Associate professor Mark Lintermans from the UC, the man responsible for the research behind the 7kms of reef rocks at the Cotter Dam. Photo: Rohan Thomson
A seven kilometre long rock wall snaking around the enlarged Cotter Dam’s contours could resolve a mystery about Macquarie perch and sustain their numbers well into the future.
Worth more than $2 million and taking two people a year to place each boulder, the wall is deigned to provide ideal habitat for the perch which survive in only six small NSW and Victorian colonies.
The enlarged dam’s level is rising one per cent a week. As it fills, scientists expect a ‘‘traffic surge’’ - a population explosion of the rare fish which are also found in the upper Murrumbidgee and Abercrombie rivers above Wyangala Dam.
University of Canberra ecologist Mark Lintermans, who has studied the Cotter’s perch for 29 years, said water rising over previously unflooded ground would trigger plenty of action.
‘‘It brings out all the grubs. All the terrestial vegetation starts to decompose. The macro invertebrates, the bugs, feed on that, so you get a surge in food resource.
‘‘You also get nutrients released from the soil.’’
When the temperature reaches 16 degrees, probably in October, about 10 per cent of the Macquarie perch will head upstream into the Cotter River to spawn.
A colony of between 600 to 1000 fish has existed for as long as the river itself.
‘‘It has been protected to some extent, because when the Cotter Dam was built in 1919, that’s kept carp and redfin and other introduced species out of the Cotter River,’’ Associate Professor Lintermans said.
A principal research fellow, Professor Lintermans said the rare perch once sheltered in a bath tub ring of reeds surrounding the old Cotter dam, emerging into exposed, deeper water at night.
Reeds won’t ring the new dam in such proliferation because of fluctuating levels, which is why the wall has been built, at various heights.
Some sections are already submerged. Netting last week revealed the perch had already moved into their new shelter.
Granite boulders more than a metre wide hauled in from a Murrumbateman quarry and rhyolite rock, quarried from the dam site, stand about two metres high, providing deep crevices and crannies for fingerlings and their precious parents.
This gives them plenty of cover from coromonts, and other predators.
‘‘They have a habit of disappearing in reservoirs over a period of time. Now they are in the streams and the plan is to have them in the river. They don’t like sediment, it smothers their eggs.’’
Sediment became an even bigger threat after the Canberra bushfires.
‘‘They will do well here for the next 20 years - after that they may disappear.’’
Previous research took advantage of the Cotter’s clear water. Six video cameras recorded the fish for 16 hours every day for a year. Months of staring at blank screens, waiting for a fish to appear, were boiled down into a few minutes of valuable study.
It found a rocky reef was superior habitat than culvert pipes, even when they were protected with wire mesh.
Environment Minister Simon Corbell said the project, a partnership with ACTEW Water was part of $10 million spent overall on the Cotter Dam’s environment.