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Integrated Multi-Trophic Aquaculture (IMTA)

A sustainable economy for people and the environment


Aquaculture, the cultivation of aquatic animals and plants, plays an increasingly important role in feeding the world. However, conventional aquaculture is often associated with major ecological problems. Many of these problems can be solved with the environmentally friendly approach of integrated multi-trophic aquaculture (IMTA). It combines different cultured animals and plants to form a natural cycle.

Explore the topics of the model

Wissenschaftsjahr 2020 – Bioökonomie  ZMT Leibniz-Zentrum für Marine Tropenforschung


This is how integrated aquaculture works

Aquaculture, the cultivation of aquatic animals and plants, plays an increasingly important role in feeding the world. It is the food sector that is growing fastest. However, conventional aquaculture is often associated with major ecological problems, such as pollution of the environment through its effluents, the spread of diseases or the destruction of entire habitats.

Many of these problems can be solved with the environmentally friendly approach of integrated multi-trophic aquaculture (IMTA). It combines different cultured animals and plants to form a natural cycle that makes the best use of feed residues and waste products.

The model shown here was developed at the Leibniz Centre for Tropical Marine Research (ZMT) in Bremen. It illustrates an IMTA in a tropical coastal landscape. Especially in developing countries, this form of aquaculture can provide a livelihood.

Tap on the individual stations in the overview picture, then information, pictures and videos on aquaculture, IMTA and new food from the sea will open up.

Or tap on the button at the end of each chapter… to go to the next chapter, so you can build up your knowledge systematically.

Image on the left: Concept for an integrated aquaculture facility (IMTA) with fish in cage, algae (left), mussels (right) and sea cucumbers on the bottom. The feed waste and excreta from the fish are used as food by the other animals and plants. This reduces the nutrient input into the surrounding ecosystems. (Graphic: Juan Pablo Carlos)

Food sources of the future: what will we be eating?

The seas are emptying…

The seas are emptying…

.. and fertile land is becoming scarce – why aquaculture is becoming increasingly important

The world population is growing. Today 7.4 billion people live on the earth; in 2050 the number is expected to be around 10 billion. To feed this many people, it is important that we all use the seas more wisely.

The maximum capacity of agriculture to feed humanity will be reached in the foreseeable future. Freshwater, arable and pasture land will hardly suffice. Since population growth and average consumption are increasing, the amount of agricultural land would have to almost double by 2050. Mineral fertilisers such as phosphorus are also becoming increasingly scarce.

Products from the sea are therefore playing an increasingly important role in feeding the world’s population, especially with protein. On average, each person consumes roughly 20 kg of fish annually.

Worldwide, 33% of reported commercially exploited stocks of wild fish are considered overfished and 60% are considered maximally exploited.

Aquaculture is therefore becoming more and more important for ensuring food security for people. Seawater aquaculture in particular has the potential to develop independently of the factors limiting agriculture.

How many tonnes of fish do you think Germany imports each year? 100? 1000? Far from it!

On Fridays there’s fish

On Fridays there’s fish

...but where does it actually come from? Here are some numbers

Fish and seafood provide people with high-quality animal protein. Sea fish are also excellent suppliers of iodine. Fatty fish like mackerel, salmon and tuna are important sources of omega-3 fatty acids and vitamin D. In addition, fish and seafood provide a variety of vitamins, minerals and trace elements.

More than 30,000 species of fish are known. About 1000 of them are used by humans as food.

In order to continue to supply the growing world population with sufficient quantities of fish for consumption, total fish production would have to be expanded to around 204 million tonnes in 2030.

The demand for fish and seafood far exceeds the production capacity of natural waters. Therefore, more than half of the fish consumed worldwide now comes from aquaculture.

Globally, 600 species of marine animals are farmed in aquaculture. Fish dominate (about 54 million tonnes per year), followed by molluscs such as mussels and snails (about 17 million tonnes) and crustaceans such as shrimps (about 9 million tonnes). However, algae (approx. 32 million tonnes) also play an important role in aquaculture farming.

The German coasts are hardly suitable for profitable aquaculture, with the exception of mussel cultures. There is a lack of adequately protected sea bays, and there are many conflicts of use, for example with tourism, fisheries, national parks or wind farms.

