The benefits of recirculating aquaculture systems

A powerful set of attributes has established recirculating aquaculture systems as a cornerstone of global temperate fish farming, writes Leslie Ter Morshuizen, owner of Aquaculture Solutions.

- Leslie Ter Morshuizen

A recirculating aquaculture system (RAS) comprises a group of fish tanks that are connected as a single system and which includes filters, aeration and water pumps. These systems offer compelling advantages over alternative infrastructure options, especially in variable climates in temperate areas.

The fish in an RAS are fed and as they grow, they also utilise oxygen, which is replenished by means of aeration; the carbon dioxide that the fish release as a by-product is simultaneously removed by the same aeration. As the fish are fed they digest their feed and excrete solid and dissolved waste.

WATER MOVEMENT

Water moves continuously through the fish tank, carrying solid waste out of the tank and into the mechanical filter where it is trapped and accumulated for regular removal, usually daily, by staff when cleaning the filters. If left in the system, this waste will accumulate and compromise water quality.

The dissolved waste is comprised mainly of ammonia, which is converted to nitrate by the biological filter, in which desirable bacteria convert the highly toxic ammonia to benign nitrate, which can be allowed to accumulate within the system without negatively impacting the fish.

Daily water changes of around 10% of the system volume occur when the mechanical filters are cleaned. This is adequate to maintain the nitrate concentration at acceptable levels for fish health.