Sustainable farming on water

One third of the Earth's surface is land, half of which is used for agriculture. Climate change is threatening this farmland, including through siltation. According to Reinier Nauta, agriculture can mitigate part of the problem by switching to crops that can withstand salt. "But we need to do more," he argues: "Where the soil is too saline, aquaculture - farming on and in water - can provide a solution."

Nauta is studying how farmers worldwide can use the sea by combining different breeding species in one cultivation system. This concept is called integrated multi-trophic aquaculture, IMTA for short. "In agriculture, we have strip cropping, with different crops next to each other. We do that with aquaculture. We put organisms from different links of the food chain in the water together so that they are connected. By mimicking the natural system in the cultivation system, we aim to increase production as well as improve sustainability."

He is involved in around fifteen projects worldwide. Seaweed usually forms the basis: "I have been working with that for over a decade. We combine it with shrimp, shellfish, sea cucumbers or fish, for example." 
In a Texel polder, Nauta and his colleagues experimented with seaweed, cockles and shrimps in one system. With varying degrees of success: "Shrimp was a big challenge; they escaped and died. There is not yet a healthy business case for seaweed in the Netherlands. But for the cockle, we did see it for breeding in basins on land. For example, in the polders between the dikes on the Wadden Sea coast in northern Groningen, where agriculture is no longer possible due to siltation."

A project in Indonesia was very different. There, aquaculture is often synonymous with large-scale shrimp farming in coastal ponds. “Everything there is focused on growing as many shrimps as possible. The sea is used as a drain, leading to extensive eutrophication (too many nutrients in the water, ed.). In addition, 80 to 90 percent of the mangrove forests along the coast have disappeared to accommodate these ponds. These are not only ecologically important, but also crucial against flooding."

IMTA experimented with seaweed, shrimp and fish in ‘low-input systems'. "We don't add feed, which meant farmers had 60 to 80 percent lower costs. Shrimp production was slightly lower, but multi-species combination kept turnover the same or even higher. And much less pollution ended up in the sea."

It has a huge impact on communities, Nauta observed: "The last experiment covered 20 acres. Farmers saw that it worked and told their colleagues. It therefore spread rapidly. That was really a turning point."

Closing the nutrient cycle, he says, is key in IMTA: "Seaweed forms the base and absorbs nutrients. These attract small organisms, which in turn provide food for larger organisms. In the Java study, these included the shrimp species P. Monodon , the tilapia and the barramundi, a brackish water fish. The natural excess of nutrients in the water served as food."

There was a lot of debate about what is the most suitable seaweed species in Javanese breeding ponds, according to Nauta. There are two species there. Farmers wanted one species because they said it grows faster. Buyers actually want the other species because the quality would be better. In a large laboratory experiment, we found that the growth rate of the two species did not differ much at all. In fact, the species preferred by the buyers because of its quality grew slightly faster. With that knowledge, both groups can move forward."

Nauta believes in IMTA's potential. He adds, however, that its economic feasibility still varies greatly by region as well as by crop. The fact that the seaweed sector is still in its infancy in the Netherlands is partly due to high labour costs, says Nauta. In the Netherlands, labour costs are high. In Indonesia, hourly wages are not yet a factor and these kinds of systems work well. We do see opportunities for cockles because mechanical fishing has been banned in the Wadden Sea while demand remains."

If fishing is further restricted in the Wadden Sea, he says, there will be more space for sustainable aquaculture. It is up to The Hague and Brussels to provide answers to such policy questions. And then it is up to the parties in the chain to work together. Something that does not come naturally: "I see it in the Dutch seaweed sector. Fifteen years ago, a handful of pioneers recognised the economic potential. But we still don't have a healthy seaweed industry in the Netherlands. And that is very much related to the diffidence to share knowledge and cooperate with competitors. Fortunately, I do see this changing, but it is still an issue."

Meanwhile, Nauta sees that in terms of IMTA a country like China is miles ahead in many areas, especially in technical knowledge. Europe needs to catch on quickly, he argues: "With IMTA, we can produce sustainable food in locations that are no longer suitable for agriculture due to siltation. It offers real prospects for a robust food system in which ecology and economics go hand in hand."

• Ministry of Agriculture, Fisheries, Food Security and Nature
• Netherlands Enterprise Agency (RVO)
• Universitas Diponegoro