EVALUATION OF THE DISPERSION OF TREATMENT SOLUTIONS OF DICHLORVOS FROM MARINE SALMON PENS

A study of the dispersion of dichlorvos (0,0-dimethyl-2,2-dichlorovinyl phosphate) is described in the sea water surrounding two salmon farms following treatment of each site with 2 mg/l Nuvan 500EC (1 mg/l dichlorvos) in tarpaulin-skirted cages. Rhodamine was added to each treatment solution as a marker dye. Dispersion was monitored by visually tracking the dispersion of the dye cloud and by water sampling and analysis of dichlorvos concentrations from within the observable dye cloud and at various points around its visible perimeter. Concentrations of dichlorvos were found to disperse rapidly below the level of detection obtained in this study (2-mu-g/l). There were no detectable levels of dichlorvos found outside a 25-m perimeter of the treated pens at any time. Twenty-four hours after treatment there were no detectable levels of dichlorvos found either within or beyond 25 m. The overall rates of dispersion were not significantly affected either by the flushing rates of the two sea lochs monitored, or by variations in tidal movement. Possible mechanisms by which such dispersion is achieved are discussed, as are the implications of this study for the environmental risk assessment of such treatments.