According to a new report, fish sludge from fish farms in Norway is comparable to the untreated sewage of millions of people and reduces oxygen levels in fjord waters.
Fish farms in Norwegian fjords cause pollution comparable to that of the untreated sewage of millions of people: this is revealed by a report by the Sunstone Institute that analysed the environmental impact of aquaculture in Norway.
Norway is the world's largest producer of farmed salmon: the fish are fed pellet feed and raised in cages placed in the fjords. Urine, faeces and unconsumed feed are discharged into coastal waters. Analysts have calculated that in 2025 Norwegian aquaculture released into the environment 75,000 tonnes of nitrogen, equivalent to that contained in the untreated sewage of 17.2 million people, 10,000 tonnes of phosphorus, equivalent to that contained in the untreated sewage of 20 million people, and 360,000 tonnes of organic carbon, equivalent to that contained in the untreated sewage of 30 million people. Since Norway has a population of 5.5 million, the fish industry produces pollution three to five times greater than that of the entire national population, and almost equal to that of the inhabitants of Australia.
The fish sludge produced by the farms can trigger the process of eutrophication, meaning it can fertilise phytoplankton and lead to destructive algal blooms that reduce oxygen levels in the water, preventing fish and other marine species from surviving. The report's authors found that pollution is highest in the summer months, when ecosystems are less able to absorb it due to rising water temperatures. Furthermore, fjords are particularly vulnerable to such effects because they are semi-enclosed bodies of water where substances accumulate. This is without considering that oxygen levels in the fjords are already declining as a result of global warming.
According to statements made to the Guardian by Tom Pedersen, the region's environmental consultant and reviewer of the Sunstone report, the data are not surprising and may even be underestimates: the main concern is precisely the falling oxygen levels in the water. Krister Hoaas, head of public relations at the Norwegian Seafood Federation, told the Guardian that a significant increase in production in some fjord systems could heighten the risk of eutrophication at a local level, but that current production is well within the ecosystem's carrying capacity and that rigorous site-by-site management is needed.