Integrated multi-trophic aquaculture (IMTA) is a farming method that uses waste, such as leftover food or animal waste from one type of water creature, as resources for another. Farmers grow animals that need to be fed (like fish or shrimp) together with plants (like seaweed) and animals that filter water (like shellfish) to create systems that help the environment, improve farming results, and make farming practices more accepted by communities.

Choosing the right types of animals and plants, and controlling how many of each are grown, helps create a balance in the ecosystem. This balance supports the natural processes that keep the environment healthy and helps all the living things in the system thrive.

In ideal situations, all the animals and plants grown together produce useful products that can be sold. Even if some of these products do not produce as much as they would in a system with only one type of crop, the overall system can still produce more total food than a single-crop system.

IMTA also helps reduce the harm that farming can cause to the environment. For example, farmed Atlantic salmon (Salmo salar) has shown how quickly aquaculture demand has grown. From 1980 to 2004, the amount of salmon farmed increased by an average of 25.6% each year, and the value of farmed salmon reached over 4 billion U.S. dollars in 2004. IMTA works by reusing waste and extra nutrients from fish farming, such as uneaten food and organic waste, to feed other species like shellfish and seaweed. This process reduces water pollution, lowers the need for chemical fertilizers, and improves the health of the environment. By combining different levels of the food chain, IMTA also helps protect more types of plants and animals and supports more sustainable ways to grow food in the ocean.

Terminology and related approaches

"Integrated" describes growing plants and animals together in a way that helps them all grow better by sharing water, nutrients, and energy. "Multi-trophic" means that different species live at different positions in the food chain, such as one eating plants, another eating the first, and so on.

IMTA is a specific type of an old farming method called aquatic polyculture, which involves raising many different water species together. In older systems, species were often raised without considering their roles in the food chain. This could lead to problems, like competition for the same food, which might reduce the growth of both species. However, some older systems, like raising certain fish in China or combining fish with land farming, can be seen as early examples of IMTA.

The term "Integrated Aquaculture" is a more general way to describe combining single-species farming systems by moving water between them. The terms "IMTA" and "Integrated Aquaculture" are similar but differ in how specific they are. Other related methods, such as aquaponics, partially separated farming, systems that mix farming and fish farming, systems that combine farming with urban areas, and systems that mix fishing with fish farming, are all types of IMTA.

Range of approaches

Today, simple traditional aquaculture methods that involve multiple types of aquatic plants and animals are more common than modern integrated multi-trophic aquaculture (IMTA). These systems often include fish, seaweed, or shellfish.

True IMTA can be built on land using ponds or tanks, or in open water such as marine or freshwater environments. Examples include combinations like shellfish and shrimp, fish with seaweed and shellfish, fish with seaweed, fish with shrimp, and seaweed with shrimp.

In open water, IMTA can use floating buoys with lines where seaweed grows. These buoys and lines are placed near fishnets or cages where fish are raised. In some tropical Asian countries, traditional aquaculture methods that combine floating fish cages, nearby fish and shrimp ponds, and oyster farming along with capture fishing in estuaries can be considered a type of IMTA. Since 2010, IMTA has been used commercially in Norway, Scotland, and Ireland.

In the future, systems with additional components or similar functions but different particle sizes may be developed. Many regulatory challenges remain unresolved.

Modern history of land-based systems

Ryther and his colleagues developed modern, combined, large-scale, land-based seawater farming. They created both theories and experiments showing how certain organisms—shellfish, tiny algae, and seaweed—can be used together to treat household wastewater. A mix of wastewater and seawater provided nutrients for phytoplankton, which became food for oysters and clams. They also grew other organisms using the farm’s organic waste. Seaweed, mainly Gracilaria and Ulva, filtered nutrients from the final wastewater. The value of the organisms grown using human waste was very low.

In 1976, Huguenin suggested ways to improve wastewater treatment for fish farming in inland and coastal areas. Tenore later added carnivorous fish and abalone, which eat seaweed, to his system.

