IVL gets to the bottom with the ascidians

IVL, Swedish environmental research institute, global sustainability goals, number 14, life below water

Ciona intestinalis. Can the lowly vase tunicate be the answer to questions like how we can fix over-fertilisation, effective biofuel production, return nutrients to our fields, sustainable protein production for animal feed and circular fish cultivation? Put simply, yes.

IVL, Swedish environmental research institute, Fredrik Norén,

We take a taxi boat out to the farm outside Djupvik on Tjörn in Bohuslän to take a look at these marine environmental heroes. Hundreds of buoys bob above the surface, holding up a full 70 kilometre long cultivation band, which at a few metres in depth is weighed down by ascidians.
Anna-Sara Krång and Fredrik Norén are marine biologists and old classmates. They are both based at the Lovén Centre Kristineberg research station in Fiskebäckskil, where IVL has a small, but growing office – even if Fredrik Norén himself is now more synonymous with Marin Biogas, the company that uses the actual ascidian farm.

"Look here, prime ascidian, healthy and fine and ready for harvest."
He pulls a few clusters from the cold water. Decimetre-long, finger-thick and slippery. For nine months, they have been growing. In a few weeks, the harvest boat will come out and pull up a few tonnes that are immediately run through a press that squeezes out the water before being taken ashore for continued processing.

Fredrik Norén pushes the small organism out of its shell. It looks a little like soft orange candy. Can it be eaten? Absolutely, marine biologists are not afraid of the fruits of the sea.

IVL, Swedish environmental research institute, Anna-Sara Krång

"It tastes like oysters. Full of protein. Anyone want a taste?"

These invertebrates grow naturally at a few metres depth along the entire west coast and have mainly been seen as a problem for mussel cultivation as they attach to the mussel bands. Besides marine biologists and environmental researchers, it has hardly any natural predators. Ascidians do their thing, filter the water of plant plankton and grow.

"It’s a huge amount of biomass with a relatively small input. They require neither feeding, watering or care," explains Anna-Sara Krång, who can't say she enjoys ascidian as a snack.

The ascidian’s soft shell or tunic as its called can be used as a raw material in various kinds of cellulose products, and the more protein-rich insides can be dried for ecologically sustainable and high-value fish or chickenfeed. The one hectare large farm in Djupvik can supply up to 160 tonnes of protein. Or the harvest can of course be converted into biogas corresponding to 65,000 litres of petrol if four times the cultivation bands are laid in the same area. The remains from the process can be converted to fertiliser that thereby returns nutrient salts to our fields.

As part of the deal, ascidians clean the ocean from over-fertilisation compounds – 26 tonnes of nitrogen and 2 tonnes of phosphorous are taken up in a year. With the environmental benefit, the possibility is now being investigated of selling nitrogen certificates, which would provide the operations a lift until profitability picks up.

One of the many on-going projects is also about the development of integrated cultivation systems.

"By combining fish farming with ascidians and macro algae, one can achieve an almost environmental impact-neutral operation since they form a kind of ecocycle between them."

"There is so much added value with the ascidian farm. The possibilities of so-called integrated aquaculture look really promising. By combining fish farming with ascidians and macro algae, one can achieve an almost environmental impact-neutral operation since they form a kind of ecocycle between them," says Anna-Sara Krång.

The interest in for example sustainable salmon farming along the Swedish coast is huge, but in contrast to the Norwegian fjords, the Swedish coastal areas are more sensitive to the effects of over-fertilisation, which traditional fish farming often brings with it. But in an integrated aquaculture, the environmental impact is reduced and it would be possible to bring about fish farming in Swedish waters. An integrated and environmentally sustainable aquaculture also provides economic benefits since at the same cost for equipment and personnel, two or three more products can be made.

The cooperation with other actors in the industry, such as feed producers and fish farmers, and of course municipalities and other researchers is therefore absolutely central. IVL is a very important party, mainly by building up knowledge in life-cycle analyses and supporting the work with industrial applications, according to Fredrik Norén.

"The method we are investigating here incites huge interest and provides both economic and ecological sustainability. Moreover, it could create more jobs in small coastal towns. For the marine industries, this could be a game changer."

For questions, please contact:
Fredrik Norén, fredrik.noren@ivl.se, phone: +46 10-788 67 41
Anna-Sara Krång, anna-sara.krang@ivl.se, phone: +46 10-788 69 12


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  • The vase tunicate – Ciona intestinalis – an ascidian, is an invertebrate that belongs to the tunicates. They grow naturally in salt water and have an extensive capacity to build biomass quickly.
  • According to Marin Biogas, one hectare of ascidian cultivation, equivalent to the one in Djupvik of a total of 4,000 tonnes, can provide 650 MWh per year at the same time that it cleans the sea from 26 tonnes of nitrogen and 2 tonnes of phosphorous.
  • The remnants that come from the biogas facility can be used as ecological fertilisation for agriculture.
  • An increased production up to 1-2 TWh per year would entail a doubling of the Swedish biogas production, according to the Swedish Energy Agency.
  • In addition to pilot cultivation projects in Bohuslän, Marin Biogas has also placed cultivation bands in Skälderviken in Skåne to investigate the cultivation possibilities in more brackish water.

IVL, Swedish environmental research institute, Anna-Sara Krång