"Co-cultivation of fed and extractive species in Swedish land-based recirculating aquaculture systems"
The Swedish aquaculture industry is expected to grow significantly in the coming years. This means an opportunity to provide a growing market with sustainably grown and nutritious food from both salt and fresh water. But there are challenges that need to be addressed. Marica Andersson, a Blue Food PhD student at the University of Gothenburg, is investigating how co-cultivation of species can contribute to better water quality, more biomass and increased animal welfare, and which species fit together.
Much of the farmed seafood consumed in Sweden is imported, even though we have good conditions for aquaculture. But this may change as several companies in the Swedish aquaculture sector start new or expand existing operations. One possibility for both existing and new aquaculture facilities is co-cultivation of species, such as fish together with mussels. In her PhD project, Marica Andersson will investigate the effects of combining species in land-based recirculating aquatic systems, or RAS.
'When you grow food fish, you add feed that is converted into biomass,' says Marica Andersson. But despite high efficiency, there is always nutrients that are not absorbed by the fish. The leftover nutrients then remain in the water as so-called POM, particulate organic matter, and dissolved nutrients. By co-culturing the fish with extractive species, such as filtering organisms or algae, these nutrients can also be consumed and converted into other types of biomass. Therefore, co-culture means that a higher proportion of the feed is utilized.
More biomass and better water quality
In the first part of Marica Andersson's project, she studies by far the most commonly farmed food fish in Sweden, the rainbow trout, and how it is affected by being farmed together with the algae sea lettuce, also known as Ulva. The rainbows will not swim around among the algae, but the species are separated in different tanks and only share the water. In this way, it is possible to separate the species and find out what the water quality is like with or without algae. In the future, she will also study species that are in greater demand by consumers, different species of fish and together with different filter feeders such as mussels and oysters.
Another term for co-culture is 'integrated multi-trophic aquaculture' (IMTA). There are several potential benefits to keeping, for example, predatory fish and bivalves in the same culture system. In addition to converting more nutrients into biomass, water quality can be improved in other ways. For example, co-cultivation with sea lettuce can make the water less acidic and provide better visibility (less turbidity), reduce problems with biological growth on the surface of algae, and filtering organisms can also reduce the concentration of bacteria and parasites. In addition, sewage sludge from co-culture systems has the potential to be used in fields and horticulture as it contains growth-promoting substances that benefit plants, in addition to the purely fertilizing effect.
Welfare and chemical signalling
An important part of the project is to study the health and welfare effects of the cultured organisms when they are co-cultured. Marica Andersson will investigate the stress response of the fish by measuring the amount of the stress hormone cortisol, as well as immune parameters and the health and barrier functions of the skin and gastrointestinal tract. She will also study how the immune system and growth of the bivalves are affected, as well as fish survival rates and visible welfare indicators. For algae, she will investigate growth and how sporulation - the ability to form spores - is affected. In general, good welfare means an increase in both quantity and quality cultivated, which is linked to a better resistance of all co-cultured species.
In one of the few experiments conducted in the past, improved growth, survival, immune system and biomass of the fish species Rohu have been observed when co-cultured with a mussel and the plant duckweed. It has also been observed that bivalves co-cultured with fish grow better than justified by the availability of nutrients.
This is probably related to chemical signaling, which I will study later," says Marica Andersson. Most marine organisms do not have eyes but communicate with information-carrying chemicals. One example is that the parasite salmon lice is attracted by a substance that salmonids release. But another substance released by turbot prevents the salmon lice from perceiving the salmon chemical. Theoretically, it would be possible to avoid salmon lice by co-cultivating salmon and turbot.
Cooperation with industry and other researchers
Blue Food partner Smögenlax is heavily involved in Marica Andersson's project. The company will build a land-based RAS facility for salmon at Kungshamn and is also building a research and development facility where Marica Andersson will be able to carry out some of her experiments.
But Marica Andersson also sees opportunities for collaboration with other Blue Food PhD students, including Mar Vall-Llosera Juanola, a Blue Food PhD student at Chalmers, who will be working on product analysis and who could investigate whether the product quality of the co-cultured organisms is affected.
Marica Andersson's project is expected to be completed in spring 2026.
Kristina Snuttan Sundell, University of Gothenburg
Henrik Pavia, University of Gothenburg
Torbjörn Johansson, IVL
Claes Lundberg, Smögenlax.