Integrated multi-trophic aquaculture (IMTA) is a method of farming aquatic animals and plants where waste and other byproducts from one species are used as resources for another. Farmers grow animals that need food (like fish and shrimp) together with plants and animals that take in nutrients (like seaweed and shellfish). This creates systems that help the environment, support economic growth, and improve farming practices.

Choosing the right species and managing their numbers helps maintain balance in the ecosystem. This balance supports the health of the organisms and improves the overall environment.

In IMTA, the species grown together should produce useful products. Even if some species produce less than they would alone in the short term, the total amount of products from the system can be higher than in traditional farming methods.

IMTA reduces the harm that aquaculture can have on the environment. For example, farmed Atlantic salmon (Salmo salar) shows how quickly aquaculture demand has grown. Salmon output increased by an average of 25.6% each year from 1980 to 2004, with the economic value of farmed salmon reaching over 4 billion U.S. dollars in 2004. IMTA works by reusing waste and extra nutrients from fish farming as food for shellfish and seaweed. This process reduces water pollution, lowers the need for chemical fertilizers, and improves ecosystem health. By combining species at different levels of the food chain, IMTA supports biodiversity and helps create more sustainable ways to farm marine food.

Terminology and related approaches

"Integrated" describes a method of growing plants and animals together closely, using water to move nutrients and energy between them. "Multi-trophic" means that different species live at different levels in the food chain, such as one species eating plants and another eating the first species.

IMTA is a specific type of an old farming method called aquatic polyculture, where many species are raised together, often without considering their positions in the food chain. In these systems, species may share resources like food, which can sometimes cause problems if they compete for the same resources. However, some older systems, like raising certain fish in China or combining fish farming with land farming, use species that fill different roles in the same environment and can be considered forms of IMTA.

The term "Integrated Aquaculture" is used more generally to describe systems where single-species farming is connected through shared water use. The terms "IMTA" and "Integrated Aquaculture" are similar but differ in how specifically they describe the systems. Other related methods include aquaponics, fractionated aquaculture, integrated agriculture-aquaculture systems, integrated peri-urban-aquaculture systems, and integrated fisheries-aquaculture systems. All of these are types of IMTA.

Range of approaches

Today, simple traditional methods of growing different types of aquatic plants and animals together are more common than modern IMTA systems. These traditional methods often involve fish, seaweed, or shellfish.

Modern IMTA systems can be built on land using ponds or tanks, or in open water such as oceans or lakes. These systems may include combinations like shellfish and shrimp, fish with seaweed and shellfish, fish with seaweed, fish with shrimp, or seaweed with shrimp.

In open water, IMTA can be created using floating platforms with lines where seaweed grows. These platforms are placed near fish nets or cages where fish are raised. In some tropical Asian countries, traditional aquaculture methods include raising fish in floating cages near shrimp ponds and oyster farms, which are sometimes combined with fishing in coastal areas. Since 2010, IMTA has been used in businesses in Norway, Scotland, and Ireland.

In the future, IMTA systems may include new components or similar functions with different sizes of particles. However, many rules and regulations about these systems are still being discussed.

Modern history of land-based systems

Ryther and his team developed a modern system for growing marine life on land, called land mariculture. They created a method to use organisms like shellfish, microalgae, and seaweed to treat household wastewater. This process was studied both through experiments and by developing theories. A mixture of wastewater and seawater provided nutrients for tiny water plants called phytoplankton, which became food for oysters and clams. Other organisms were grown using organic waste from the farm. Seaweed, mainly Gracilaria and Ulva, helped filter nutrients from the final wastewater. The value of the organisms grown from human waste was very low.

In 1976, Huguenin suggested ways to improve the treatment of wastewater from fish farming in both inland and coastal areas. Later, Tenore combined this system with fish that eat meat and abalone that eat seaweed.

In 1977, Hughes-Games described the first practical system for raising marine fish, shellfish, and phytoplankton together. In 1981, Gordin and others studied this further. By 1989, a system in the Gulf of Aqaba (Eilat) on the Red Sea raised seabream and grey mullet in a pond with a density of 1 kilogram of fish per square meter. This system supported large numbers of tiny algae called diatoms, which were good food for oysters. Hundreds of kilograms of fish and oysters were sold from this location. Researchers measured water quality and nutrient levels in ponds with 5 kilograms of fish per square meter. The phytoplankton helped keep the water clean and converted about half of the nitrogen waste into algae. Experiments with shellfish showed they grew quickly. This technology supported a small farm in southern Israel.

Sustainability

IMTA helps protect the environment and supports the economy by turning leftover food and waste from farmed animals into useful crops. This process reduces water pollution and creates more variety in what farmers can produce.

Carefully managed multi-level farming systems help organisms grow faster without harming the environment. This improves the environment's ability to handle the farmed organisms, which lowers harmful effects on nature.

IMTA allows farmers to grow different types of crops and animals by using waste from lower levels of the food chain, often without needing new land. Studies since 1985 show that IMTA can increase profits and reduce financial risks from problems like bad weather, disease, or changes in market prices.

Growing many different species helps keep the economy strong. Farmers can spread their risks by having multiple sources of income. For example, if the price for one type of product drops, farmers can still sell other products like shellfish or seaweed. This reduces the financial problems that happen when only one type of crop or animal is farmed. Also, producing a variety of food and materials helps the system stay strong over time.

