Open net pens or cages enclose fish in offshore coastal areas or in freshwater lakes. This is the most common method for salmon farming.
Net pens are considered a high-impact aquaculture method because they are in open contact with surrounding waters. Diseases and parasites are easily transferred to wild fish and vice versa and fish escapes occur. Also waste from the fish, chemicals and antibiotics can pass freely into the surrounding environment, polluting wild habitats.