In 2016, 2.2 million tonnes of fish and seafood were consumed in Germany. Only 300,000 tonnes were caught or bred here. The rest – a total of 1.9 million tonnes – was imported.

Salmon, the most popular food fish in Germany, comes almost exclusively from Norwegian aquaculture. We obtain farmed trout from Eastern Europe. Increasingly, we also import from China and Southeast Asia, especially shrimps and pangasius from aquaculture.

The cultivation of aquatic animals has an ancient tradition in Asia. China is the country with the most aquaculture facilities, followed by India, Vietnam, Indonesia and Bangladesh.

Does this mean that aquaculture should be a way of securing the world’s food supply? But you always hear so many negative things about it…

The environmental polluter

The environmental polluter

How unsustainable aquaculture damages other habitats

Traditional aquaculture often struggles with a number of problems. This is often the case with intensive farming of predatory fish like salmon in Norway, Canada and Chile. Low hygiene and environmental standards exacerbate the problems. This is particularly true in the Asian region, where almost 90% of the world’s aquaculture production takes place.

The growing demand for aquaculture products and strong competitive pressure is leading many fish farmers to intensify farming. This means that production efficiency is increased through monocultures with densely packed animals.

Feed is often used in an untargeted and too massive way. The feed residues and faeces of the animals accumulate in the aquaculture water. The addition of hormones and medicines to the breeding stock puts additional strain on the water.

From open net enclosures in the sea or from ponds near the coast, which release their water unfiltered into coastal waters, the harmful substances and the surplus of organic material are released into the open seawater.

There they lead to algal blooms and oxygen deficiency in the water. This stresses neighbouring ecosystems, such as coral reefs or seagrass beds, which can cause considerable economic and ecological damage.

For example, ZMT researchers investigated the impact of intensive shrimp aquaculture on seagrass beds in Chinese coastal areas. There, the species diversity and growth density of the grasses declined sharply, with small algae overgrowing the grasses and taking away their light.

The sea is also home to countless bacteria. Most of them are harmless to the marine fauna, but this can change if the water quality deteriorates.

Tiny creatures with big effects

Tiny creatures with big effects

What pathogens from aquaculture can infect

Aquaculture is often carried out in open net enclosures or ponds close to the coast, with unfiltered wastewater entering the coastal sea. Intensive farming of fish or shrimps in monocultures can then lead to the rapid spread of pathogens and parasites from infected farmed animals.

Often too much food is fed, which leads to eutrophication of the waters. Pathogens such as bacteria multiply very quickly in water that contains a lot of organic material.

Disease-causing bacteria or viruses can infest the wild forms of the cultured animals or neighbouring aquaculture facilities. Frequently, antibiotics are administered in an uncontrolled manner, which promotes the development of resistant pathogens. As a result, for example, almost the entire shrimp industry in Southeast Asia has collapsed in recent decades.

When aquaculture products are consumed, the pathogens can infect humans. The problem is particularly acute in Asia, from where many industrial nations – including Germany – obtain fish and seafood.

Whether harmful bacteria from breeding enclosures can also reach neighbouring ecosystems, such as coral reefs, and directly damage the corals is still being researched.

However, the nutrient-rich wastewater from the enclosures promotes the growth of algae. Researchers at the ZMT have found that some of the algae species release certain sugars into the water, which boost the reproduction of harmful bacteria in the reef.

Reefs dominated by algae therefore often contain a high number of corals that suffer for example from white pox or white or black band disease.

Another ecosystem also suffers particularly from the methods of conventional aquaculture, but for other reasons.

Once upon a time there was a mangrove forest

Once upon a time there was a mangrove forest

Countless mangrove forests have fallen victim to conventional aquaculture

For the creation of breeding ponds on coasts, other ecosystems are affected or destroyed. Very often this affects mangroves.

Mangroves occur on the coasts of more than 100 countries. They are coastal forests at the transition between land and sea, which tolerate salt water and the strong tidal range. Mangroves are true survival specialists in this extreme habitat and are of great benefit.

They provide protection against coastal erosion and storms, act as filters to trap environmental toxins and sediment from the land before they reach the open sea, bind large quantities of carbon dioxide and provide millions of people with firewood, building materials and food.

Driven by high prices and strong global demand, huge stretches of coastline were reshaped in the 1990s, particularly in Southeast Asia, mainly to create shrimp breeding ponds. Many square kilometres of valuable mangroves were sacrificed for this.

