Australasian Science Magazine

By Colin Creighton

Improved water flows have reopened the Murray River’s flow to the sea, but much more needs to be done to restore the fisheries in Australia’s largest estuary.

Juvenile barramundi, mangrove jack and banana and tiger prawns face at least 1500 barriers as they try to move upstream to their preferred habitat on the Burdekin floodplain. In the rivers between Ingham and Port Douglas the last count was over 5500 barriers. The recent flood events in Queensland and NSW saw fish kills as anoxic and acidic water was purged from barraged floodplain wetlands.

All around Australia’s coast, fisheries productivity has lost out to urban and agricultural development. Yet we all like to eat prawns, oysters and fish, so why have we been so blind to the destruction of fisheries habitat and the benefits these habitats provide to all Australians? And what can we do about it?

The plight of the Coorong, Lower Lakes and Murray River’s mouth as an example of the total disregard for fisheries was re­inforced to me when I led the National Land and Water Resources Audit in the late 1990s. In a draft Audit Report I provocatively added a photo and caption suggesting the likelihood of the Murray’s mouth closing and the further loss of fisheries. After a friendly chat with the then Murray–Darling Basin Commission’s CEO, I retained the photo but agreed to tone down the caption.

Thanks to the collective vision of our governments we now have more water flows promised for Australia’s largest estuary. How can we now start the process of improving fisheries productivity?

The Coorong, Lower Lakes and Murray River’s mouth was an estuarine and lagoon system with a diverse range of freshwater, estuarine and marine habitats. Porpoises travelled up past the lakes and Wellington in times of low freshwater flow. So did their food – juvenile mullet and herring. After a nursery phase in the estuary, juveniles of many species repopulated environments to the east in South Australia and Victoria.

Until the 1930s about 100 mulloway fishers in the Coorong supplied Adelaide with its seafood requirements. Since then a series of barrages with connecting roadways has changed the Lower Lakes into a high-evaporation weir pool for irrigated agriculture, reducing the area of habitat available to estuarine fish by around 99%.

As freshwater flows increase into the lower Murray, now is the time to reap the benefits to provide certainty for farmers, restore the estuarine habitat and increase fisheries productivity.

Agriculture would be far better served by a ring main drawing freshwater on demand from upstream of Wellington.

Restoring a significant portion of the estuarine habitat can be achieved by simple and inexpensive engineering interventions to increase the residence time of freshwater in the estuary, improve connectivity to the Lakes, and increase freshwater flows to the southern Coorong.

To improve freshwater residence time, the barrages closest to Lake Alexandrina, well south of the ocean entrance, should be fitted with automatic control gates. Automation would encourage regular opening to allow more freshwater to enter the Coorong, maximising the highly productive brackish water zone that ensures high rates of fisheries productivity.

The small islands that span the Murray River’s mouth between Goolwa and Pelican Point were once traversed by creeks through which fish travelled to migrate further upstream. Most of these were blocked when roads were built to connect the barrages. Simply installing culverts to reopen the creek lines would restore the habitat without compromising the road network.

Rationalisation of drainage systems as part of further manipulation and improvement of the Upper South-East Drainage Scheme could deliver increased freshwater flows to the often hypersaline southern Coorong. As well as enhancing the fishery, this would especially benefit migratory bird species.

Revitalising estuaries around Australia would be as simple as these actions for the Murray.