Cage Culture (Net Pen)

Cage culture, or net pen farming, holds fish in flexible or rigid net enclosures moored in open water — lakes, reservoirs, rivers, or coastal seas — relying on the surrounding water body for all oxygen supply, waste dispersal, and temperature control, with the farmer responsible only for feeding, stock protection, and net maintenance. Its economic logic is compelling: capital and operating costs per kilogram produced are far below land-based systems because no water treatment, pumping, or heating infrastructure is needed, which is why the great majority of farmed salmon and a large share of seabream, seabass, and pangasius are cage-farmed. The corollary is near-zero control: dissolved oxygen, temperature swings, algal blooms, and turbidity events arrive unfiltered, and dissolved and particulate waste — nitrogen, phosphorus, organic solids that settle beneath the cage — is discharged directly into the receiving ecosystem, making site carrying capacity, depth, and current flow critical regulatory variables. Other operational risks follow from the open boundary: fish escapes through storm-damaged or predator-torn nets introduce farmed genetics to wild stocks; sea lice and pathogens transmit both directions between farmed and wild fish; and net fouling by algae and invertebrates must be managed by in-situ cleaning, antifouling nets, or regular net changes. Modern regulation couples cage farming with biomass limits, fallowing requirements, environmental monitoring of the benthos, and increasingly closed or semi-closed containment designs that separate the fish from the surrounding water.