It’s as if the equivalent of the entire Australian population – almost 30 million people – relieved themselves directly into Norwegian fjords. That’s the scale of nutrient pollution caused by Norwegian fish farms, according to a recent report by the Oslo-based Sunstone Institute.

Norway is the world’s largest producer of salmon, feeding over 400 million salmon per year with little to no waste management. The problem is their poop.

Farmed salmon, unlike salmon in the wild, is fed nutrient-dense pellets designed to maximise growth. But they only absorb half of those nutrients, the report found.

The rest becomes faeces, urine, or uneaten feed that spreads directly into the surrounding ecosystem. The result is a nutrient-filled cloud of dissolved nitrogen, phosphorus, and organic carbon.

While other factors are involved in creating nutrient pollution in Norwegian fjords – such as agricultural and industrial run-off, municipal wastewater, and sewage – fish farming is the single largest contributor along the country’s coastline.

One of the report’s authors, Aleksander Nordahl, has spent years diving in Norway’s fjords and documenting the damage. He describes how excess nutrients cause algae to take over, smothering other marine life. When they die in autumn and sink to deep waters to rot, they deplete the ocean bed of oxygen, leaving too little for fish, shellfish, and other marine life to survive.

This is called eutrophication, a global problem that plays out around intensive salmon farms just as it does in any over-fertilised pond: when you add too much fertiliser and food scraps to it, algae will bloom and suffocate everything beneath it.

The second longest fjord in Norway, Hardangerfjord in Vestland county, is an example of how alarming the situation is. Oxygen levels in the deeper parts of the fjord have dropped by a third since 1955.

“Our deepest concerns are with the depleting oxygen levels in the fjord basin,” Tom Pedersen, a senior advisor at the county governor’s office, told us. In 2023, nine new fish farming applications were put on hold for this reason.

The office announced in March this year that those fish farming applications were rejected to prevent further deterioration in the fjord. However, “There has been no improvement in the environmental conditions in the fjord,” Pedersen said. “The measurements are still showing a negative trend.”

That’s not to say improvement is impossible. By 1990, the Black Sea faced large-scale eutrophication, turning 40,000 sq kilometers effectively dead – roughly the size of Switzerland and the Netherlands combined.

When the Soviet Union collapsed, fertilisers became too costly. Investments were made into pollution-reduction and phosphorus runoff in half was cut in half, while nitrogen dropped by 20%. This has led to partialrecovery of bottom-dwelling marine life.