Aquaculture is the farming of fish, shellfish, and aquatic plants in controlled or semi-controlled water environments. It works by managing every stage of an organism’s life cycle, from breeding and hatching through feeding and growth to final harvest. Global aquaculture production hit 130.9 million tonnes in 2022, now supplying 51% of all aquatic animal production worldwide. That makes farmed seafood, for the first time in history, a larger source than wild catch.
The Production Cycle: Hatchery to Harvest
All finfish aquaculture starts with broodstock, the adult fish selected for breeding. When these adults reach maturity, workers collect and fertilize the eggs, then place them in temperature-controlled incubation trays. Within about 30 days, the embryos develop visible eyes, a milestone called “eyeing up.” Roughly 10 days later, the eggs hatch into tiny larvae called alevin, which carry a yolk sac that feeds them for their first weeks of life. From fertilization to the free-swimming fry stage typically takes around 105 days, though this varies by species and water temperature.
Once the fry absorb their yolk sacs, they move into rearing tanks where they’re fed multiple times a day. Farm workers monitor growth continuously, removing uneaten food and waste, and splitting fish into additional tanks as they grow to prevent overcrowding. Depending on the species, this grow-out phase can last anywhere from several months to two or more years before the fish reach market size.
Open Water Net Pens
The most visible form of aquaculture is the net pen, also called a sea cage. These are enclosed floating structures anchored in coastal waters that hold fish while allowing ocean currents to flow freely through the mesh. A typical net pen has two layers: an inner net with mesh small enough to contain the fish, and an outer net strong enough to keep predators like seals from breaking in. Bird netting covers the top. The design lets the surrounding water supply fresh oxygen and carry away waste naturally, which is the system’s main advantage and its main environmental limitation.
Net pens are most commonly used for salmon, sea bass, and sea bream. Because waste disperses directly into the surrounding water, site selection matters enormously. Farms need strong enough currents to flush waste but calm enough conditions to protect the structures. This balancing act is one reason regulators closely manage where net pens can be placed.
Recirculating Aquaculture Systems
Recirculating aquaculture systems, or RAS, take the opposite approach: they’re indoor, land-based facilities that continuously clean and reuse their water. A RAS has five core components. The growing tank holds the fish. A clarifier tank acts as a settling basin, using slow flow rates to let solid waste (feces, uneaten feed) sink to the bottom for removal before it consumes oxygen or clogs downstream equipment. A biofilter, often described as the heart of the system, contains plastic sheets, beads, or sand grains coated with a film of beneficial bacteria that convert toxic ammonia from fish waste into less harmful compounds. An oxygenation system injects pure oxygen into the water. And a circulation pump keeps everything flowing.
RAS facilities can reuse 90% or more of their water, making them viable in locations far from the coast. The tradeoff is cost: these systems require significant energy and infrastructure compared to net pens. They’re increasingly used for high-value species like Atlantic salmon and are growing in popularity as the technology improves.
Shellfish and Seaweed Farming
Not all aquaculture involves feeding the animals. Bivalves like oysters and mussels are filter feeders that pull nutrients directly from the water, and seaweed absorbs dissolved nutrients through its fronds. This makes them far less resource-intensive to farm than finfish.
Shellfish farming uses two broad approaches: bottom culture and off-bottom culture. Bottom culture is the simpler method. Farmers scatter hard surfaces like old shells, stones, or other collectors across the seabed in areas where larvae naturally settle. Off-bottom methods suspend the shellfish above the seafloor using poles, racks, rafts, or longlines. In Spain, mussel farmers hang ropes from wooden rafts; seed mussels are wrapped in a dissolvable mesh that holds them against the rope until they attach on their own. In Japan, oyster farmers suspend wire collectors carrying hundreds of scallop shells from bamboo poles set below the tide line. In France, mussels grow on oak poles staked into the seabed, a centuries-old technique called bouchot culture.
Seaweed farming follows similar principles, using ropes or nets suspended in the water column where the algae can access sunlight and nutrients. Global algae production reached 37.8 million tonnes in 2022.
Keeping the Water Right
Water quality is the single most important variable in any aquaculture operation. Three parameters matter most. Dissolved oxygen needs to stay above 5 milligrams per liter for most species; below that threshold, fish become stressed, stop eating, and become vulnerable to disease. Ammonia, which fish excrete through their gills, is toxic even in tiny amounts and should remain below 0.1 parts per million. And pH, a measure of how acidic or alkaline the water is, generally needs to stay between about 6.5 and 8.5.
In open water systems, natural currents handle much of this regulation. In RAS and pond systems, farmers rely on biofilters, aerators, and constant monitoring to keep these numbers in range. Even small deviations can cascade quickly: a spike in ammonia damages gill tissue, which reduces oxygen uptake, which stresses the fish, which suppresses their immune system.
Feed Efficiency Compared to Land Animals
Fish are remarkably efficient at converting feed into body mass, mainly because they’re cold-blooded and don’t spend energy maintaining body temperature. Atlantic salmon have a feed conversion ratio of about 1.1 to 1.2, meaning it takes roughly 1.1 kilograms of feed to produce 1 kilogram of fish. For comparison, poultry typically requires about 1.8 to 2 kg of feed per kg of meat, pigs around 3 kg, and beef cattle 6 to 8 kg. This efficiency is a major reason aquaculture has expanded so rapidly as a protein source.
Modern fish feeds have also shifted substantially. Where early salmon farming relied heavily on fishmeal and fish oil from wild-caught forage fish, today’s feeds increasingly substitute plant-based proteins, insect meal, and algae-derived oils. This reduces pressure on wild fish stocks, though the sourcing of feed ingredients remains one of the industry’s most debated sustainability questions.
Managing Disease and Parasites
Raising animals in close quarters creates disease risks, and aquaculture is no exception. In salmon farming, sea lice are the most persistent challenge. These small parasites attach to fish skin and feed on mucus and tissue, causing stress and open wounds. Farmers use four main strategies to manage them: medical treatments (antiparasitic drugs added to feed or bath treatments), mechanical removal (flushing fish with warm water or pressurized jets), cleaner fish (small species like wrasse or lumpfish that eat lice off the salmon), and barrier technologies (specialized skirts or enclosures that prevent lice larvae from reaching the fish). Research on public perception has found that cleaner fish and barrier methods are viewed more favorably than chemical or mechanical approaches.
Integrated Multi-Trophic Aquaculture
One of the more promising developments in the industry is integrated multi-trophic aquaculture, or IMTA. The concept is straightforward: farm multiple species together so the waste from one becomes food for another. Finfish or shrimp (the “fed” species) produce nutrient-rich waste. Shellfish like mussels, positioned nearby, filter out the solid organic particles. Seaweed absorbs the dissolved nitrogen and phosphorus. The result is a system where nutrients cycle through multiple harvests rather than accumulating as pollution. NOAA and Canadian researchers have been developing this approach, though it remains more common in research settings than in large-scale commercial operations.
Scale and Economic Reach
The global aquaculture market was valued at roughly $204 billion in 2020, with projections reaching $262 billion by 2026. The Asia-Pacific region dominates production, with China alone responsible for more farmed seafood than the rest of the world combined. South Asia and Europe round out the top producing regions, together accounting for over 70% of global output. The species farmed range from tilapia and catfish in tropical freshwater ponds to Atlantic salmon in Norwegian fjords to shrimp in Southeast Asian coastal farms, each with its own production system tailored to the biology of the species and the geography of the site.