Integrated multi-trophic aquaculture (IMTA) is a farming method where waste and other byproducts from one aquatic species are used as resources, such as food or fertilizer, for another species. Farmers combine farming of animals like fish and shrimp with farming of seaweed and shellfish to create balanced systems that help protect the environment, improve economic results, and support better farming practices.

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

In ideal conditions, the species grown together all produce valuable products. IMTA can increase the total amount of products compared to farming only one species, even if some products are less productive in the short term.

IMTA also helps reduce the negative effects of aquaculture on the environment. For example, farmed Atlantic salmon (Salmo salar) shows how quickly aquaculture demand has grown. From 1980 to 2004, the amount of farmed salmon increased by an average of 25.6% each year, and its economic value reached over 4 billion U.S. dollars in 2004. IMTA works by creating a system where waste and extra nutrients from fish farming are used by other species, like shellfish and seaweed. This process reduces water pollution, lowers the need for chemical fertilizers, and improves ecosystem health. By combining different levels of the food chain, IMTA supports greater biodiversity and more sustainable ways to produce food in the ocean.

Terminology and related approaches

"Integrated" means growing different plants and animals together in a way that helps each other grow, using water to move nutrients and energy between them. "Multi-trophic" describes how different species live at different levels 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, where many species are grown together, often without considering their positions in the food chain. In these systems, species might share some resources, but this can sometimes cause problems if they compete for the same food. However, some traditional systems, like raising certain fish in China or combining fish farming with land farming, can be examples of IMTA.

The term "Integrated Aquaculture" describes a system where single-species farming is connected through water movement between different parts of the system. The terms "IMTA" and "Integrated Aquaculture" are similar but differ in how specific they are. Other examples of IMTA include aquaponics, partial aquaculture, systems that combine farming on land and in water, systems near cities, and systems that mix fishing with aquaculture.

Range of approaches

Today, traditional or incidental aquaculture that involves multiple species, such as fish, seaweed, or shellfish, is more common than modern Integrated Multi-Trophic Aquaculture (IMTA). These traditional systems are often simple in design.

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

In open water, IMTA can be set up using floating devices with lines where seaweed grows. These devices are placed near fishnets or cages where fish are raised. In some tropical Asian countries, traditional methods like raising finfish in floating cages near fish and shrimp ponds, and oyster farming combined with fishing in estuaries, are considered forms of IMTA. Since 2010, IMTA has been used commercially in Norway, Scotland, and Ireland.

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

Modern history of land-based systems

Ryther and his colleagues developed modern systems for raising sea life on land. They created methods to use shellfish, seaweed, and tiny algae to treat wastewater from homes. This process used wastewater mixed with seawater to feed phytoplankton, which became food for oysters and clams. Other sea creatures were grown using organic waste from the farm. Seaweed, like Gracilaria and Ulva, helped filter nutrients from the final wastewater. The value of the sea life grown from human waste was very low.

In 1976, Huguenin suggested ways to improve wastewater treatment for fish farming in both inland and coastal areas. Tenore later combined this with systems that included carnivorous fish and abalone, which eat seaweed.

In 1977, Hughes-Games described the first practical system for raising marine fish, shellfish, and phytoplankton together. Gordin and others followed in 1981. By 1989, a system in the Red Sea near Eilat raised seabream and grey mullet in ponds with high fish density. This system supported large numbers of diatoms, which helped feed oysters. Hundreds of kilograms of fish and oysters were sold from this farm. Researchers measured water quality and nutrient levels in ponds with 5 kilograms of fish per square meter. Phytoplankton helped keep water clean and converted about half of the waste nitrogen 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 support the economy by turning leftover food and waste from animals into crops that can be harvested. This process reduces water pollution and helps farmers earn income from different sources.

When managed correctly, raising multiple types of aquatic plants and animals together can help them grow faster without harming the environment. This method allows the area to handle more life without causing harm to nature.

IMTA lets farmers produce more types of products by using waste from lower-level species instead of buying new materials. This often doesn’t require new land. Studies since 1985 show that IMTA can increase profits and lower risks from problems like bad weather, disease, or changes in the market.

Growing many different species helps keep the business financially stable. If the price for one type of product drops, farmers can still sell other products, like shellfish or seaweed. This reduces the risk of losing money that happens when only one type of crop is grown. Producing both 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 soluble ammonia and phosphorus (ortho phosphate) into the water. Seaweeds and similar plants can take these inorganic nutrients directly from the environment. Fish and shrimp also release organic nutrients that help feed shellfish and organisms that eat particles from the bottom of the water.

Shellfish, which are often in the middle of the food chain, have two roles. They filter organic organisms from the water and also produce some ammonia. Leftover feed can add more nutrients, either through direct consumption or by breaking down into individual nutrients. In some cases, waste nutrients are collected and reused in the food given to farmed fish. This can happen by turning grown seaweed into food.

IMTA systems can greatly improve how efficiently nutrients are recovered. This is especially important in areas where traditional farming methods cause nutrient imbalances, leading to problems like excessive algae growth (eutrophication) and areas with little to no oxygen (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.

How well nutrients are recovered depends on factors like technology, harvesting schedules, management practices, how the system is arranged in space, the amount of production, the types of species used, the balance of organisms at different food chain levels, the availability of natural food, particle size, how easily nutrients are digested, the season, light, temperature, and water flow. These factors vary by location, so recovery efficiency also changes.