In 1977, Hughes-Games described the first practical system for raising marine fish, shellfish, and phytoplankton. Gordin and others did similar work in 1981. By 1989, a system in the Gulf of Aqaba (Eilat) on the Red Sea raised seabream and grey mullet in a semi-intensive pond (1 kg of fish per square meter). This system supported large numbers of diatoms, which helped feed oysters. Hundreds of kilograms of fish and oysters were sold there. Researchers also measured water quality and nutrient levels in ponds with 5 kg of fish per square meter. Phytoplankton usually kept water quality good and converted more than half of the waste nitrogen into algae. Experiments with large-scale bivalve farming showed high growth rates. This technology supported a small farm in southern Israel.

Sustainability

IMTA helps protect the environment and support the economy by turning leftover food and waste from farmed animals into crops that can be harvested. This process reduces water pollution and creates more ways for people to earn money.

When multi-trophic aquaculture is managed properly, it allows more animals to grow in the same area without harming the environment. This helps the area handle more life without causing damage.

IMTA helps farmers produce more types of food by using waste from lower-level sea life instead of buying new supplies. This often does not require new land or water areas. Studies since 1985 show that IMTA can increase profits and lower financial risks caused by bad weather, sickness, or changes in the market.

IMTA is more economically stable because it grows many different species. Farmers can spread their financial risks and have more ways to earn money. For example, if the market for one type of sea life drops, farmers can still sell other products like shellfish or seaweed. This reduces the financial problems that happen when only one type of crop is grown. Growing many kinds of food and materials also helps the system stay strong over time.

Nutrient flow

In Integrated Multi-Trophic Aquaculture (IMTA) systems, carnivorous fish and shrimp are usually at the highest levels of the food chain. These animals release ammonia and phosphorus (ortho phosphate) into the water as waste. Seaweeds and similar plants can take in these nutrients directly from the environment. Fish and shrimp also produce organic nutrients that support shellfish and organisms that eat from the seafloor.

Shellfish, which are at middle levels of the food chain, often do two things: they filter organic matter from the water and release some ammonia. Uneaten feed can also add nutrients, either through direct consumption or by breaking down into smaller nutrients. In some IMTA projects, these nutrients are collected and reused in the food given to farmed fish. This can include processing seaweed into food for fish.

IMTA systems improve how efficiently nutrients are recovered. This is especially important in areas where traditional farming methods cause nutrient imbalances, leading to problems like eutrophication and dead zones. Shellfish, such as mussels and oysters, remove excess nitrogen and phosphorus from water. Seaweeds absorb carbon dioxide and release oxygen, improving water quality and creating a more stable marine environment.

The efficiency of nutrient recovery depends on factors like technology, harvest timing, management practices, how systems are arranged, production levels, species chosen, the balance of organisms at different food chain levels, natural food availability, particle size, how easily nutrients are digested, season, light, temperature, and water flow. These factors vary by location, so recovery efficiency also changes.

In a hypothetical small-scale farm that combines fish, microalgae, bivalves, and seaweed, based on data from small tests, at least 60% of nutrients used became commercial products. This is about three times more than in modern net pen farms. For a hypothetical 1-hectare (2.5-acre) system, yearly production might include 35 tonnes (34 long tons; 39 short tons) of seabream, 100 tonnes (98 long tons; 110 short tons) of bivalves, and 125 tonnes (123 long tons; 138 short tons) of seaweed. These results required careful water quality control and attention to bivalve nutrition, as maintaining consistent phytoplankton populations was difficult.

In systems on land, seaweeds can absorb 2% to 100% of nitrogen. However, how efficiently seaweeds take in nitrogen in open-water IMTA systems is not yet known.

Food safety and quality

Putting waste from one animal into another's food could cause contamination, but this hasn't been seen in IMTA systems. Since 2001, mussels and kelp near Atlantic salmon cages in the Bay of Fundy have been tested for contamination from medicines, heavy metals, arsenic, PCBs, and pesticides. Levels of these substances are either not found or much lower than allowed by regulations from agencies like the Canadian Food Inspection Agency, the U.S. Food and Drug Administration, and European Community Directives. Taste tests show these mussels don't have a fishy taste or smell and are hard to tell apart from wild mussels. These mussels have more meat because there are more nutrients available. Recent studies show mussels near salmon farms are better for winter harvest because they keep high meat weight and a good meat-to-shell ratio. This is important because in the Bay of Fundy, mussels grown alone (monoculture) usually have low meat-to-shell ratios in winter, and paralytic shellfish poisoning (PSP) often limits harvesting to winter.

