In Ireland sea cages, also called sea pens or net pens, are usually made of mesh framed with steel or plastic. They can be square or circular, 10 to 32 metres across and 10 metres deep, with volumes between 1,000 and 10,000 cubic metres. A large sea cage can house up to 90,000 fish.
They are usually placed side by side to form a system called a seafarm or seasite, with a floating wharf and walkways along the net boundaries. Additional nets can also surround the seafarm to keep out predatory marine mammals. Stocking densities range from 8 to 18 kilograms per cubic metre for Atlantic salmon.
In contrast to closed or recirculating systems, the open net cages of salmonon farming lower production costs, but provide no effective barrier to the discharge of wastes, parasites and disease into the surrounding coastal waters. Farmed salmon in open net cages can escape into wild habitats, for example, during storms.