In a hypothetical small-scale farm that includes fish, microalgae, bivalves, and seaweed, based on pilot data, at least 60% of the nutrients added to the system became commercial products. This is nearly three times more efficient than in modern net pen farms. For a hypothetical 1-hectare (2.5-acre) system, the expected yearly output would be 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 control of water quality and attention to feeding bivalves properly, because maintaining consistent phytoplankton populations was difficult.

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

Food safety and quality

Feeding waste from one species to another could cause contamination, but this has not been seen in IMTA systems. Mussels and kelp growing near Atlantic salmon cages in the Bay of Fundy have been studied since 2001 for contamination by medicines, heavy metals, arsenic, PCBs, and pesticides. Levels of these substances are either not found or much lower than limits set by the Canadian Food Inspection Agency, the United States Food and Drug Administration, and European Community Directives. Taste tests show these mussels do not have a fishy taste or smell and are similar to wild mussels. These mussels produce more meat because nutrients are more available. Recent studies show mussels grown 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 in winter have lower meat-to-shell ratios, and paralytic shellfish poisoning (PSP) usually limits mussel harvesting to winter months.

Locations

Historic and ongoing research projects include:

Japan, China, South Korea, Thailand, Vietnam, Indonesia, Bangladesh, and other countries have raised fish, shellfish, and seaweed together in marine, brackish, and fresh water environments for many years. These species are often grown in bays, lagoons, and ponds. Over time, people have learned better ways to combine these species through trial and error. The percentage of Asian aquaculture production that uses integrated multi-trophic aquaculture (IMTA) systems is not known.

After the 2004 tsunami, shrimp farmers in Aceh Province, Indonesia, and Ranong Province, Thailand, were trained in IMTA. This was important because raising only shrimp in large numbers was found to be unsustainable. Farmers now also grow tilapia, mud crabs, seaweed, milkfish, and mussels. This work is supported by the AquaFish Collaborative Research Support Program.

In Bangladesh, Indian carps and stinging catfish are raised in ponds and cages. However, these methods could be more productive. Current practices focus only on fish and do not use other parts of the food chain to improve efficiency. Expensive artificial feeds are used to provide protein for the fish. Costs could be reduced by raising freshwater snails, like Viviparus bengalensis, alongside fish. These snails could increase available protein. Waste from fish farming could also be reduced by adding freshwater snails and aquatic plants, such as water spinach.

A demonstration project in Gran Canaria, Spain, called Pharos, studies how waste from commercial aquaculture can reduce pollution and help restore biodiversity. The project surrounds a fish cage with seaweed, abalone, sea cucumbers, and artificial reefs. Pharos 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) started the first seaweed farm in the country. The farm is named "De Wierderij" and is used for research.

The Scottish Association for Marine Science in Oban is researching ways to raise salmon, oysters, sea urchins, and seaweed together. Studies focus on how living things and natural forces work, as well as how to manage costs and protect coastal areas. Researchers include M. Kelly, A. Rodger, L. Cook, S. Dworjanyn, and C. Sanderson.

In Bantry, Ireland, a Pharos project compares a seaweed farm near a salmon farm with a control site in Roaringwater Bay. The seaweed farm grows Alaria esculenta, Saccharina latissima, and Laminaria digitata and is designed to produce large amounts of seaweed. The study examines changes in biodiversity, carbon dioxide absorption, and how seaweed might help reduce ocean acidification.

Industry, universities, and governments are working together to expand IMTA systems for commercial use. Current systems include Atlantic salmon, blue mussels, and kelp. Researchers are also studying deposit feeders. AquaNet, a Canadian research network, funded the first phase of the project. The Atlantic Canada Opportunities Agency is funding the second phase. Thierry Chopin of the University of New Brunswick and Shawn Robinson of the Department of Fisheries and Oceans are leading the project.

In British Columbia, the Pacific SEA-lab project studied raising sablefish, scallops, oysters, blue mussels, urchins, and kelp together. "SEA" stands for Sustainable Ecological Aquaculture. The project aimed to balance these species. Stephen Cross led the project under a British Columbia Innovation Award at the University of Victoria’s Coastal Aquaculture Research & Training network.

In Chile, the i-mar Research Center at the Universidad de Los Lagos in Puerto Montt is working to reduce the environmental impact of raising large numbers of salmon. Early research included trout, oysters, and seaweed. Current studies focus on raising salmon, seaweed, and abalone in open water. Alejandro Buschmann is leading the project.

SeaOr Marine Enterprises Ltd. in Israel raised gilthead seabream, seaweed (Ulva and Gracilaria), and Japanese abalone. The company used fish waste to grow seaweed, which was then fed to abalone. Water was cleaned enough to be reused in fishponds and to meet environmental regulations.

PGP Ltd. in southern Israel raises marine fish, microalgae, bivalves, and Artemia. Waste from seabream and seabass is collected in ponds, where microalgae grow. Clams, oysters, and Artemia filter the algae from the water, producing clean water. The farm sells fish, bivalves, and Artemia.

Three farms in land-based tanks grow seaweed using water from abalone farming. Up to 50% of the water is passed through seaweed tanks. These farms do not raise fish or shrimp as the top species in the food chain. The goal is to avoid overharvesting natural seaweed beds and prevent red tides, rather than reducing nutrients. These projects were developed through research by Irvine and Johnson Cape Abalone and scientists from the University of Cape Town and Stockholm University.

Gallery

• Carp (Labeo rohita) is grown in an IMTA pond.
• Snails are raised on bamboo pieces in the IMTA system.
• Snails are also grown on the bottom of the IMTA pond.
• Water spinach and snails are collected from the IMTA pond.
• Shrimp are raised in cages within the IMTA system.