Recreational fishers are alarmed about reeling in fish with red raw ulcers on their sides in the Noosa and Maroochy rivers.
"There are so many sick fish in the Maroochy River right now," Wade Beasley said.
"We are picking up bream and flathead alive out of the water with our bare hands with sores on them."
Tony Richings said he caught and released an unsightly bream in the Maroochy River late in April, with a nasty ulcer it was unlikely to have survived.
Further north, Steve Ozoux shared a picture of an infected flathead to the Sunshine Coast Fishing Facebook Page.
"Caught this poor bugger Noosa River over the weekend, released him but it was in bad shape," Mr Ozoux told the ABC.
Flesh destroying fungus
Aquatic animal health specialist Ben Diggles said the fish were infected with red spot disease or epizootic ulcerative syndrome (EUS), caused by the exotic necrotic fungus, Aphanomyces invadans.
Dr Diggles said when poor water quality compromised the animal's immunity, the invasive fungus attacked one scale, then spread, causing patches of tissue to die.
"They can recover from smaller lesions but once the lesions get beyond a certain size the fish becomes very compromised," Dr Diggles said.
"Its immune system can't cope and its osmoregulation, its water balance, is all thrown out of whack and unfortunately, many of them die."
Dr Diggles filmed a helpless bream floating on the surface of the Brisbane River late last year.
It was suffering from red spot disease and a bacterial lesion on its head and gills called saddleback syndrome.
Loading...Exotic invader
Potentially introduced to Australia on ornamental fish about 50 years ago, red spot disease has been reported in freshwater catchments and estuaries in New South Wales, Queensland, Western Australia, South Australia, Victoria, and the Northern Territory, as well as inland, in the Murray Darling system.
It has impacted on wild fish and aquaculture farms around the world, with more than 100 susceptible estuarine and freshwater fish species in Australia.
Acidic water
Dr Diggles said red spot disease was particularly bad after heavy flushing rains in coastal plains with underlying layers of acid sulfate soils that released sulphuric acid into waterways when they were exposed to oxygen by clearing, draining, and development.
He explained that the sulphuric acid erodes the mucus-secreting cells in the epidermis, or outer skin of fish, leaving them more vulnerable to the fungus.
Dr Diggles said sediment, suburban, and agricultural chemical run-off had significant impacts on water quality.
"The rivers end up just being drains and the fish tell us what the ramifications of that are if you ignore those things," Dr Diggles said.
"The fish don't lie, fish never lie, you just need to understand what they're trying to tell you."
Don't eat them
Although red spot disease is not deadly to humans, people are being warned not to eat any fish that have been severely infected.
"The advice with sick aquatic animals is not to eat them because they may have a variety of bacterial or other microbial pathogens in them," Dr Diggles said.
Report red spot
Australia's only fishing conservation charity, OzFish, is asking people to help it better map red spot disease by reporting when and where they find infected fish and uploading photos, using an online form.
Water temperatures of 18-22 degrees Celsius favour the fungus, which can survive for at least 19 days without a host and spread with fish including bream, which Future Fisheries Veterinary Service director Matt Landos said migrated hundreds of kilometres each year.
From pristine to polluted
Dr Landos advises OzFish and he recently released an international report for IPEN, a network of more than 600 non-governmental organisations working in more than 120 countries to reduce and eliminate harm to human health and the environment from toxic chemicals.
Dr Landos said while fisheries management often focused on over-fishing as the main cause of declines in productivity, the effects of degrading water quality and habitat were not given the same consideration.
"This struck me as being inconsistent with the evidence of disease expression and mortality incidents which had nothing to do with fishing activities," Dr Landos said.
Dr Landos said environmental pressures included run-off from stormwater, wastewater from sewage treatment plants, beef, dairy, cattle grazing, sugar cane production, irrigated cropping, intensive horticulture, agricultural fertilisers, floodgates, dams, weirs, loss of vegetation, and increased erosion and sediment.
He commended Sunshine Coast Regional Council for its Blue Heart initiative, encompassing an area of more than 5,000 hectares on a natural flood-plain including the council's Coolum Creek environment reserve network, the state-owned Coolum Creek conservation park, and Unitywater's Yandina Creek wetland.
Blue Heart project
The council has purchased properties and reopened tidal floodgates that once drained land for cane production to gradually transition to an estuarine wetland habitat.
In a statement, a council spokesperson said that, unlike freshwater, tidal waters could act as a natural treatment for acid sulfate soils.
"Once acid sulfate soils are exposed, tidal exchanges will continue to neutralise the acidity of the soil; by contrast, excess freshwater runoff associated with acid sulfate soils can lead to water quality problems in waterways," the spokesperson said.
Wetland restoration
Dr Landos said seawater contained little oxygen to fuel acid sulfate soil oxidation and carried in calcium carbonate minerals secreted by marine organisms which "stops the further creation of acid".
"These wetlands in their original format before we started farming were not producing acid, back in time these were not areas that generated toxic plumes of water," Dr Landos said.
"Quite the opposite, they were the most productive habitats in our estuaries, they generated hundreds, and in many cases, thousands of tonnes of seafood because they were the nursery areas where the base of the food web was created and where baby fish and crustaceans would go."
Farmers affected
But the removal of floodgates has been met with increasing concern from surrounding farmers and residents who said their land was being devalued by the rising water table and increased salinity, with no compensation.
"I think we do need a scheme to support farmers in the transition, it's not something that we should expect of them just to give it up and hand it over, that's not appropriate," Dr Landos said.
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