Effects of organic waste from marine fish farms on the seabottom beneath the cages

Seven fish farms and a control station were investigated four times during one year. The farms represented a gradient of increasing accumulation of organic waste on the sea bottom. The thickness and the organic content of the waste increased up to 20 cm with increasing sedimentation. When the thickness of the accumulated waste exceeded 20 cm the larger fauna ( > 5 mm) disappeared, and the decomposition rate of the material as a function of the accumulated amount decreased. The spontaneous gas ebullition from the waste increased linearly with increasing accumulation. Preliminary analysis of stable carbon isotope ratios of the sediment suggests that there is a return to original background isotope ratios within 30 m of a well-run fish farm