How Recirculating Aquaculture Systems Move Fish Farming Onshore

For most of its history, fish farming has meant putting cages in the sea. Net pens moored in coastal bays are cheap, they use the ocean's own currents to bring in oxygen and carry away waste, and they have made farmed salmon one of the great food-production success stories of the past fifty years. They also come with problems the industry has never fully solved: sea lice, disease outbreaks, nutrients pouring onto the seabed beneath the pens, and farmed fish escaping into wild populations. Recirculating Aquaculture Systems, universally known as RAS, propose a radical alternative. Instead of putting the farm in the ocean, bring the ocean indoors. Raise the fish in enclosed tanks on land, clean the water continuously, and reuse it almost indefinitely. The technology genuinely works, and it has quietly become indispensable to modern aquaculture. Whether it can replace ocean farming at industrial scale is a much harder question, and one the industry is still expensively learning the answer to. Here is how the system works, step by step.

 

1. Water Is Pumped Into the System

 

The defining feature of a RAS is right there in the name. Fish are raised in enclosed tanks where the water is continuously circulated through a treatment loop and returned, rather than flowing through once and being discharged. A conventional flow-through farm takes clean water from a river or the sea, passes it over the fish, and sends it back out carrying the waste. A RAS closes that loop.

Everything else in the system exists to make this possible. Once you stop flushing waste away, you have to deal with it yourself, which means the farm needs its own life-support infrastructure to remove solids, neutralise toxins, replenish oxygen, and control temperature. A RAS is, in effect, a water-treatment plant with fish living inside it, and the fish are almost the simplest part of the operation.

 

2. Fish Are Fed and Grow

 

Inside the tanks, conditions are engineered rather than inherited. Water quality, temperature, oxygen levels, lighting, and feeding are all held at the optimum for the species being raised, and because the environment never fluctuates with the seasons or the weather, growth is steady and predictable year-round.

This control is the source of the model's most compelling advantages. Fish raised in an indoor tank are shielded from storms, predators, algal blooms, jellyfish, and the sea lice that plague open-net salmon farms. Production can be scheduled with industrial precision, and a farm can be sited almost anywhere, including places where the climate would never permit the species to be farmed outdoors, which is how Atlantic salmon come to be grown in Florida and in the deserts of the Middle East. The trade-off is that control must be total. In the sea, the ocean is the life-support system and it never switches off. In a RAS, if the power fails or the oxygen supply is interrupted, a tank of fish can die within hours.

 

3. Solids Are Removed

 

The first stage of treatment is physical. Mechanical filters, typically rotating drum screens combined with settling devices, capture the solid waste, meaning fish faeces and uneaten feed, before it can break down in the water. Getting this right matters more than it sounds, because organic solids left to decompose consume oxygen and release ammonia, undoing the work of everything downstream.

Solids removal also determines what the farm does with its waste, and this is one of the model's genuinely elegant features. In an ocean net pen, waste simply falls to the seabed, where it can smother benthic life and fuel nutrient pollution. In a RAS it is collected in concentrated form, which turns a pollution problem into a manageable by-product that can be processed into fertiliser, digested for biogas, or otherwise put to use. The waste does not disappear; it becomes something you can handle.

 

4. Water Is Biologically Treated

 

The most critical step is invisible. Fish continuously excrete ammonia through their gills, and ammonia is highly toxic even at low concentrations, so it must be removed constantly. The system does this with a biofilter, a chamber packed with plastic media that provides an enormous surface area for colonies of nitrifying bacteria. Those bacteria oxidise ammonia into nitrite, and a second group converts nitrite into nitrate, which is far less toxic and can accumulate to much higher levels before it causes harm. This two-stage process is called nitrification, and the biofilter is the beating heart of any recirculating system.

Two consequences follow. First, the farm depends on a living bacterial population that takes weeks to establish and can be disrupted by temperature swings, disinfectants, or antibiotics, which is why a RAS cannot simply be switched on and why chemical treatments must be used with great care. Second, nitrate steadily builds up, and it is largely to flush out this accumulated nitrate, along with maintaining mineral balance, that the system exchanges a small percentage of its water each day. The biofilter is what makes recirculation possible, and it is also the component most likely to bring a farm down when it fails.

 

5. Water Is Purified

 

After biological treatment, the water passes through a final conditioning stage. Degassing units strip out the carbon dioxide the fish exhale, which would otherwise acidify the water. Oxygen, often pure oxygen rather than air, is injected to keep dissolved levels high enough to support dense stocking. Ultraviolet light and ozone disinfect the water, killing pathogens and controlling the bacterial load.

