Aquaculture Methods
The environmental impact of farmed seafood is largely determined by the farming method used.
Aquaculture, or fish farming, may take place in the ocean, lakes, or on land. Here is a summary of common aquaculture methods.
High-Risk Systems
Open-net Pens: Found offshore, in coastal areas or in freshwater lakes, open-net pens, or “cages”, are considered a high-risk aquaculture method, since they allow for free exchange between the farm and the surrounding environment. Salmon are typically farmed this way. Open-net pens allow free exchange of waste, chemicals, parasites and disease. There is also the potential for farmed fish to escape. Farms can also attract predators, such as marine mammals, that can get tangled in fish farm nets and drown.
Ponds: Ponds are semi or fully-enclosed bodies of water. Tilapia and shrimp are typically farmed this way. Discharged waste must be filtered and treated to be considered a “low risk” method. “High-risk” pond farms discharge untreated wastewater, which pollutes the surrounding environment. They can also cause devastating habitat damage. For example, shrimp ponds are a leading cause of mangrove destruction.
Low Risk Systems
Closed Systems: Closed systems, or closed containment farming methods, use a barrier to control the exchange between farms and the natural environment. This significantly reduces pollution, fish escapes, negative wildlife interactions, and parasite and disease transfer from farms to marine and freshwater ecosystems. The most common types of closed systems are raceways and recirculating systems.
Raceways: Flowing water is diverted from natural streams or a well. Raceways are typically used for raising rainbow trout. To be considered a low-risk method, waste must be treated and fish escapes prevented.
Re-circulation Systems: Water in these systems is treated and re-circulated, with minimal wastewater discharge. Almost any type of finfish can be raised in recirculating systems. Common species farmed this way include Arctic char, striped bass, barramundi, sturgeon, and increasingly, salmon. These systems are designed to treat effluent before it is discharged to natural water bodies, which reduces pollution, disease and parasite transfer. Fish escapes are virtually impossible, with appropriate barriers designed into the facilities.
Suspended-aquaculture: Farmers grow shellfish on beaches or suspend them in water on ropes, plastic trays or in mesh bags. The shellfish farmed using these methods are filter feeders and require only clean water to thrive. Oysters, scallops, mussels and clams are cultured using suspension systems. Shellfish farming in suspended-aquaculture is often low risk, if the farmed species is native to the area, and if the farm has sufficient water flow to prevent waste accumulation.
