Open Net-Pens

Circular net-pens, typically 10 metres deep and up to 150 metres wide, are placed in arrays of 12 or more along ocean coastlines. Most net-pens have mesh bottoms and sides, while some also have mesh tops, usually for offshore locations. Marine waters as well as any substances or other organisms can pass through the mesh, while the salmon are meant to be contained inside. 

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The aquaculture salmon are fed fish feed with automatic feeding machines, farmers maintain fish health with periodic treatments, and any waste is left in the marine environment. The growing period is about two years depending on the location, before fish are harvested for processing and sale. The net-pens at the site are either left for a period (fallow) or re-stocked for another growing cycle depending on the site lease and license requirements.

Why are open net-pens bad?

• Escapees – Salmon escapes can occur during salmon transfer and transport activities to or from the net-pens, as well as directly from net-pens that are damaged by storms, wildlife, or deterioration. Escaped salmon may affect wild salmon populations by competing for food and habitat, and by impacting their reproductive fitness through interbreeding. 

• Sea lice– Open net-pen operations are notorious for sea lice infestations due to the large, dense congregations of fish. The sea lice are not contained to the net-pens and are transmitted to wild Atlantic salmon, putting pressure on already-fragile populations. It only takes seven sea lice to kill a juvenile salmon.

• Diseases – The transfer of diseases and pathogens from net-pen salmon to salmon outside net-pens puts pressure on wild populations. There are many diseases that are concerns such as ISA, piscine reovirus, and amoebic gill disease. Resistant and new diseases in the future are a concern. These diseases impact not only salmon but sea trout and Atlantic herring too.

• Marine pollution – The release of harmful chemicals and substances into the marine environment may include anti-fouling products used to keep net-pen cages clean. Chemicals used to treat diseases and feed ingredients are also released to surrounding waters where they can negatively affect other organisms. Salmon feces and unused feed fall to the seafloor beneath net-pens causing oxygen depletion in the water as it biodegrades.