Modelling output and retention of suspended solids in an integrated salmon–mussel culture

The most marked effect of cage aquaculture in an environment is the output of suspended solids and dissolved nutrients, which in some cases have been shown to cause environmental degradation. Instead of using traditional methods built on technological solutions, integrated farming methods have been put forward as a mean for treating nutrients and particulate wastes from fish cage farming. However, the results from integrated experiments, where filter feeders have been used to absorb suspended particles from fish cages, have not in many cases resulted in the expected beneficial effects. By modelling the output of waste from a salmon cage cultivation and particle filtration by mussels, the following main constraints for using filter feeders for removing particles from fish cages were identified: (1) Suspended solids from the fish cages will be highly diluted by the large volume of water passing through the cages. During continuous feeding (demand feeding) the concentration of released suspended solids was low (0.06–0.3 mg l−1). Only when fish density in the cages was high prior to harvest, and water currents were slow (0.03–0.05 m s−1), the increase in released concentrations were >0.1 mg l−1. (2) Addition of feed in pulses showed a 3–30-fold increase in water particle concentration, but the short duration of a pulse and saturation of mussel feeding would make long term seston concentration more important for mussel growth. (3) The potential particle retention from a rich pulse was limited by mussel pseudofaeces threshold level. It is concluded that, in a co-cultivation of mussels and salmons, the ambient seston concentration is of greater importance in controlling mussel growth, and increases in suspended solids from the fish cages may contribute significantly only during periods of low plankton production.