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Rare and chronic fish disease mycobacteriosis found for the first time in salmon farmed in Tasmania’s Macquarie Harbour
A Department of Primary Industries internal discussion paper, obtained by ABC Rural through Right to Information laws, reveals that an increased rate of the disease was recorded last year.
The paper links the disease to a degraded environment and warns Macquarie Harbour’s low oxygen levels and complex marine environment increases the risk of disease spread among salmon.
Senior Lecturer in veterinary pathology at the University of Adelaide, Dr Stephen Pyecroft, said a degraded environment would worsen the incidence of the disease.
“You get an increased number of those bacteria if you have low dissolved oxygens, high levels of detritus, and you get into what we call an eutrophic environment (high amount of nutrients from fish waste),” he said.
“[A degraded environment] would promote the presence of mycobacteria, [with] any degrading in the environment you’re going to predispose animals to infection.”
A vet in the Department of Primary Industries, Parks, Water and the Environment (DPIPWE) in Tasmania wrote the discussion paper as advice for the Chief Veterinary Officer Rod Andrew-Arthur.
The paper said that producers began reporting increased incidence of inflammation in the kidneys of fish or ‘granulomatous kidneys’; which was caused by an infection with Mycobacteria salmoniphilium.
The report, which had sections related to stocking density redacted before it was released under Right to Information laws, said that it was uncommon for the disease to be found in Atlantic salmon.
Report links disease to environmental conditions
A Department of Primary Industries, Parks, Water and the Environment (DPIPWE) spokesperson, said mycobacteria is a common environment bacterium, found in soil and water.
“There has been an incidental recording of it in farmed salmon previously as well in the state’s south east,” the spokesperson said.
“At this stage the incidence of its detection in samples from Macquarie Harbour is quite low, with seven of 79 submissions to the Animal Health Labs indicating its presence.
“The detection of this bacteria does not represent a disease outbreak, but rather that some fish have picked up this infection.”
The disease is also commonly called “fish tuberculosis” and can spread between fish through contaminated water sources, consumption of contaminated feed, and cannibalism of infected or dead fish.
Fish can harbour the disease, which is not fatal in all cases, for long periods of time and it is possible for them to recover if the environmental conditions change.
Infection from fish to human is extremely rare and has been associated with infected cuts on hands or people with compromised immune systems.
The disease can be transferred to humans through breaks in the skins when in contact with contaminated water or infected fish.
Dr Pyecroft also worked for the Department of Primary Industries, Parks, Water and the Environment in Tasmania for 10 years until 2012 as a veterinary pathologist.
He said that it was difficult to link the emergence of mycobacteriosis to the expansion of fish farming in Macquarie Harbour.
“Relating that to expansion of farming and other things is a bit of a leap of faith, because if you’re managing those risks, then you should be okay,” he said.
“But if you see increased incidence of this, you need to start looking at the bottom of the cages, what is the ground underneath the cages looking like?
“[Do some] harbour management and harbour surveillance and say, ‘Yes, we have an environment that is not looking as optimal as we would like’.”
Warning against fish farming expansions in Macquarie Harbour
Tasmania’s half a billion-dollar salmon industry is rapidly expanding and is a major source of jobs in the state.
But concern among environmentalists and some scientists is also growing, and one of those flashpoints is the impact of fish farming in Macquarie Harbour, which is on the doorstep of the World Heritage Area.
Greens Senator Peter Whish-Wilson said information in the internal DPIPWE discussion paper about disease risks and low dissolved oxygen levels is concerning.
“It’s just another piece of a jigsaw puzzle which suggests to us that unless we diffuse it Macquarie Harbour is a ticking time bomb for the salmon industry and for potentially the wider and broader eco-system,” he said.
Mr Whish-Wilson said that it’s disappointing the DPIPWE didn’t provide this information to a recent Senate inquiry into the salmon industry in Tasmania.
“There is information out there, there are scientists conducting scientific studies now, there have been numerous over the last five years especially since the expansion…but all that science goes into the black hole that is DPIPWE,” he said.
Professor of Marine Ecology and Director of the Centre of Environmental Sustainability at the University of Technology in Sydney, Dr David Booth, said Macquarie Harbour was a very vulnerable marine environment
“It’s such a stratified estuary, the bottom layers of the water have incredibly low levels of oxygen, this means it’s already a situation where it’s almost at tipping point,” he said.
“If the salmon farms aren’t correctly managed not only could they damage themselves but it could have a flow on effect to the environment.”
Three companies; Tassal, Huon Aquaculture, and Petuna have been expanding fish farming in Macquarie Harbour over the last five years.
This report comes after the leaked emails from Petuna and Huon Aquaculture from late year, revealed concerns about the health of Macquarie Harbour, plunging dissolved oxygen levels and disease risk among fish.
The salmon industry has since reported that oxygen levels have improved, but the discussion paper obtained by ABC Rural, reveals that it remains an ongoing concern.
Dr Booth raised questions about continued expansions in Macquarie Harbour.
“It’s already at the environmental carrying capacity or beyond, so with a bit of clever husbandry the current stocking and profit levels could be maintained,” he said.
“But an expansion, from what I’ve read, could be disastrous for Macquarie Harbour.”