Integrated aquaculture offers potential for salmon farming

Dutch marine biologist Henrice Jansen has been working on the ecological effects and sustainability of aquaculture for the past ten years.

Currently Henrice Jansen works for Wageningen Marine Research, where her research focuses on shellfish cultivation, and in Norway, at the IMR, with salmon culture and development of Integrated Multi Trophic Aquaculture (IMTA).

“My research at IMR is connected to salmon farming, in collaboration with the industry,” explains Jansen.

“At the moment we study the dispersion of fish waste, and we explore the possibilities for re-using the nutrients in salmon waste. Together with salmon company Leroy and environmental organisation Bellona, we are working on the Ocean Forest project. In that project research is put into practice in integrated systems, with for example seaweed and polychaete (marine worm) cultivation around salmon cages. Hereby we investigate the ecological and commercial perspective of full scale IMTA systems.”

Reuse of fish waste
The whole idea behind IMTA is to extract waste nutrients and reuse them to grow additional species. 60 to 70 percent of the nutrients fed to a farmed fish will be returned to the water, via faeces, urine and uneaten feed.

“With an integrated aquaculture system you try to imitate nature, by building an ecosystem that consist of several species. Monoculture is rather unnatural.”

“I am looking at the effect of both inorganic and organic nutrients. Inorganic nutrients are for example ammonia, organic nutrients consist of faeces and uneaten feed pellets.”

The effect of the two different nutrient streams are very different, she explains.

“Inorganic nutrients dissolve in the water, and are available as food for plants, like seaweeds and phytoplankton, through a process called diffusion. Organic nutrients on the other hand consist of small pieces of particulate material, that will sink to the seabed. Organic nutrients can be a food source for sea cucumber, sea urchins and worms. Often shellfish, such as blue mussels, are proposed to filter out the particulate waste material from the water column. However, research shows that the waste material is often too big for them to absorb. So shellfish do not seem to be an efficient option for waste removal within IMTA systems.”

An alternative to shellfish is the recycling of inorganic nutrients through the intervention of phytoplankton.

“Phytoplankton would grow on the inorganic nutrients, and shellfish in their turn will feed on phytoplankton.”

Another important factor in the development of IMTA systems is the spatial scale of the processes, says Jansen.

“Fish faeces and uneaten feed pellets may spread 100 to 500 meters outside the cages. You’d have to farm other species up to that distance, to counter the negative impact.”

But inorganics also have their challenges, she continues: they dissolve very quickly in the water.

Water sampling in Norway. credit: Marit Nederlof

“Where do these nutrients go? So we have to look at the whole picture: do we farm seaweed close to the cages, or do we look at a larger area of water, for example a fjord? In that case you would look at the nutrient balance of the region: how much nutrient needs to be absorbed by seaweeds to remove the waste nutrients produced by salmon culture? There is no agreement yet on how large this should be.”

Current development shows that most companies prefer to farm other species close to their cages, explains Jansen.

“But from an ecological point of view it seems it’s not absolutely necessary to cultivate seaweeds and shellfish right next to the cages in order to realize a mitigating effect.”

The concept of IMTA has its origin in Asia.

“For hundreds of years small fish farmers in Asia have been combining fish with other species to achieve the highest possible return. The country that at this moment applies IMTA most intensively is China,” Jansen explains.

“We also collaborate with scientists from China on this topic, and we can definitely learn from the large scale of operations there: how do you get as much as possible out of your farming system? I recently visited Qingdao; the bays over there are completely filled with all kinds of farming systems, mostly shellfish with seaweed. You’d think it is not possible, but it works!”

IMTA development in Norway
Jansen came across IMTA fifteen year ago, when she was studying in Canada, with professor Chopin.

“It is very interesting to see that after many years of research the concept is now starting to be picked up by the industry.”

IMTA is not yet widely applied in Norwegian salmon farming, says Jansen.

“Only on a small scale and at experimental level. So far, mostly in combination with seaweed production. This trend is similar in other countries, like Canada.”

Seaweed cultivation in Norway. credit: Julia Fossberg

One driver that has spiked the interest in IMTA in Norway is the so called ‘green licenses’, which mean that companies have to be committed to solutions that reduce environmental impacts . “IMTA development could be one of these solutions. Another driver for IMTA seems to be the recent interest in seaweed production in Norway, and Europe. The expansion of a viable seaweed sector, monoculture or within an IMTA setting, largely depends on progress in bio-refinery and processing of seaweeds. In other words: it is essential to increase the price of seaweed products by creating added value, in order to make production in Europe profitable.”

Henrice Jansen believes IMTA has potential for the salmon industry, but there are several issues to overcome.

“The reasons why IMTA is not yet widely adopted are related to technical and socio-economic issues, but the ecological puzzle also remains to be solved. Knowledge about how to cultivate the other species needed in IMTAs is still limited. Also, the salmon biomass produced of the Norwegian salmon industry is large: you would have to farm enormous amounts of extractive species to be able to absorb all the waste that salmon produces. For example; to absorb the waste from growing 1 kilo of salmon, you would need to produce 7-13 kilos of seaweed. For that quantity you require a vast amount of sea space. Will that be accepted in Norway? Social acceptability as such is less of an issue in countries such as China.”

“And there is also the matter of seasonal mismatch: salmon grows fastest and thus produces most waste nutrients in summer when water temperatures are high, while seaweed (kelp) has its major need for nutrients in early spring…”

Fish feed
With the limited availability and increasing prices of fish meal and fish oil, the fish feed industry is continuously searching for new (marine) ingredients. IMTA could play a role in that, says Jansen.

“Both seaweeds and marine worms have been identified as potentially valuable food sources, as seaweeds seem to boost fish health, and some marine worms contain fatty acids that are beneficial for fish growth.”

“Though IMTA systems are not explicitly designed to recycle waste nutrients and transfer them back to the salmon, there might be potential to include part of the biomass of seaweeds, shellfish and worms in fish feed.”

Read also: Cooperation between environmentalists and industry in Ocean Forest project




Related Articles