Aquaculture

When wild salmon meet farms

Open net-pen farms put wild salmon in the path of disease and pollution.

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Aquaculture has long been part of coastal food systems in British Columbia.

For millennia, First Nations managed and enhanced marine resources through practices such as clam gardens and fish traps that helped sustain abundant seafood harvests.^† Industrial salmon aquaculture emerged in the 1970s and expanded rapidly. By the early 2000s, hundreds of open net-pen farms operated along the coast, creating new pathways for the exchange of water, waste, chemicals, pathogens, and parasites between farmed fish, the marine environment, and wild salmon.

What happens in
the pen doesn’t
stay in the pen

Open net-pen salmon farms, which primarily raise Atlantic salmon, are an ideal place for parasites, viruses, and bacteria – collectively referred to as pathogens – to multiply and spill over to wild Pacific salmon.

Farms can introduce new pathogens with eggs imported from other countries or concentrate and spread pathogens that are found naturally in our oceans.^†

The global emergence of Piscine orthoreovirus

A virus called Piscine orthoreovirus (PRV) has spread from Europe across the world with the expansion of salmon farming. PRV can infect wild Pacific salmon^† and lead to jaundice, anemia, and muscle and heart inflammation. This, in turn, can hurt their swimming performance and migration success, make them more vulnerable to other stressors and diseases, and contribute to lower overall survival of a population.^†^†

Schematic representation of the global emergence of PRV-1. Arrows depict estimated translocations of two different variants of PRV-1 (solid and dashed lines). Adapted from Mordecai et al. (2021).^†

Stylized world map of the global emergence of PRV-1, with dated arrows tracing estimated transmission of the virus from Norway to Iceland, British Columbia and Washington, and Chile

Wild salmon migrating in the water past an open net-pen salmon farm — Wild salmon migrate past an open net-pen farm.

A familiar bacteria, an unfamiliar risk

Other pathogens naturally occur within Pacific salmon populations, but the location and conditions on salmon farms can amplify natural infection levels. The bacterium Tenacibaculum can result in ulcers and mouth rot in Chinook salmon^† with reduced body condition and higher mortality in infected fish.^† Wild sockeye experience infection pressure 12.7× higher than background levels as they migrate past salmon farms in the Discovery Islands^†, raising concerns about how farm-amplification of Tenacibaculum may affect wild salmon survival.

A year-round reservoir of sea lice

Sea lice are parasites that feed on fish skin and blood and are commonly found on adult wild salmon. However, small, scaleless juvenile salmon are far more vulnerable to the impacts of these parasites.^†

When wild salmon die after spawning, it naturally breaks the cycle of sea lice infection – their parasites die with them. But farmed salmon are present year-round and continue to carry sea lice through the winter, acting as a reservoir that can re-infect the next generation of salmon when they enter the ocean.

In salmon farming regions, juvenile wild salmon leaving their home streams in spring may encounter sea lice from farms at levels 73 times higher than in areas without farms,^† leaving them vulnerable to an infection their bodies aren’t prepared for.

Small, scaleless juvenile salmon are far more vulnerable to sea lice than adult salmon. Inset: a single louse, magnified.

Macro close-up of a single sea louse, magnified from the juvenile salmon shown above. — Small, scaleless juvenile salmon are far more vulnerable to sea lice than adult salmon. Inset: a single louse, magnified.

How farmed salmon keep sea lice in the system year-round.

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Increase in sea lice infection pressure near salmon farms relative to ambient levels.^†

Open net-pens transfer costs to the environment

While often heralded as a sustainable way to produce seafood without taking from natural populations, open net-pen salmon farms do take a toll on ecosystems both locally and globally.

Aerial view of an open net-pen salmon farm in a forested inlet in the Broughton Archipelago — Open net pens, Broughton Archipelago, 2022.

The clean-up

Salmon farms produce large amounts of organic waste that is not contained in open net-pens where fish are raised, taking advantage of the marine environment’s natural ability to dilute and recycle it. When organic material breaks down, it consumes oxygen, creating “dead zones” in the water or on the sea floor below the net pens where there is not enough oxygen to support marine life. Chemicals, feed, and excess nutrients from the farms also accumulate in the sediment and affect other species.^†

What farmed salmon eat

Farming carnivorous fish like salmon requires specialized feed made largely from wild-caught forage fish: anchovies, sardines, and herring. Those same forage fish feed seabirds, marine mammals, and wild salmon. Harvesting too many of these small fish disrupts the food web that wild salmon and other species depend on, impacting ecosystems worldwide. Salmon aquaculture is often held up as a solution to global protein demand given declines in wild-capture fisheries – but the reality is that it may do more harm than good.

A dense catch of small, silvery forage fish — Wild-caught forage fish, harvested largely for aquaculture feed.

Proportion of marine capture fishery production that was made into fish meals and fish oils, primarily for feed in aquaculture, in 2024.^†

Ending open net-pens: a commitment worth keeping

A substantial body of research has established the connection between salmon farming and weaker wild returns, backed by studies of Atlantic salmon across Europe.^†^†

Even when infections aren’t severe enough to kill salmon directly, they impact the ability of salmon to migrate, avoid predators, and compete for food – all of which translates to fewer returning spawners for the population. In recognition of this threat, the Government of Canada has committed to ending open net-pen salmon farming in B.C. by June 29, 2029. Transitioning to fully closed containment salmon farms that eliminate the transfer of pathogens and waste to the environment is the path forward.

The planned phase-out of open net-pen salmon farms is a win for wild salmon, but the risks posed by salmon farms will remain until the transition is fully complete.