Farmocean AB, Datavägen 14 A, S-436 32, Askim, Sweden

To develop such a cage/system, the requirements of the fish, the fish farmer, the insurance companies, the authorities and the moorings must be considered. The existing offshore concepts can be classified into: (1) simple flexible one-net bag cages; (2) integrated stiff semi-submersible one-net bag constructions; (3) simple stiff poly-net bag cages; (4) integrated stiff poly-net bag constructions; (5) submersible and submerged cages. An example from each group is described.

The Bridgestone cage and the Farmocean system, the only offshore concepts that have been in commercial use for some years, are compared with respect to strength and management. The other concepts are evaluated in various parts.

A suitable offshore concept should, in general, consist of a stable, strong and safe platform that should be easy and safe to tow, moor and board. It should allow a proper attachment of the net bag (preferably round, 15 m deep and 5000 m³ in volume), and guarantee its strength and shape during all weather conditions. The fish should be fed by means of a computer-controlled automatic feeder.

Faster growth, lowered mortality and reduced visceral fat content are reported among offshore farmed fish compared with those farmed inshore. A lower degree of self-pollution will also be a consequence of moving larger and deeper cages/systems offshore.

The tentative rules for the type approval of floating fish farming units that have been presented by “Det norske Veritas” (DnV) in Norway will be positive for the fish farming business if it starts to be valid and cover the rest of the fish farming countries.

Despite the higher investment required for the offshore cages/systems, it has been indicated that there is a better economic result and that the cost of producing 1 kg of fish is lower than in conventional fish farms.

As the single-net bag concept represents a lower risk for the fish farmer, insurance company and veterinary authority, it will probably be the preferred concept.