Natural Resources Institute Finland (Luke) is studying best methods to cultivate the common duckweed in aquaculture facilities – and to use it as fish feed, closing the nutrient circle.
Sustainable development of the growing aquaculture industry requires new means to decrease and compensate nutrient emissions. Common duckweed is a diverse plant which sequesters both nitrogen and phosphorus effectively. Thus, it may prove helpful in recycling nutrient in inland fish farming.
Common duckweed or Lemna minor is a small, floating plant which blooms in naturally eutrophic waters like ponds and ditches. Rich in protein, the plant is rather simple to grow and harvest, unlike algae. Thus, it is a perfect plant for cultivation in fish tanks or runoff waters of agriculture.
Traditionally, the Finnish aquaculture industry has been rather dependent on imported nutrients. Most of them come from animal feed; another alternative has been imported plant proteins, particularly soy and sunflower seeds. The country’s self-sufficiency in fodder plants is only 25%, which means that a lot of imported nutrients end up in the environment and our nutrient balance.
“When we recycle the nutrients in common duckweed as fish feed, imported nutrients can be replaced with domestic alternatives,” says Pasi Laajala, project manager of Luke’s Pikkuli project.
Pikkuli is joint project with a Finnish aquaculture company Kalankasvatus Vääräniemi. The aim is to study how the plant really manages in fish tanks. The start was promising, and the cultivation pools were soon full of light green culture. However, the nature gave its surprise earlier than expected.
“We estimated that birds could harm the culture only in the autumn, but a goldeneye family showed up for supper already in early-summer. We learned that some birds can land even in very small ponds to eat the plants. Now we have paid our dues and the cultures have been protected from airstrikes,” Laajala says.
Lemna minor on a plate – or in the fuel tank?
Feed producer Raisioagro is an associate partner and a feed specialist in the project. They are the only Finnish producer of fish feed, accounting for approximately half of the fish feed used in Finland.
”Our partnership with Raisioagro dates back many years. Together we have developed many new, sustainable feed products which are rich in nutrients,“ Laajala explains.
But there could be even more to common duckweed than feed: in addition to protein, it contains good fat acids and carbohydrates, such as beta-glucan, which has positive effects to human health.
“The last use case would be to gasify the plant in a bioreactor or use it in the compost. Then the nutrients could circulate back to the fields, and the need for industrially made fertilisers would diminish,” Laajala points out.
”The side product, biogas, could then be used to heat up buildings or refined as fuel.”