Salmon farming and the environment

. The growth of the aquaculture industry and the general awareness of the environment have increased the concern of’ the environmental impact from aquaculture. As fish farms are open systems, all material that is not harvested as fish is released to the environment. The most important effluents are solid particles like feed and faeces, dissolved substances like nutrients and excretion products, and antibacterial agents. The effect of these substances varies with their nature and quantity and with local conditions. Production and biomass of phytoplankton seem to be little influenced. The benthic community beneath and close to the cages is often altered. At poorly flushed sites the sea‐bed may be anoxic and methane and hydrogensulphide are released from the sediments. Wild fauna, like fish, otters and birds are attracted to the farms. The respiration and excretion of the fish directly influence water quality. Usually the oxygen tension is somewhat reduced in the net cages, while the concentration of ammonia is elevated in the net. Such conditions are assumed to influence fish growth and health. Most of the antibiotic agents used in fish farming are persistent and may remain in high concentrations in the sediments up to one year after medication. The frequency of resistant bacteria in the sea‐bed is normally increased, and there is a temporary drop in sediment metabolism after medication. Wild fish have been found to have high residues of antibiotic agents. The organic loading of the sea‐bed, the evolution of resistant bacteria and residues in the wild flsh are considered to be the most severe environmental impact of aquaculture. These are all associated with effluent of large particles, and thus mainly restricted to the site area.