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Rising salinity levels in the world-renowned Gippsland Lakes is putting the wetlands at risk, with a recent government report acknowledging it did not know the long-term risks of the problem.
Salinity levels have risen dramatically in recent years, threatening the ecosystem. The shoreline has been eroded and vegetation and fish species are dying.
In an effort to combat the problem, the Gippsland Lakes Ministerial Advisory Committee has suggested replacing freshwater vegetation with salt-resistant plants.
However, former government bureaucrat and environmental activist Ross Scott said a salinity barrier was needed to help stop levels from increasing. He suggested more regular monitoring to ensure authorities knew the scope of the problem.
"The saltwater wedge has now progressed up the lake chain as far as the Port of Sale, and the high salinity in the Latrobe River threatens the adjoining Dowd and Heart Morass," he said.
The community had spent $2.5 million recently to buy low-lying farmland to protect and expand the Heart Morass, only to have it at risk now from salt, he said.
The Environment Protection Authority tests salinity levels at five spots in the lake monthly.
The Gippsland Lakes cover 600 square kilometres in Victoria's south-east and support the local economy through fishing, tourism and recreation.
The recent Gippsland Lakes Ministerial Advisory Committee's environment strategy said it the long-term impact on the ecosystem of increasing salinity was still not known.
It recommended an assessment of whether installing salinity barriers would increase understanding of the problem.
Managing director Martin Richardson said the lakes were made up of various ecological values, some of which were naturally marine. Monitoring was undertaken.
"Variability in salinity in the Gippsland Lakes is now almost solely the result of variability in freshwater inflows from the main rivers and periods of low river discharge are strongly correlated with periods of high lakes salinity, and vice versa," he said.
"There's no suggestion there is above acceptable level and it's not causing a major problem. But there are subtle changes."
The wetlands are listed under the Ramsar Convention because of their importance to many endangered and threatened waterbird species. They are also listed in the Japan Australia Migratory Bird Agreement and China Australia Migratory Bird Agreement.
Salinity levels have rapidly increased since dredging in 2008, which almost doubled the port depth to 5.5 metres.
A lack of freshwater inflows, evaporation and the permanent entrance to the ocean via Lakes Entrance have all contributed to the problem.
The latest Natural Assets Report Card graded the lake's health as "moderate" - 62 per cent of the water meets guidelines, but fish were considered "poor" because the numbers have plummeted since the 1990s.