Locations

Historic and ongoing research projects include:

Japan, China, South Korea, Thailand, Vietnam, Indonesia, Bangladesh, and other countries have raised aquatic species together in marine, brackish, and fresh water environments for many years. Fish, shellfish, and seaweeds have been grown together in bays, lagoons, and ponds. Over time, people have learned better ways to combine these species. It is unknown how much of Asian aquaculture production happens in integrated multi-trophic aquaculture (IMTA) systems.

After the 2004 tsunami, many shrimp farmers in Aceh Province, Indonesia, and Ranong Province, Thailand, were trained in IMTA. This was important because raising only marine shrimp was not sustainable. Farmers now also grow tilapia, mud crabs, seaweeds, milkfish, and mussels. AquaFish Collaborative Research Support Program

In Bangladesh, Indian carps and stinging catfish are raised, but the methods could be more productive. Current pond and cage cultures focus only on fish. These methods do not use other species to improve productivity. Expensive artificial feeds are used to provide protein for the fish. Costs could be reduced if freshwater snails, such as Viviparus bengalensis, were raised alongside the fish. This would increase available protein. Organic and inorganic waste from fish farming could also be reduced by raising freshwater snails and aquatic plants, such as water spinach.

An IMTA demonstration from the Pharos project in Gran Canaria studies how pollution reduction and biodiversity restoration happen when nutrient-rich waste from commercial aquaculture is used. The project surrounds a commercial aquaculture cage with polyculture macroalgae, high-value abalone lines, sea cucumber cages, and artificial reefs. The Pharos project is funded by the European Union under Horizon Europe and led by Dr. Gordon Dalton.

In the Netherlands, Willem Brandenburg of UR Wageningen (Plant Sciences Group) created the first seaweed farm in the country. The farm is called "De Wierderij" and is used for research.

The Scottish Association for Marine Science in Oban is developing co-cultures of salmon, oysters, sea urchins, and brown and red seaweeds through several projects. Research focuses on biological and physical processes, production economics, and effects on coastal zone management. Researchers include: M. Kelly, A. Rodger, L. Cook, S. Dworjanyn, and C. Sanderson.

A Pharos project demonstration in Bantry, Ireland, studies the co-location of polyculture seaweed with a large salmon farm to assess environmental benefits. The study, conducted by Bantry Marine Research Station and the Ultfarms project, compares two sites: a 6-hectare Bantry farm growing Alaria esculenta, Saccharina latissima, and Laminaria digitata near a salmon farm, and a control site in Roaringwater Bay. The demonstration examines differences in biodiversity, CO₂ sequestration, and potential buffering of ocean acidification. The Bantry site has 17 lines of 110 meters each designed for high biomass production.

Industry, academia, and government are working together to expand production to commercial scale. The current system combines Atlantic salmon, blue mussels, and kelp. Deposit feeders are being considered. AquaNet (one of Canada’s Networks of Centres of Excellence) funded phase one. The Atlantic Canada Opportunities Agency is funding phase two. The project leaders are Thierry Chopin (University of New Brunswick in Saint John) and Shawn Robinson (Department of Fisheries and Oceans, St. Andrews Biological Station).

Pacific SEA-lab researched and was licensed to co-culture sablefish, scallops, oysters, blue mussels, urchins, and kelp. "SEA" stands for Sustainable Ecological Aquaculture. The project aimed to balance four species. The project was led by Stephen Cross under a British Columbia Innovation Award at the University of Victoria Coastal Aquaculture Research & Training (CART) network.

The i-mar Research Center at the Universidad de Los Lagos in Puerto Montt is working to reduce the environmental impact of intensive salmon farming. Early research involved trout, oysters, and seaweeds. Current research focuses on open waters with salmon, seaweeds, and abalone. The project leader is Alejandro Buschmann.

SeaOr Marine Enterprises Ltd., which operated on the Israeli Mediterranean coast north of Tel

Gallery

• Carp (Labeo rohita) is grown in an IMTA pond
• Off-bottom snails are raised on bamboo pieces in an IMTA system
• Snails are grown on the pond bottom in the IMTA system
• Water spinach and snails are harvested from the IMTA pond
• Shing is raised in cages within the IMTA system