Nutrient flow

In Integrated Multi-Trophic Aquaculture (IMTA) systems, carnivorous fish and shrimp are at higher positions in the food chain. These animals release ammonia and phosphorus (ortho phosphate) into the water. Seaweeds and similar plants can take in these nutrients directly from their environment. Fish and shrimp also release organic nutrients that support shellfish and deposit feeders.

Shellfish, which are at intermediate positions in the food chain, have two roles. They filter organic matter from the water and also produce some ammonia. Unused feed can provide extra nutrients through direct consumption or by breaking down into smaller nutrients. In some systems, these nutrients are collected and reused in fish feed. This can involve turning seaweed into food for fish.

IMTA systems help recover nutrients more efficiently. This is 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 the water. Seaweeds absorb carbon dioxide and release oxygen, improving water quality and creating a more stable marine environment.

Nutrient recovery efficiency depends on factors like technology, harvest timing, management, layout, production levels, species chosen, the balance of organisms at different food chain levels, available natural food, particle size, digestibility, season, light, temperature, and water flow. These factors vary by location, so recovery efficiency also varies.

In a test-scale IMTA system with fish, microalgae, bivalves, and seaweed, at least 60% of nutrients became usable products, three times more than in modern net pen farms. For a hypothetical 1-hectare (2.5-acre) system, expected yearly harvests included 35 tonnes of seabream, 100 tonnes of bivalves, and 125 tonnes of seaweed. These results required careful water quality control and attention to bivalve nutrition, as maintaining consistent phytoplankton populations was challenging.

Seaweeds in land-based systems absorb nitrogen at rates ranging from 2% to 100%. However, nutrient absorption rates in open-water IMTA systems are not yet known.

Food safety and quality

Using waste from one species to feed another could cause contamination, but this has not been observed in integrated multi-trophic aquaculture systems. Scientists have checked mussels and kelp growing near Atlantic salmon cages in the Bay of Fundy since 2001 for harmful substances such as medicines, heavy metals, arsenic, PCBs, and pesticides. Levels of these substances are either not found or much lower than allowed by food safety rules in Canada, the United States, and Europe. Taste tests show these mussels do not have a fishy taste or smell and are hard to tell apart from wild mussels. These mussels have more meat because they get more nutrients from the salmon waste. Recent studies show that mussels near salmon farms are better for winter harvest because they keep their meat weight and meat-to-shell ratio high. This is important because in the Bay of Fundy, mussels grown alone (monoculture) usually have lower meat-to-shell ratios in winter. Also, during winter, a type of poison called paralytic shellfish poisoning (PSP) is common, which usually stops mussel harvesting. However, mussels near salmon farms avoid this problem.

Locations

Historic and ongoing research projects include:

Japan, China, South Korea, Thailand, Vietnam, Indonesia, Bangladesh, and others have grown aquatic species together in marine, brackish, and fresh water environments for centuries. Fish, shellfish, and seaweeds have been raised together in bays, lagoons, and ponds. Learning through experience has improved these methods over time. The percentage of Asian aquaculture production that uses IMTA systems is not known.

After the 2004 tsunami, shrimp farmers in Aceh Province, Indonesia, and Ranong Province, Thailand, learned about IMTA. This was important because growing only marine shrimp was widely seen as not sustainable. Production of tilapia, mud crabs, seaweeds, milkfish, and mussels has been added. AquaFish Collaborative Research Support Program

In Bangladesh, Indian carps and stinging catfish are raised, but the methods could be improved. Pond and cage cultures focus only on the fish. They do not use other levels of the food chain, which could increase productivity. Expensive artificial feeds are used to provide protein. Costs could be reduced by raising freshwater snails, such as Viviparus bengalensis, at the same time. This would increase available protein. Organic and inorganic waste from raising fish could also be reduced by adding freshwater snails and aquatic plants, such as water spinach.

An IMTA demonstration from the Pharos project in Gran Canaria studies how waste from commercial aquaculture cage operations can reduce pollution and restore biodiversity. The project surrounds a commercial aquaculture cage with polyculture macroalgae, high-value abalone, 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 includes studying biological and physical processes, production economics, and effects on coastal zone management. Researchers involved include M. Kelly, A. Rodger, L. Cook, S. Dworjanyn, and C. Sanderson.

In Bantry, Ireland, a Pharos project demonstration studies the co-location of polyculture seaweed with a large salmon farm to evaluate environmental benefits. Conducted by Bantry Marine Research Station and the Ultfarms project, the study 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 absorption, and potential reduction 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 grow production to commercial scale. The current system includes 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. 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. It 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 culture. Early research included 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 Aviv, raised marine fish (gilthead seabream), seaweeds (Ulva and Gracilaria), and Japanese abalone. Its method used local climate and recycled fish waste to grow seaweed biomass, which was fed to abalone. It also purified water enough to reuse it in fishponds and meet environmental regulations.

PGP Ltd. is a small farm in southern Israel. It raises marine fish, microalgae, bivalves, and Artemia. Waste from seabream and seabass collects in sedimentation ponds, where dense microalgae—mostly diatoms—grow. Clams, oysters, and sometimes Artemia

Gallery

• Carp, a type of fish called Labeo rohita, are raised in IMTA ponds
• Snails are grown on bamboo pieces placed above the pond bottom in IMTA systems
• Snails are also raised directly on the pond bottom in IMTA systems
• Water spinach and snails are collected from the IMTA ponds
• Shing are raised in cages within IMTA systems