Salmon farming raises serious environmental concerns, from genetic damage to wild fish populations to chemical contamination of the ocean floor. Open-net pen farms, which produce the vast majority of farmed Atlantic salmon worldwide, operate by suspending large mesh cages directly in coastal waters. This design means everything inside the farm (waste, parasites, chemicals, and the fish themselves) can interact freely with the surrounding ecosystem.
Escaped Fish Weaken Wild Populations
Farm-raised salmon regularly escape from open-net pens during storms, equipment failures, and routine handling. When these fish breed with wild salmon, the genetic consequences ripple through populations for generations. Farmed salmon have been selectively bred for fast growth, which sounds beneficial but creates a dangerous mismatch with survival in the wild. Research from the North Atlantic Salmon Conservation Organization shows that genetically farmed salmon and farmed-wild hybrids have reduced survival rates in natural rivers and oceans.
The core problem is that breeding for growth makes fish more vulnerable to predators, which partly explains why juvenile survival drops in wild populations exposed to farm genetics. But the damage goes deeper than individual survival. When farmed genes spread through wild populations, the fish begin maturing faster, spending fewer years in freshwater before migrating to sea, and producing smaller eggs relative to their body size. Wild salmon with higher levels of farmed genetic influence are more likely to migrate to sea a year earlier than they normally would, without being any larger at the time of migration. Their growth rate at sea also increases, particularly in the first year.
This “faster pace of life” might seem harmless, but wild salmon populations have evolved their specific growth rates, migration timing, and reproductive schedules over thousands of years to match local conditions. Accelerating these traits disrupts the delicate fit between fish and habitat, reducing the long-term resilience of wild stocks that are already under pressure from climate change and habitat loss.
Sea Lice and Parasite Amplification
Open-net pens concentrate tens of thousands of salmon in a small area, creating ideal conditions for sea lice to reproduce in enormous numbers. Sea lice are naturally occurring parasites, but farms amplify their populations far beyond what wild fish would encounter. Juvenile wild salmon migrating past coastal farms can pick up lethal loads of lice before they ever reach the open ocean. In regions like British Columbia, Norway, and Scotland, elevated sea lice pressure from farms has been linked to declines in wild salmon and sea trout populations.
To control lice, farms deploy a range of chemical treatments that bring their own problems. One of the most widely used is a pesticide sold under the brand name SLICE, which is mixed into feed pellets and excreted by the fish into surrounding waters. Research from the University of Victoria found that this compound persists in marine sediment between treatment periods and is measurable near farm sites long after application. At higher concentrations, it increased mortality in spot prawns by preventing them from completing their molting process. Prawns exposed to it also lost their ability to detect and orient toward food, a behavioral disruption that could cascade through local crustacean populations.
Waste and Nutrient Pollution
A single large salmon farm can hold over a million fish. Their feces, uneaten feed pellets, and chemical residues fall directly through the mesh and settle on the seabed below. This concentrated organic waste depletes oxygen in bottom sediments, smothers organisms living on the seafloor, and triggers algal blooms in the water column. The effect is comparable to dumping raw sewage: a farm producing 2,000 tons of salmon generates waste loads roughly equivalent to a small city, but with no sewage treatment system.
Over time, the seabed beneath and around farm sites can shift from a diverse community of worms, crustaceans, and shellfish to a bacterial mat with very little life. Recovery after a farm is removed or fallowed can take years, and in areas with poor water circulation, the damage accumulates faster than the environment can process it.
What Farmed Salmon Eat
Salmon are carnivores, and feeding them at industrial scale requires massive inputs of marine protein and oil. As of 2020, salmon feed still contained about 12% fishmeal and 10% fish oil sourced from wild-caught forage fish like anchovies, sardines, and herring. To account for the nutritional quality of what goes in versus what comes out, researchers use a metric called the nutrient Fish In: Fish Out ratio. For long-chain omega-3 fatty acids (the primary nutritional reason people eat salmon in the first place), the ratio sits at about 2.17. That means it takes roughly 2.2 kilograms of wild fish nutrients to produce 1 kilogram of salmon nutrients.
The industry has tried to reduce this dependency by shifting toward plant-based ingredients. Modern salmon feed is now about 41% vegetable protein and 20% vegetable oils, with only small additions of newer alternatives like insect meal and microalgae (about 0.4% combined). This plant-heavy diet comes with its own footprint: soy and other crops used in feed are linked to deforestation and agricultural land conversion, particularly in South America. The shift also changes the nutritional profile of the final product, reducing the omega-3 content that makes salmon nutritionally valuable.
Antibiotic Use Varies Wildly by Region
Antibiotic use in salmon farming depends heavily on where the fish are raised. Scotland’s salmon industry reported record-low antibiotic use in 2024 at 4.9 milligrams per kilogram of fish produced, a dramatic drop from 24.8 mg/kg just the year before. Norway has similarly pushed usage to very low levels through vaccines and improved husbandry.
Chile tells a different story. Chilean salmon farms have historically used antibiotics at rates hundreds of times higher than Norway, driven by persistent bacterial infections in warmer waters and denser farming conditions. High antibiotic use accelerates the development of drug-resistant bacteria, which can spread from farm environments into surrounding waters and potentially into the broader food chain. The gap between best and worst practices in the industry is enormous, and consumers typically have no way to distinguish between salmon raised with minimal antibiotics and salmon raised with heavy use.
Alternatives Exist but Face Barriers
Land-based recirculating aquaculture systems (RAS) eliminate most of the environmental problems with open-net pens. They contain waste, prevent escapes, block parasite transfer to wild fish, and can be built close to markets. A comparative financial analysis found that production costs for land-based systems are only about 10% higher than open-net pens ($5.60 versus $5.08 per kilogram), and the day-to-day operating costs are nearly identical ($4.37 versus $4.30 per kilogram).
The real barrier is upfront investment. Building a land-based facility costs roughly $54 million compared to $30 million for an open-net operation, an 80% premium that makes financing difficult. There’s also a carbon tradeoff: land-based production generates about double the emissions of ocean-based production when you look at farming alone (7.01 versus 3.39 kg CO2 per kilogram of live fish). However, when you factor in transportation, a land-based facility near its market can actually cut total emissions in half compared to Norwegian ocean-farmed salmon shipped by air to the same destination (7.41 versus 15.22 kg CO2 per kilogram).
The economics suggest land-based farming is viable, but the industry has little incentive to switch while open-net operations can externalize their environmental costs onto public waterways. Without stronger regulation, the transition will remain slow.