Nutrient Pollution — Seafish

Atlantic Halibut

Hippoglossus hippoglossus

Nutrient Pollution

Both on-shore and net-pen halibut farms use commercial aquafeeds. Feed is a major component of the operational costs in aquaculture and the efficient use of feed is an important aspect of farm management – to ensure that as much of the feed as possible is consumed by the fish and that as little as possible is wasted. Efficient monitoring and management of feed regimes reduces the potential for pollution of the freshwater or marine environment arising from solid (including uneaten feed) and dissolved nutrient discharges from halibut farms. This is important as the accumulation of uneaten food and faecal matter around discharge points of RAS and PAS and beneath net-pens, has the potential to affect aquatic life through de-oxygenation and algal blooms which can be associated with nutrient increase. The key nutrients likely to cause problems for receiving waters are nitrogen and phosphorus.

Through licensing and management, farms can be located in areas where the environmental impact of nutrient release is minimal. Where applicable, this includes undertaking full environmental impact assessments and modelling impacts of release of nutrients before licenses are issued. In Scotland, for example, the discharge of wastes is regulated through Controlled Activities Regulations1.

RAS and PAS generally use feed more efficiently which in turn reduces the generation of excess wastes. In addition, waste is easier to control in land-based aquaculture facilities compared to open culture systems such as net-pens2.

Any waste generated in land-based aquaculture must be appropriately dealt with, and many clean-up technology options are now available, for both RAS and PAS facilities.

Solid waste materials can be removed from the effluent prior to discharge via mechanical filtration (e.g. drum filters), settlement ponds, and draining the water through salt reed beds, which additionally extract dissolved nutrients3, 4. Unlike freshwater aquaculture systems where the collected solid waste can be applied to agricultural land, in marine RAS and PAS this is more complicated because of the salt content. Alternative opportunities to utilise solid wastes from land-based marine aquaculture systems are being explored (e.g. production of bio-gas)5, 6.

There are significant differences between PAS and RAS in terms of water usage and discharge. The amount of water required by a RAS facility is significantly lower than PAS for the same amount of fish produced, consequently generating less wastewater and potentially serving as an advantage in complying with discharge regulations.

The lower levels of discharge from RAS means there can be even less of an environmental impact than from RAS; in fact RAS are often perceived as having strong ‘green credentials’ as they abstract little water from natural water bodies, produce minimal effluent, and have readily managed waste streams2, 7.


  1. Marine Scotland
  2. SARF
  3. Castine, S.A. et al, 2013. Wastewater treatment for land-based aquaculture: improvements and value-adding alternatives in model systems from Australia. Aquaculture Environment Interactions 4: p285–300
  4. AQUAETREAT Project
  5. Nofima
  6. Zhang, X. et al, 2014. Potentials and limitations of biomethane and phosphorus recovery from sludges of brackish/marine aquaculture recirculation systems: A review. Journal of Environmental Management, Vol 145, 2014, p394-395
  7. Seafood Watch