Open-Net Salmon: Sea Lice, Disease and Mass Mortality

Atlantic salmon are farmed in open-net pens in coastal waters, mainly in Norway, Chile, Scotland and Canada. Norway's Veterinary Institute estimates that 16.7% of salmon in the sea phase died in 2023 and 15.4% in 2024. In Scotland, the BBC reported more than 17 million farmed salmon deaths in 2023, citing government data.

Sea lice (Lepeophtheirus salmonis) are small parasitic crustaceans that feed on salmon skin and mucus, and controlling them is a constant task on open-net farms. Treatments include warm-water (thermal) and mechanical delousing, and the institute ranks injuries from delousing among the leading health problems, alongside complex gill disease and winter ulcers.

In the 2024 survey, injuries from jellyfish ranked third among reported problems — a reminder that open-net pens are exposed to whatever drifts through the surrounding sea.

Under the EU's slaughter regulation (1099/2009), fish are covered only by the general duty to spare animals avoidable pain, distress and suffering. The detailed stunning rules that apply to mammals and birds do not apply to them.

For anyone deciding what to eat, the figures show that a significant share of farmed salmon do not survive to harvest. Algae-oil supplements provide the long-chain omega-3 fats EPA and DHA without fish farming.