Cage and Net-Pen Aquaculture: A Guide
How cage and net-pen fish farming works: cage types, site selection, stocking and feeding, net maintenance, environmental considerations, and the species cultured.
Overview
Cage and net-pen aquaculture raises fish in enclosures held within an existing body of water—lakes, reservoirs, rivers, or coastal and offshore marine sites. The surrounding water provides exchange and oxygen, so cages can use a natural water body without diverting it. FAO and NOAA describe cage culture as a widely used method in both inland and marine aquaculture, from small reservoir cages to large coastal net pens.
Cage types and materials
- Fixed (staked) cages: net hung from posts driven into the bottom, used in shallow water with gentle flow.
- Floating cages: a net bag suspended from a buoyant frame (drums, foam or plastic floats), used in deeper standing water.
- Rigid cages: framed in bamboo, wood or metal where local materials are used.
- Net cages: netting of nylon or similar material on a galvanized, aluminium or wooden frame.
Marine sea cages used for salmon are typically 10 to 32 m across and about 10 m deep, with volumes between roughly 1,000 and 10,000 cubic metres; a single large cage can hold up to about 90,000 fish, according to the salmonid aquaculture literature.
Site selection
FAO guidance stresses adequate water depth, sufficient water circulation, and good, unpolluted water quality, because the exchange of the surrounding water controls how densely a cage can be stocked. Dissolved oxygen is generally less limiting than in ponds because currents renew the water around the net, but sites must still be assessed for the natural tolerance of the chosen species. Shelter from strong waves and storms is important for floating marine cages.
Stocking and feeding
Stocking density is expressed per unit volume and varies widely by species and system. Reported examples from FAO reservoir work include silver-carp fingerling densities of several hundred individuals per cubic metre and net yields on the order of tens of kilograms per cubic metre. Some cage culture relies on natural plankton drifting through the net, while intensive marine cages depend on formulated pellet feed delivered to the fish.
Fouling and net maintenance
Nets accumulate algae, invertebrates and other fouling organisms that block water flow and reduce oxygen exchange. Operators clean or change nets, and FAO notes that in some freshwater systems a small number of fish that graze the cage walls are stocked to help control growth on the netting. Net integrity is also maintained to prevent escapes.
Environmental considerations
- Effluent and nutrients: uneaten feed and fish wastes are the main source of excess nutrients, monitored under water-quality regulations.
- Benthic impact: organic matter settling under cages can affect the seabed, so impact zones are limited and monitored.
- Escapes: farmed fish that escape can compete with, and genetically affect, wild populations.
- Mitigation: good site selection, fallowing, and integrated multi-trophic aquaculture reduce impacts.
Species cultured
Inland cage culture commonly rears tilapias (Oreochromis niloticus, O. mossambicus) and carps such as Cyprinus carpio and the Chinese carps. Marine and coastal cage culture rears Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), European sea bass, gilthead seabream and cobia, among others. Norway and Chile together account for the majority of farmed salmonid production, most of it grown in sea cages after a freshwater smolt stage.