Aquaculture

Picture a net-pen salmon farm anchored in a coastal bay. Fish are packed into cages sitting in open water, fed pellets made partly from ground-up wild fish, and the pens sit directly in the current. Uneaten feed and fish waste fall straight through the net to the seafloor below, loading the water with nitrogen and phosphorus that can trigger algal blooms and smother bottom habitat. Fish also escape through torn nets, and farmed escapees can breed with wild populations or spread disease and parasites like sea lice into them. None of this is contained, because the system is open to the surrounding ocean.

A closed recirculating aquaculture system fixes the containment problem. Fish are raised in tanks on land, where water is filtered, treated, and reused rather than flushed straight into a bay. Waste gets captured and removed instead of drifting onto the seafloor, and there is no net for fish to escape through. This cuts local pollution and the risk of genetic or disease spillover to wild populations, though it costs more energy to run the pumps and filters.

The feed problem is separate from containment and hits carnivorous species hardest. Farming salmon or shrimp often means grinding up wild-caught anchovies and sardines into fishmeal, and if it takes more than a kilogram of wild fish to produce a kilogram of farmed fish, the farm is a net drain on ocean biomass, not a relief from overfishing. Coastal shrimp farming adds a land-use cost too: ponds are frequently built by clearing mangrove forests, destroying nursery habitat for wild fish, storm protection for the coast, and a major carbon sink, all at once.