MODELING OUTPUT AND RETENTION OF SUSPENDED-SOLIDS IN AN INTEGRATED SALMON-MUSSEL CULTURE

The most marked effect of cage aquaculture in an environment is the ou tput of suspended solids and dissolved nutrients, which in some cases have been shown to cause environmental degradation. Instead of using t raditional methods built on technological solutions, integrated farmin g methods have been put forward as a mean for treating nutrients and p articulate wastes from fish cage farming. However, the results from in tegrated experiments, where filter feeders have been used to absorb su spended 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 fol lowing main constraints for using filter feeders for removing particle s from fish cages were identified: (1) Suspended solids from the fish cages will be highly diluted by the large volume of water passing thro ugh the cages. During continuous feeding (demand feeding) the concentr ation of released suspended solids was low (0.06-0.3 mg l(-1)). Only w hen fish density in the cages was high prior to harvest, and water cur rents were slow (0.03-0.05 m s(-1)), the increase in released concentr ations 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 durati on of a pulse and saturation of mussel feeding would make long term se ston concentration more important for mussel growth. (3) The potential particle retention from a rich pulse was limited by mussel pseudofaec es threshold level. It is concluded that, in a co-cultivation of musse ls and salmons, the ambient seston concentration is of greater importa nce in controlling mussel growth, and increases in suspended solids fr om the fish cages may contribute significantly only during periods of low plankton production. (C) 1998 Elsevier Science B.V. All rights res erved.