The fish farms pollute the water with nutrients, methane and hydrogen sulphide, and chemical residues which threaten both farmed fish and other marine life. Growth promoters and antibiotics are added to food. Pesticides are used to treat infestations of sea lice, for example.
In Denmark, 70 to 80 percent of drugs administered in fish farms end up in the environment. Usually they are added to feed which falls out of cages into rivers or the seas without being eaten. Studies of sediments beneath the fish cages have found concentrations of up to 11 parts per million of the antibiotic oxytetracycline.