The excrement from fish farms can be used to sustain other organisms. For instance, the excretions from shrimps serve as food for large marine algae. Haddock feed on faeces particles and shrimp shells. This integrated multi-trophic aquaculture (IMTA) is now found in many different countries. It is operated mainly in breeding facilities along coastal areas.
Another type of integrated feeding used in inland breeding facilities is the aquaponic process. Effluent is used to fertilize crop plants; uneaten food, faeces and fish excreta provide the plants with nutrients. The plants in turn clean the water, thus closing the loop. Bacteria are often a part of the system which converts the food, faeces and excreta into chemical compounds that the plants can utilize. When animals and plants are combined with skill, such aquaponic facilities can be quite self-sufficient: operators neither have to feed the fish nor process the water. Tilapia, flowers and vegetables, among others, are farmed in aquaponic facilities. To date such facilities have seldom been operated on an industrial scale. The technology still needs optimization.

Fig. 4.11 > Impressive aquaculture: fish and vegetables are produced together in this facility in the USA. Fish excrements provide nutrients for the plants. The plants purify the water. Such a closed-loop system is called “aquaponic”. © Jon Lowenstein/Noor/laif