"Effect of next generation feed sources on aquaculture manure in plant/microbial production, on gut biota and metabolic effect on the fish as food"

The feed used in fish farms has a significant impact on environmental impact, costs and fish growth. But it can also affect how the fish tastes, the nutritional content of the fish meat and how useful the fish's manure is for plant cultivation. Pontus Gunnarsson, Blue Food PhD student at SLU, takes a holistic approach to the feed issue.

In the first part of the project, Pontus Gunnarsson studied rainbow trout fed different diets. The fish were fed either mussels, soldier flies, mealworms, sole or mycelium and were compared with a group of fish raised on soy-based feed. The experiment was carried out at Blue Food partner Aquaculture Center North's research station in Kälarne, Jämtland. In addition to calculating growth, based on initial and final weight, Pontus Gunnarsson took samples of feces and liver, but above all he gutted, filleted and diced the fish.

Tasting in Grythyttan

holding a fish
Photo: School of Restaurant and Hotel Management, Grythyttan.

The fish were then transported to the Grythyttan campus for sensory experiments under the auspices of Örebro University. There, people who have taken courses in sensory science have assessed aspects such as colour, smell, texture and taste according to a test scheme (RATA, "rate-all-that-apply"). The results will be used in a first article in the project.

"They had to taste raw fish, like sashimi, and assess how different flavors such as 'mineral', 'sea' and 'umami' appear", says Pontus Gunnarsson. Only after the test were they told what they had eaten so as not to affect the experience. The taste is crucial, the fish must be good, everything else is secondary. I have tried the fish myself and it tastes excellent.

Gunnarsson also took samples to see how the muscle was affected by the feed and what substances, known as secondary metabolites, were produced in the different groups. When the muscle samples - and also the liver and stool samples - have been analyzed, it will be possible to see if there are any additional benefits from different feeds and if the taste can be linked to, for example, the microbiota.

Alternative feeds and use of manure

In the future, data from the first trial will be compiled in parallel with other experiments. So far, there are indications that sole is a good feed, while the results for soy are worse. There will probably also be a complementary growth trial with rainbow trout, when new feeds will be tested. Among other things, Pontus Gunnarsson would like to investigate whether it is possible, to a greater extent, to feed the fish with, for example, flies and mealworms that have only been dried, without further preparation. If so, this would mean a cost saving.

The next step is to focus on the fertilizer obtained from fish farming. Experiments will be carried out at SLU Alnarp to investigate how fertilizer from different feed states works with different plants and affects their growth and health. Pontus Gunnarsson will also investigate which growth-promoting substances can be found in different fertilizers to see which crops can be suitable. All the options Pontus Gunnarsson is studying should be suitable for organic farming.

Holistic approach to fish farming

Pontus Gunnarsson sees a high ecological potential in his research.

'The whole point of this is based on the import of fertilizer to the Baltic Sea catchment areas,' says Pontus Gunnarsson. 'Instead, we want to reuse the nutrients we have around us. And this can be done, for example, by harvesting mussels in the Baltic Sea and making feed from them.

To get answers to the total effect, the last part of the project will be a summary of the total effect of feed and fertilizer from a life cycle perspective. Here he hopes to collaborate with Blue Food PhD student Kristina Bergman, who works with life cycle analysis at KTH.

Pontus Gunnarsson also sees opportunities for collaboration with Marica Andersson, at the University of Gothenburg, who studies co-cultivation of species.

The project is particularly relevant to Blå Mat's research areas 1. Primary production, 5. Sustainability analysis and 6. Health and welfare.

Pontus Gunnarsson's project is expected to be completed in summer 2026.

Principal supervisor

Johan Dicksved, SLU

Assistant supervisor

Anders Kiessling, SLU
Jean Yong, SLU
Markus Langeland, Rise
Åsa Öström, Örebro University
Wenche Hansen, Matfiskodlarna.

Pontus Gunnarsson's contact details on slu.se