Combining fish and vegetable growth produces food in a uniquely sustainable way. Researchers want to enable aquaponics to make a breakthrough.

The biggest freshwater fish in the world: a fully grown Pirarucu measures up to two and a half metres and weighs about 250 kilograms. The specimens swimming in the holding tanks at the Leibniz-Institut für Gewässerökologie und Binnenfischerei (IGB) (Leibniz Institute for Freshwater Ecology and Inland Fisheries, IGB) in Friedrichshagen in Berlin are not yet fully grown – but they are already impressively large.

"They're fascinating fish – fast-growing, robust, and stress resistant," says Werner Kloas, Professor of Endocrinology at the Humboldt University, Berlin and scientific Department Head at the IGB. "They also taste great," he adds, as this is not irrelevant to his research. Werner Kloas and his team are investigating the Amazonian fish, also known as Giant Arapaima, because it is perfectly suited to aquaponics – a specific form of food production. The concept combines fish and vegetable farming in such a way that the leafy or vegetable plants can use the fish excrement as fertiliser. DThis can reduce chemical fertiliser use by approximately 75 percent (research article).

In 2007, long before the Giant Arapaimas arrived, aquaponics research had already begun at the IGB with tilapia and tomatoes. Tilapia – a member of the Cichlid family – have been considered one of the most important fish species in aquaculture for decades. They can tolerate the warmth of the greenhouses, they breed easily, they are robust, and they are a popular culinary fish all over the world. The scientists also chose the tomato variety deliberately. "If I can make a tomato happy, I've done my job," say Werner Kloas with a wink, referring to the particular requirements of that crop and its demand on the supply of nutrients. "Then it will also work with any other plant."

How successful the researchers were with their work can be seen today at the institute's aquaponics facility: an abundance of fruit hangs on the tomato vines, the tendrils reach all the way up to the ceiling of the greenhouse. In the round basins, schools of tilapia are swimming. To optimally supply the tomato plants, the aquarium water, enriched with nutrients and CO2, passes through various filters and treatment stations before a perfectly measured amount is directed to the roots of the plants, where nutrients and water are absorbed. The water evaporates through the stomata in the leaves, is collected again in condensation traps in the ceiling and pumped back into the aquariums. The system has been well-established here for several years and it is known as 'Tomatofish'. By now, there are even companies that professionally operate an aquaponics system based on this model and use it to produce food for consumers - and yet the principle of aquaponics still only has niche status.

The researchers are hoping to change this and are therefore taking their work to the next phase. Aquaponics needs to finally make a breakthrough ( report from the IGB))and provide more people than ever with sustainably produced fish and vegetables. "Soon almost 80 percent of the global population will live in cities," explains Werner Kloas. Especially in urban areas, aquaponics facilities could produce food very efficiently in a limited space. "We have big problems in agriculture," says Kloas. He is convinced that aquaponics could solve many of these problems. After all, food production could hardly be more efficient in terms of water and resources.

To boost production, the principle of aquaponics would need to become more widely known, and would also need to be more closely examined. How big the facilities would have to be in order to be economically profitable, which legal requirements promote or inhibit aquaponics in various countries and how aquaponics can be integrated into urban planning to ensure the necessary infrastructure are questions being investigated by Berlin researchers, together with research partners from the USA, the Netherlands, Brazil, Norway and Sweden. It is also a matter of the local and cultural characteristics of the respective countries. In Germany, for example, aquaculture - and therefore aquaponics - is not considered agriculture. Anyone wishing to operate such a facility has to go through a licensing procedure for a commercial enterprise - an obstacle that Werner Kloas finds annoying and unnecessary. A facility that is economically viable in Brazil may have to have different dimensions to meet the same goals in North America. In addition, very different types of fish and vegetables end up on the plates of Europeans and South Americans.

For this reason, researchers are trialling the use of other fish and vegetables in their system. The Giant Arapaimas, for instance, are already lined up and ready to go. Werner Kloas has high hopes for this fish in particular. "It's wonderfully good at making use of available food, and can reach up to 15 kilograms in the first year." It will not endanger the natural ecosystem, in case it should ever escape the facility: at temperatures under 23 degrees Celsius, the fish dies. But there is still one catch: breeding the fish in captivity has not yet worked out – a basic requirement for a successful aquaponics facility. "This is a question that we will be looking into in the future," says Werner Kloas.

• Chances and Challenges of Sustainable Aquaculture in Germany – a Report by the IGB
• Research Article on Saving Inorganic Fertiliser