The tropical coasts, where mangroves grow, offer ideal conditions for the facilities: fresh seawater is not far away and can be pumped into the ponds to exchange the water or be washed in through the tides.

Over many kilometres, the ponds in India and Southeast Asia now line up close together along the coast. In 2004, many countries in Southeast Asia felt the consequences: the great tsunami hit the coast with a force that healthy mangrove forests could have slowed down.

The wild catches of fish and shrimps in the region have also declined significantly, because mangroves are the natural nursery for the juvenile stages of many shrimp and fish species.

There are therefore a multitude of problems associated with conventional aquaculture. How can these be brought under control?

IMTA: Underwater flatshare

IMTA: Underwater flatshare

Who gets along with whom in integrated aquaculture?

Many of the negative impacts of conventional aquaculture, which the individual chapters address, can be minimised with the integrated aquaculture (IMTA) approach. Modelled on natural ecosystems, it offers a sustainable alternative.

It combines a variety of very different farmed animals and plants, which complement each other perfectly in terms of nutrition. The surplus feed of fish or shrimps fattened with pellets and their nutrient-rich excreta are used by other farmed organisms that do not need to be fed separately.

For example, mussels and algae filter the food remains and excreta of fish or shrimps out of the water and feed on them. Sea cucumbers and sea worms burrow through the sediment on the seabed and eat the waste that has settled there.

The co-cultured animals and plants can also be marketed, which creates an overall income stability by enhancing product diversity. The potential utilization of co-cultured species as feed for fish and shrimps could create a natural cycle.

As a result, less waste is released into the environment and the feed is used very efficiently. Unlike monocultures, the diversity of farmed organisms in an IMTA can also reduce their susceptibility to disease.

The ZMT is researching which animals and plants would be interesting candidates for an IMTA in order to achieve the best synergy effects. For example, a system with milkfish, salt-tolerant plants and sea cucumbers is currently being tested.

Thus, sea cucumbers could play a prominent role in IMTA. What are these creatures and how do we use them?

The ocean invites you to the table

The ocean invites you to the table

Sea cucumbers and sea worms on the menu

One highly interesting candidate for integrated aquaculture (IMTA) is the sea cucumber. Like starfish and sea urchins, sea cucumbers, of which there are about 14,000 species, are echinoderms. They are found in all seas from the Arctic to the tropics.

Like earthworms on land, sea cucumbers burrow through the sand on the seabed, swallow it, digest the organic material and excrete the grains of sand again. Researchers at the ZMT have found out that a sea cucumber can work through around 10,600 kilos of sediment in an area of 1,000 square metres in one year.

As such “hoovers of the seas”, sea cucumbers prevent too much decayed organic matter from settling in the sea sand and over-fertilising the water.

This is also where their value for IMTA lies: if they are kept on the bottom below fish cages, for example, they use the food leftovers and excreta of the farmed fish that end up in the sediment on the bottom and do not need to be fed themselves.

Sea cucumbers are an important source of protein and trace elements in many tropical countries. In Asia, especially in China, they are traded as expensive superfood and added to soups or stews.

They are also sold there as a cure for high blood pressure, are said to be able to suppress cancer and have an aphrodisiac effect. The extent to which they actually contain health-promoting substances is the subject of current research.

They are easy to collect and can bring in several hundred dollars per animal, which contributes to their enormous overfishing. Every year, 400,000 tonnes of sea cucumbers are caught from the sea and many species are considered to be endangered.

In its experimental aquaria, the ZMT is investigating the conditions under which certain sea cucumber species grow best in order to be able to keep them in an IMTA. This would help to protect them from overfishing.

Like sea cucumbers, the protein-rich sea worms, which also circulate and clean the sand on the seabed, could be suitable for IMTA and consumption.

For people in our latitudes, shellfish like mussels are more appetizing than sea cucumbers or sea worms. For an IMTA mussels have many advantages.

Living water filters

Living water filters

Mussels are also efficient cleaners

Mussels are highly efficient filter-feeders that use their gills to filter organic material from the water for food intake. Their gill arches produce a mucous in which the particles are trapped and transported to the mouth by means of cilia.

