Environmental guidance for your business in Northern Ireland & Scotland

Marine cages for fish farming at sea

Fish farming: preventing pollution from marine cages

Waste from fish cages can have a negative impact on the environment. It can:

  • smother bottom-dwelling animals and plants
  • remove oxygen from the water
  • increase the concentration of nutrients such as ammonia, nitrogen and phosphate.

Your discharge consent or authorisation will limit the number of fish held in cages to help minimise the impacts of fish waste.

Discharges to water or sewer

What you must do

If you use chemical treatments in cages, you must comply with any limits on chemical use included in your discharge consent or authorisation.

Good practice

Place your cages in an area with good tidal flushing and water exchange so that waste is quickly diluted and dispersed.

Use fallow periods to allow the seabed to recover and help reduce sea lice populations. You can maximise environmental benefit by co-ordinating fallow periods at all fish farms that are close to one another.

New methods of dealing with fish lice are being developed. You could investigate the use of heat treatment or the use of cleaner fish.

Minimise the use of chemicals on your fish farm in order to reduce their impact on the environment. For example, you may be able to reduce the frequency of chemical application for sea lice treatment if all farms in an area treat their fish within a short timescale.

In Scotland, voluntary Area Management Agreements between farms have been used to co-ordinate sea lice treatments and stocking. This has maximised the effectiveness of the treatments and therefore reduced overall use of chemicals.

You should take care to ensure that oil or fuel used for boats is not spilt onto land or into watercourses.

When you harvest fish, avoid spilling dead fish or blood into the sea. Collect and remove all blood and fish waste. Take care not to spill any ice or water slurry into the sea.

Frequently monitor cages for dead fish and remove them from the water as soon as possible.

Recent developments in fish farming technology have introduced floating closed systems, where all wastes from the fish farm are collected, pumped ashore and treated. This greatly reduces the environmental impacts from wastes and also reduces the need for sea lice treatments. 

Read about More ways to prevent water pollution on NetRegs

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