Tasmanian Inquirer – Antibiotic traces found in eels, water samples and sediments near Tasmanian salmon hatchery on Derwent River

Antibiotic residues have been found in eel, sediment and water samples from a freshwater salmon hatchery on the banks of the Derwent River, according to a report submitted to the Tasmanian Environment Protection Authority.

An interim report by Marine Solutions Tasmania, a consultant for Huon Aquaculture, revealed the company used almost 52 kilograms of the controversial antibiotic oxytetracycline (OTC)  at its Meadowbank hatchery, in the Derwent Valley,  between January 21 and February 9 to combat an outbreak of bacterial enteritis. The World Health Organization has warned overuse of OTC in the food industry could lead to the development of antibiotic-resistant “superbugs”, which it described as “one of the biggest threats to global health”.

The report attributes the initial disease outbreak to a 5C increase in the temperature of water drawn from the Derwent River. The company used 38.28 kg of OTC to treat salmon in seven tanks at the hatchery. Ten days after the initial disease outbreak, Huon applied a further 13.42 kg of OTC to fish in two other tanks. The Meadowbank plant is one of 10 “flow-through” freshwater hatcheries in the state that draw water from and discharge effluent into a river.

The company and EPA did not release details of the antibiotics use for more than two weeks until Tasmanian Inquirer made inquiries after receiving a tip-off. Companies with environmental licences for freshwater salmon hatcheries are not required to monitor antibiotic residues or report their use, but Huon voluntarily reported its use to the EPA.

The EPA finalised [Pdf] the monitoring schedule for water, sediment and fish samples on January 23, two days after Huon started using OTC at the hatchery. As a result of the delay, sediment samples were not collected at five sites in the Derwent River until six days after the first use of the antibiotic. Sheenagh Neill, a spokesperson for environment organisation Marine Protection Tasmania, criticised the EPA over the delay in establishing the monitoring schedule. “This report shows that once again the EPA has been caught unawares and left scrambling to try and catch up with antibiotic pollution of a major river by a salmon company,” she said.

“Hiding the numbers of dead fish behind claims of commercial disadvantage is unacceptable and does not meet community expectations”.

Meg Webb, independent MP

Eight weeks of sampling revealed no OTC residues at the hatchery water intake at or above the reporting threshold of 0.01 micrograms per litre (µg/l). It was a different story elsewhere.

One week after the last use of OTC, sediment from the effluent sludge tank showed a peak reading of 1800 milligrams per kilogram (mg/kg), exceeding the reporting threshold of 0.01 mg/kg. Levels well in excess of the reporting limit continued for three months. Huon disposed of sludge from the tank at Southern Waste Solutions’ waste management plant at Copping.

Sediments at the point of discharge into a small stream leading to the Derwent River remained six times higher than the reporting threshold for OTC in early May. Residues were recorded at lower levels just downstream of the discharge point and exceeded the reporting threshold only once at a point 500 metres downstream. The report, completed in June and published by the EPA late last week, said sediment sampling was “ongoing” because the regulator required monitoring to continue until consecutive samples returned results below the reporting threshold.

Twelve water samples from three sites exceeded the reporting threshold for OTC residues. One collected 500 metres downstream of the hatchery six days after antibiotic use began was 0.5 ug/l, 50 times the reporting threshold.

Three samples from short-finned eels had OTC residues above the limit. Two of these occurred three weeks after the end of OTC use. The highest OTC concentration in an eel was 0.05 mg/kg, five times the reporting threshold, but well below the 0.2 mg/kg maximum residue standard for human consumption.

A spokesperson for Huon said the use of antibiotics on veterinary advice was “one of the most important tools for best practice animal health”.

Unanswered questions

In January, the EPA said new monitoring requirements for antibiotic residues would apply to all freshwater hatcheries and told the ABC the new standard would be in place by the end of March. An EPA spokesperson told Tasmanian Inquirer last week the introduction of new standards “is currently underway” but did not provide a specific timetable.

Meg Webb, an independent member of the Legislative Council and the chair of a 2020 inquiry into finfish farming, said the agency needed to explain the delay. “Full disclosure of antibiotic use and subsequent monitoring data must be a licensing requirement for freshwater fisheries,” she said. “If the EPA has failed to put that in place close to six months after the Huon incident, the community deserves an explanation for the delay and a firm commitment to those licensing changes being expedited.”

The number of fish that died in the disease outbreak has not been publicly released. Huon said it could not disclose details during the two-year production cycle of fish due to “anti-competitive considerations”. Webb said the industry operated in public waterways and must be held accountable for animal welfare. “Hiding the numbers of dead fish behind claims of commercial disadvantage is unacceptable and does not meet community expectations,” she said.