A blue mussel (Mytilus edulis) for example, filters about four litres of water per hour, removing the smallest particles such as plankton, i.e. tiny algae and animals, and other suspended matter.

Thus, mussels are suitable candidates for integrated aquaculture (IMTA), because when they filter-feed, they clean the aquaculture water by taking up food residues and excreta of the fish or shrimp and do not need to be fed separately.

However, unlike sea cucumbers, which are active on the sea floor, they filter out the material floating in the water column.

Mussels are rich in nutrients because, like other seafood, they provide high-quality protein and many vitamins and minerals. If pearl oysters are cultivated in an IMTA, the pearls can also be marketed.

However, their way of feeding also has disadvantages: they can also filter environmental toxins, which accumulate in the mussels and are ingested by humans when they eat them.

IMTAs that rely on mussels for water purification should therefore be set up in clean waters and not near river estuaries or sewage from coastal settlements. Many toxins as well as fertilisers that promote the mass reproduction of toxic small algae are often released into the sea there.

In Europe, blue mussels, oysters, clams and scallops are particularly popular. Around 100,000 tonnes of blue mussels are consumed annually.

Algae are also popular flatmates of the IMTA flat-sharing community.

What is green caviar?

What is green caviar?

A special culinary experience that is considered a superfood

Macro-algae are an ideal complement to the other waste processors in an IMTA system. While sea cucumbers feed on organic particles from marine sediment and mussels filter them out of the water column, algae and other aquatic plants use the decomposed, dissolved nutrients to fuel photosynthesis.

As shown in the model, in an IMTA mussels and algae are therefore cultivated on lines hanging in the water in the immediate vicinity of the fish cage, while sea cucumbers stay on the seabed under the cage.

Algae can be used in many different ways. They have a high protein and sugar content and many mineral salts, vitamins and trace elements. Moreover, with their very fast growth rates, algae can take up tremendous amounts of CO2 from the atmosphere.

In East Asia they have been an integral part of the diet since 2500 BC. Nowadays they are boiled, fried, steamed or pickled in vinegar, processed into sushi and salads or added to soups, eaten as a vegetable garnish or dried as chips, serve as a spice or tea.

The broad spectrum of their ingredients makes them interesting not only for the food industry, but also for the pharmaceutical, health and cosmetics industries, as well as for use as fertiliser or biofuel. In the cosmetics sector, algae are in demand, for example, as softeners, skin brighteners, anti-aging and skin protection agents.

At the ZMT, research is being conducted on a type of algae colloquially known as “sea grape” or “green caviar”. It is characterised by its special form, has a high nutritional value, lots of iodine and a high antioxidant potential. The small, round balls, which hang from a panicle, taste slightly salty and burst in the mouth like caviar.

Green caviar comes from the Indo-Pacific, it is in great demand in Southeast Asia, especially in Japan, Vietnam and China. The alga has great potential to contribute to food security. It could also be suitable for rearing in integrated aquaculture (IMTA).

What would happen if the algae were also fed to IMTA fish?

Can predatory fish become vegetarians?

Can predatory fish become vegetarians?

...or at least flexitarians, who eat very little animal protein

We are choosy. When we eat fish, we usually prefer the larger predatory fish, such as salmon, tuna or halibut.

However, predators need much more energy to grow than herbivores. On land we therefore usually breed animals that eat mainly plants, such as beef, pork or chicken.

As there is a great demand for the large predatory fish, these are also mainly bred in aquaculture. However, this is not very sustainable, as their diet consumes many times the amount of smaller wild fish, such as sardines or anchovies, which are thus overfished.

Global aquaculture production is forecast to need around 85 million tonnes of fish feed in 2025. This amount will no longer be covered by stocks of wild prey fish.

Due to declining stocks of small wild fish, the world market price of fishmeal and fish oil has increased four to five times in recent years.

As a result, attempts are now being made to accustom large farmed fish to vegetable proteins from, for example, maize, rape, lupine, soya and other legumes. But micro- and macroalgae will also become increasingly important sources of protein and oil for fish feed in the future.

Thus, the proportion of fishmeal can already be greatly reduced. Some predatory fish, such as salmon and trout, even manage almost without fishmeal.

Ideally, the protein and the oil in fish feed also comes from aquaculture. In the case of integrated aquaculture (IMTA), the organisms bred along with the fish, such as mussels and algae, which utilise the food residues and excreta of the fish, could then serve as fish feed themselves. This would create a sustainable cycle.

