Antibiotic use on Tasmanian fish farms 'unlikely to have affected environment'
Maximum residue found in samples was 10 times below safe limit, says watchdog
The use of the antibiotic florfenicol to treat disease at nine salmon farms in southern Tasmania between November 2025 and March 2026 is unlikely to have affected the health of the marine environment, the state’s Environmental Protection Authority (EPA) has concluded.
The EPA carried out an environmental risk assessment on the use of florfenicol to combat salmonid rickettsial septicaemia (SRS), caused by the bacteria Piscirickettsia salmonis.
EPA director Catherine Murdoch said the rigorous and contemporary risk assessment used the largest and most complete set of data ever collected to examine how antibiotics affect the environment in marine aquaculture worldwide, and continued to support the initial findings released by the EPA that indicate the majority of the antibiotic administered has been taken up by salmon. Measured concentrations in the environment remained very low and consistent with the results previously reported.
Extensive field work
“This risk assessment brings together extensive field work and a detailed review of florfenicol and its breakdown product florfenicol amine,” said Murdoch.
“It outlines how the EPA developed interim Default Guideline Values (DGVs) for florfenicol to help identify harmful impacts, looks at 4,240 results from water and sediment samples, and uses several types of evidence together to assess environmental risk.”
Murdoch said the EPA has followed national water quality guidelines for protecting aquatic ecosystems and set an interim guideline level of 7 µg per litre (7 parts per billion) for florfenicol in pristine Tasmanian marine ecosystems, and 50 µg/L for florfenicol in modified Tasmanian marine ecosystems, such as the D'Entrecasteaux Channel.
Florfenicol was found in water and sediment samples collected both inside and outside nine treated fish farm sites, the EPA reported. The highest levels of florfenicol and its breakdown product were found in surface waters and surface sediment samples collected within the farm leases. Levels dropped quickly with distance from the farm leases, with depth from surface waters to the seabed, and with time after treatment finished.
Highest sample was 10 times below limit
Of the 4,240 results, fewer than 12% had any measurable amount of florfenicol or its breakdown product. None of the 4,240 sample results showed florfenicol levels above the interim DGV.
Of the 171 water samples where florfenicol was detected, the highest level found was 5.2 µg/L (or 5.2 parts per billion). This sample was collected during florfenicol treatment, right next to a fish pen. The highest result was approximately 10 times lower than the EPA’s interim DGV, and the average level (0.4 µg/L, or 0.4 parts per billion) was approximately 125 times lower.
Florfenicol amine was found in 297 water samples. The highest level measured was 4.2 µg/L (or 4.2 parts per billion), also collected during treatment next to a fish pen. While there is no guideline value set for the breakdown product florfenicol amine, EPA's risk assessment has considered that it may be as toxic as florfenicol. Both the highest and average levels (0.6 µg/L, or 0.6 parts per billion) are considerably lower than the interim DGV set for florfenicol.
The presence of florfenicol or florfenicol amine within sediments was highly localised to treated fish farm leases, with only 29 out of 1,680 sediment samples containing florfenicol or florfenicol amine. 24 of these samples were collected within the lease boundary. Florfenicol did not persist in the sediment beyond 14 days after treatment.
Low risk of unacceptable harm
“The findings of this environmental risk assessment highlight that the use of florfenicol in Tasmania's marine waters between November 2025 and March 2026 posed a low risk of causing unacceptable environmental harm. Overall, the results indicate that impacts were limited and unlikely to affect the health of the marine environment," said Murdoch.
Salmon Tasmania, which represents the state's three salmon growers - Huon Aquaculture, Tassal, and Petuna - said in a social media post that the risk assessment findings align with the best available science consistently showing no impact on waterway health or fish safety.
"Tasmanian salmon growers operate under the strongest regulatory regime in the world and rigorous, independent monitoring like this is important to give the community confidence in our operations."
