Psetta maxima

Nutrient Pollution

Both RAS and PAS turbot production use commercial aquafeeds. Such systems generally use feed more efficiently which in turn reduces the generation of excess wastes. However, excretory products and uneaten feed are still generated in turbot rearing facilities, and if released into the environment in an uncontrolled manner, could potentially cause problems to coastal waters through increased eutrophication (via dissolved nutrients) and deposition of organic matter around discharge points.

Therefore, efficient monitoring and management of feed regimes are essential to minimise nutrient discharges from on-shore turbot farms and reduce the potential of pollution arising from uneaten food entering the marine environment. In addition, feed is a major component of the operating costs of intensive aquaculture and its economical use is an important aspect of the financial sustainability of the farm.

Waste is easier to control in on-shore aquaculture facilities compared to open culture systems such as net-pens1. Many options of clean up technologies are now available to deal appropriately with the wastes generated in turbot farms. Solid waste materials can be removed from effluent prior to discharge via mechanical filtration (e.g. drum filters), settlement ponds, and saline tolerant reed beds (which also extract dissolved nutrients)2, 3. 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)4, 5.

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 volumes of water exchange and discharges from RAS means there can be even less of an environmental impact than from PAS and RAS are often perceived as having strong ‘green credentials’1, 6.


  1. SARF
  2. 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: pp285–300
  3. AQUAETREAT Project
  4. Nofima
  5. 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
  6. Seafood Watch