This disinfection step delivers the biosecurity that is arguably the strongest argument for the whole approach. A closed, sterilised system can genuinely exclude the diseases and parasites that circulate freely in coastal waters, which in turn reduces or eliminates the need for the chemical and antibiotic treatments used in open farming. The catch is that these processes are also among the largest consumers of power in the facility, alongside pumping and temperature control, and energy is where the economics of RAS become difficult.

 

6. Clean Water Is Reused

 

Having been stripped of solids, detoxified, degassed, oxygenated, and disinfected, the water flows back into the tanks and the cycle begins again. Well-run systems recirculate the great majority of their water, commonly in the range of ninety to ninety-nine percent, with only a small daily exchange to remove nitrate and top up minerals. Compared with a flow-through farm, the reduction in water consumption is dramatic, which is what allows fish farming in water-scarce regions and far from any coast.

It is worth being precise about what this figure means, since it is often misread. The percentage describes how much water is recycled within the loop, not that the facility barely uses any water at all. Large commercial plants still draw and discharge substantial volumes daily for cleaning, quarantine, processing, purging fish of off-flavours before harvest, and emergency tank refills. RAS is extraordinarily water-efficient per kilogram of fish, but a big facility is not a closed box that never touches the outside world.

 

7. Fish Are Harvested Near Markets

 

Because a RAS creates its own environment, it can be built where the customers are rather than where the water is. A facility outside a major city can supply fresh fish to that market within hours, eliminating the air freight that carries salmon from Norway or Chile to consumers on the other side of the world, cutting transport emissions and delivering a fresher product with a longer shelf life.

This proximity is one of the model's more commercially persuasive arguments, since it converts an environmental benefit into a market one, allowing producers to command premium prices for local, fresh, traceable fish. It also fits the wider push for food security and shorter supply chains. The economics are real but partial, though, because savings on freight have to be weighed against the far higher cost of building and powering the facility in the first place.

 

8. Environmental Impacts Are Reduced

 

Set against open-net farming, the environmental case for RAS is genuinely strong on several fronts. Water use per kilogram of fish is minimal. Nutrient discharge is captured rather than released into coastal ecosystems. Disease and parasite risks are contained, cutting chemical use. And because the fish are physically enclosed on land, they cannot escape and interbreed with or transmit disease to wild populations, which is one of the most persistent criticisms of sea-cage salmon farming.

Two honest caveats belong alongside that list. The first is energy. Running pumps, oxygenation, filtration, and above all temperature control consumes a great deal of electricity, and studies of energy use in RAS report figures spanning a very wide range depending on species, climate, and system design, with water circulation alone accounting for up to nearly half of demand in some facilities and temperature control for more than half in others. Whether a RAS farm has a smaller carbon footprint than a net pen therefore depends heavily on how clean its electricity is, which is precisely why Iceland and Norway, with abundant geothermal and hydropower, have become favoured locations.

The second caveat is commercial. Building a land-based farm can cost many times more per unit of capacity than a sea-cage operation, with some estimates putting initial investment as much as an order of magnitude higher and operating costs materially above conventional farming. The past few years have delivered a hard lesson in what that means. Atlantic Sapphire, the most prominent land-based salmon venture in the world, raised close to a billion dollars for its Florida facility and then suffered repeated mass mortality events, system failures, emergency harvests, and mounting losses, culminating in a rescue financing and a take-private at a small fraction of its former value by 2026. It is not alone; a number of large grow-out projects have missed production targets, and financing for the sector has become markedly harder to obtain. Where RAS has succeeded decisively is at smaller scale and in specific niches, above all in hatcheries and smolt production, where nearly the entire salmon industry now relies on recirculating systems to raise juvenile fish before transferring them to sea. The realistic near-term picture, then, is not that land-based farming replaces the ocean, but that it takes over the parts of the cycle where control matters most, while the challenge of growing full-sized fish to harvest on land profitably at scale remains, for now, unresolved.

 

Did You Know?

 

Modern recirculating aquaculture systems can reuse in the region of ninety-five to ninety-nine percent of their water, making them among the most water-efficient methods of commercial fish farming ever devised, and allowing salmon to be raised in a Florida warehouse or a desert facility thousands of kilometres from the sea. The engineering, in other words, is no longer the hard part. Farmers have proven they can build an ocean indoors and keep fish healthy inside it. What the past decade has shown is that the far more stubborn problem is doing so at industrial scale for less than the sea does it for free, and that is the question on which the future of onshore fish farming now turns.

 

Note: This article reflects the state of recirculating aquaculture as of mid-2026, drawing on sources including peer-reviewed aquaculture engineering research, industry reporting, and company disclosures. Figures for water reuse, energy consumption, and capital costs vary widely by species, climate, and system design, and the commercial performance of large land-based projects continues to evolve.