Integrated Multi-Trophic Aquaculture - Seaweed and Salmon

Research: Can seaweed farming absorb nutrients from salmon farming? This question was raised by Agnes Mols Mortensen, a researcher at Fiskaaling, in a presentation at the large WEFTA conference held at Hotel Føroyar these days.

Agnes explained the question of whether seaweed grown in a salmon farm has the capacity to absorb nutrients from salmon farming in the Faroese fjords.

We know that seaweed absorbs nutrients. We also know that many nutrients come from active aquaculture productions, but also that many nutrients come from the open sea into Faroese fjords. It is very interesting to know if the seaweed will utilize or absorb the extra nutrients that the farmed fish produce when the seaweed is placed within the aquaculture cages. Based on this, it was investigated whether seaweed grows better in an integrated multi-tropic aquaculture system (IMTA) compared to when seaweed grows away from salmon farming.

The results showed neither better nor worse seaweed growth, when placed in the salmon cages. The conclusion is, therefore, that seaweed, grown in an integrated system together with salmon in Faroese fjords, is unable to absorb the additional nutrients that salmon farming secretes in nature. The reason is that the natural fjord environment is sufficiently supplied with nutrients, so that the seaweed does not need the nutrients that the salmon production secretes.

On the other hand, we can conclude that Faroese fjords have great development potential to carry commercial seaweed production. This means that with seaweed production in the fjords, the nutrients are utilised to produce a biomass with commercial potential, while improving the nutrient budget.

However, seaweed production will only improve the nutrient budget of the fjords if the seaweed is harvested and taken out of the fjord system before it wears and pieces fall off, as well as before the sea snail Lacuna vincta starts eating the seaweed. Lacuna vincta is a small sea snail with an unusually large capacity to consume seaweed. If the planted seaweed is not harvested before the sea snail begins its activities in the summer, the seaweed will end up as a snail's food, and then the seaweed production will add nutrients and carbon to the fjord budget, instead of taking it out.

In connection with the SUREAQUA project, Fiskaaling, Hiddenfjord and TARI-Faroe Seaweed collaborate on the study of seaweed's capacity to absorb salmon farming nutrients in an integrated multi-tropic aquaculture system. Seeded ropes with Saccharina latissima was deployed inside aquaculture cages in Vestmanna and on the Sørvágsfjord. The brown algal species Alaria esculenta and Saccharina latissima were seeded on rope and deployed at a distance of < 500 m from a full­scale salmon farm in Sørvágur.