Another measure is to breed more herbivorous or omnivorous fish species, such as the freshwater fish tilapia, catfish and carp, instead of predatory fish. It would also make sense if we ourselves ate the small fish from wild stocks, such as sprats, anchovies or sardines, instead of processing them into fish food.

By the way, the idea of an IMTA has ancient roots, a preform of it was already practised in Asia in the Bronze Age.

The family business

The family business

IMTA: A source of income that can provide a livelihood for families in the tropics

Sustainable aquacultures such as integrated aquaculture (IMTA) are not yet very widespread in Europe. However, awareness of the importance of healthy seas for the environment and biodiversity is growing steadily.

Canada, Chile, Israel and South Africa are already working hard on IMTA. In Southeast Asia, mainly a preform of IMTA can be found.

There, co-cultivation has been practiced for hundreds of years, in which the cultured organisms benefit from each other: examples are fish farming in flooded rice fields or poultry and pigs kept above fish ponds.

IMTA is basically a new edition of historical knowledge from Asia, but it uses more modern methods and hygiene standards, has better balanced farming communities and is more environmentally compatible.

For most coastal inhabitants of the tropics, fish, mussels and crustaceans are the main source of animal protein. Trade in fish is also an important economic factor for developing countries, generating net annual revenues of around USD 36 billion.

IMTA is associated with a variety of different tasks: feeding the fish, harvesting mussels and algae, drying fish, sea cucumbers and algae on land, storing and transporting the goods to market.

It offers families or village communities in tropical coastal areas a reliable source of income where everyone has his or her role to play. The economic risk is lower than with monocultures because if one product fails due to “crop damage” or if demand collapses, the others can still be marketed.

The by-products, such as algae, mussels or sea cucumbers, can fetch high prices, but are undemanding and can be cultivated with little investment. They are therefore very profitable.

What can we do to eat the fish on our plates with a clear conscience?

What does all this have to do with me?

What does all this have to do with me?

The seas are getting emptier, aquaculture practices are often harmful to the environment – what can we do about it?

Buy small fish rather than big fish: Consumers prefer large predatory fish like salmon or tuna. As a result, they are being cultivated in aquaculture more and more often, but they are fed small fish. It makes more sense to eat the small fish, such as herring, sardines or anchovies directly, if possible from sustainable fisheries.

Expand the menu: The most sustainable way would be to eat mainly those marine products that are at the bottom of the food chain, such as the nutrient-rich algae and other aquatic plants or filter feeders like mussels. These do not need to be fed at all and even purify the water around them.

Local is the best choice: If possible, buy locally farmed or caught fish that has not been transported long distances. In Germany, mainly freshwater fish such as carp or trout are farmed, so they should be preferred to sea fish. Regarding fish from the sea and depending on current stock status, local species from the North and Baltic Seas should be preferred, such as herring and plaice, as Germany has fairly strict fishing regulations.

Demand the origin of fish products: As far as possible, take into account information on fishing methods or type of aquaculture when purchasing. You can find out which fishing methods and aquaculture production are ecologically sustainable on the Internet, for example on the website of the Food and Agriculture Organisation (FAO).

WWF and Greenpeace provide information in their purchasing guides on which edible fish and seafood can be eaten with a clear conscience.

Suppliers such as Followfish select the products in their range with a focus on sustainability and make it possible to reliably trace their origin.

Labels can be good guides: Labels on the package such as MSC (for catch fish) or ASC (for aquaculture fish) and Naturland stand for sustainable production and can help in the purchasing decision. However, it should be kept in mind that certification is also a business and the standards for evaluation can sometimes vary considerably.

Overview of all topics

The IMTA and traditional aquaculture

  1. Introduction
  2. 1 The seas are emptying…
  3. 2 On Fridays there’s fish
  4. 3 The environmental polluter
  5. 4 Tiny creatures with big effects
  6. 5 Once upon a time there was a mangrove forest
  7. 6 IMTA: Underwater flatshare
  8. 7 The ocean invites you to the table
  9. 8 Living water filters
  10. 9 What is green caviar?
  11. 10 Can predatory fish become vegetarians?
  12. 11 The family business
  13. 12 What does all this have to do with me?