On top of the risk of death from disease, salmon are sensitive to environmental changes such as temperature spikes and dips in oxygen levels. The build-up of waste on the sea floor under fish farms can negatively impact oxygen levels, as can the stocking density of fish, water flow, water temperature and a range of other factors. When oxygen levels drop, fish become stressed and struggle to breathe. In 2015, 85,000 salmon suffocated to death in a salmon farm in Tasmania due to a change in oxygen levels.
Atlantic salmon prefer cool waters and in the wild, they can migrate huge distances to find temperatures where they can thrive. In farms, their movement is restricted to pens, and when the temperature rises, they have no escape.
In 2025, at least four million salmon died prematurely in Tasmanian fish farms. More than one million died in February alone during a mass mortality event in the lower D’Entrecasteaux Channel. The Tasmanian Government says unusually warm waters and the bacterium Piscirickettsia salmonis contributed to the event.
During Tasmania’s 2025 mass mortality event, drone footage showed live salmon being siphoned into containers alongside dead fish and the lids sealed. RSPCA Australia described the handling as inhumane and, following an investigation, withdrew Huon Aquaculture’s RSPCA Approved certification.