DESPITE decades of research and centuries of dependence on the Murray Darling Basin for agriculture, some basic elements of the river system’s ecology are yet to be uncovered.
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Monitoring of outcomes from environmental watering has increased since 2012 under the Basin Plan, producing some fascinating fish insights and new insights into bankside erosion.
Although environmental flows, which in the Southern Basin means targeted releases from water storages by the Commonwealth Environmental Water Office, comprise a relatively small portion of overall river flow they can be timed to benefit key environmental outcomes.
Ecologist Dr Angus Webb from the University of Melbourne headed a study in the Goulburn River under the Basin Authority's Long Term Intervention Monitoring initiative.
Golden perch are a particular focus. The popular angling species’ is strongly linked to river health by their spawning events, which are triggered by flow events. But only under conducive conditions.
“After the study, we now have a good idea of how much water to deliver, when to do it and how warm the river needs to be to induced gold perch spawning,” Dr Webb said.
“During the Millennium Drought, I don’t think there was any spawning detected in the Goulburn. But when floods broke the drought in 2010 we saw good spawning.
“Since then, we have seen the largest spawning on managed flow, in late 2014. The following year (2015) was dry and the decision was taken to deliver Spring water early, because vegetation needed more help than the fish.
“But last year there was minor flooding, and we saw another spawning when the water temperature warmed up.”
Scientists have attached acoustic transmitters to about 60 perch in the Goulburn so their movements can be detected by an array of listening stations along the river and into the Murray.
“We can see they move just prior to, or at, spawning and the hypothesis they move downstream to spawn.”
Other research programs that monitor fish movement in the Basin use the same tags as Dr Webb’s team, which means the Goulburn River fishes’ far-flung travels can be recorded.
“We saw one particularly intrepid perch move from the Goulburn into the Edward-Wakool River, that is 600 kilometres - a personal best for our monitoring so far.”
Scientists are also analysing fish earbones, called otoliths, which can be read like tree rings. Every layer of bone denotes age and carries a chemical signature from the water the fish was swimming in at the time.
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“Different rivers have different chemical signatures,” Dr Webb said.
The research in the Goulburn showed some fish from the Goulburn went to the Darling River in Far West NSW to spawn, spent some time in the Murray River and ended up back in hte Goulburn.
A separate research initiative to Dr Webb’s, to track golden perch in the Loddon, Campaspe and Goulburn Rivers, collaborated with the Basin Authority to limit flow in a section of the Murray when the Goulburn was flush and in good condition.
Monitoring revealed the canny flow management lured the young fish which were migrating up the Murray at that time to take a right turn and venture up the Goulburn.
Dr Webb said the lifecycle of Murray cod is very different to golden perch.
“Cod are interesting. They operate on a large scale and undertake big migrations for breeding, up to hundreds of kilometres.”
Cod spawn independently of flow events.
“For Murray cod, the primary benefit from environmental flows is better habitat for adults,” Dr webb said.
“Deep pools with better food resources can lead to better breeding and survival outcomes for young fish, but that can longer for us to detect,” he said.
Recent monitoring showed Murray cod in reasonable numbers in the Goulburn, and the threatened trout cod further upstream than expected.
The Millennium Drought had left bankside vegetation in a sorry state and other hardier species were moving in to displace them.
“The river was heavily regulated. The Water in Eildon Dam went to stock, domestic and a little bit of agriculture. Water-dependent species had died right back and the grasses and trees from the floodplain were making way down bank,” Dr Webb explained.
The situation has improved markedly, with better flows over recent years allowing bankside species to regrow.
“We can tell how high flows got last year and the year before got by how far vegetation got up the bank, and by what species had shown up,” Dr Webb said.
Vegetation cover can help hold the bank together and resist erosion. Some landholders have criticised environmental flow regimes over bank erosion at various points across the Basin.
Dr Webb said often overlooked erosion factors are the rate of retreat from a high flow event, and deposition of silt.
“There are good records around maintaining slow recession rates, the rate a discharge drops over time, showing a quick retreat is more likely to encourage erosion.
“We have seen environmental water can be used to manage the rate of recession from natural flows.
“We are finding that there is erosion and deposition all the time, these are natural processes in every river.
“We are also seeing the amount of bank activity associated with environmental flows is a reasonably one part of the story - but reasonable small - because they are a small part of total amount of water in the river.”
