian bass to gain access to hundreds of kilometres of freshwater habitat upstream of the Clarence gorge. The gorge, located approximately 150 km upstream of the Clarence river mouth, has a series of waterfalls between 2 and to 8 metres in height which provide a natural barrier to migratory movements of Australian bass on their long (up to 300km) return trips from breeding in the estuary.
Modelling of flow velocities under different flood events combined with knowledge of maximum bass swim speed and Fish Track data of actual bass movements showed that a slightly larger than an average annual flood event is required to facilitate the upstream movement of bass through the gorge. However, during extended dry periods these "average" annual flood events often do not occur particularly in the drier months of Spring when the bass are moving upstream to their freshwater habitats.
This has helped explain the relatively low numbers of bass found in rivers upstream of the gorge during the dry years leading up to 2009. The abundance of bass enjoyed by recreational fishers upstream of the gorge during the wet years (2009 and 2013) puts to rest the concern expressed by many that the eastern freshwater cod was eating all the bass. Bass simply could not return upstream until river flows provided passage. The study also draws attention to the uniqueness of the Orara River, which flows into the Clarence downstream of the gorge providing passage to and from the estuary free of natural barriers.
Flows also play an important role in triggering bass migration to the estuary for breeding. Understanding how much flow constitutes a 'trigger flow' for downstream movement is critical for environmental flow management. Downstream movements of tagged bass to the estuary were recorded during large flood events in late autumn to early winter (e.g. 2009 and 2011). Interestingly, there were no downstream movements recorded during 2010 and 2012, presumably due to the absence of rise in the river.
"We are keenly awaiting the analyses of bass movement data from Autumn this year during which a relatively small flood event occurred because it is very likely to help answer how much flow is required to trigger bass migration." Tony concluded.
Media contact: Tony Broderick, Team Leader, Land Services , Phone 0409 225 798
Photo caption: A bass ready to be tagged.
Clarence River Gorge