Can seaweeds be part of the diet for salmonids?
Whether it’s social media influencers preaching about nutrition or celebrity chefs trying to excite us with new ingredients, there always seems to be a lot of talk about what’s in our food.
And it seems the discussion around feed ingredients in the aquaculture industry is not too different. Having just published a comprehensive review of over 300 studies on the use of seaweed in finfish diets (“The use of macroalgae in feeds for finfish aquaculture”, Frontiers in Aquaculture, June 2025) we thought it might be interesting to look specifically at what the benefits and limitations might be for the salmon and trout industry.
This is becoming increasingly relevant as the aquaculture industry faces increasing pressure to identify not only sustainable and cost-effective alternatives to traditional feed ingredients like fishmeal (FM) and fish oil (FO), but also searches for feed materials that can supply additional benefits including physiological functions such as immunostimulatory effects, physical properties like binders, or enhance flesh quality like pigments.
The idea of using seaweed and their derivatives in salmonid feeds is not particularly new, with studies dating back to the late 1980s. However, with the increased interest in seaweed culture in Scotland and the search for more circular feed ingredients, there is a resurgence of interest in using seaweed and seaweed-derived products in feeds for salmon and trout.
Nutritional contributions and performance enhancement
Seaweeds contain a diverse range of nutritionally beneficial compounds, including proteins, lipids (notably the key omega-3 fatty acids, EPA and DHA), carbohydrates, vitamins, minerals and pigments. While the protein content in whole seaweed biomass can be moderate and may require concentration to match traditional protein sources, some species offer protein quality comparable to FM and soybean meal, and can have higher proportions of essential amino acids.
Nutritional feed trials with Atlantic salmon and rainbow trout indicated that seaweeds can be incorporated successfully into diets as partial substitutes for traditional protein sources, typically at levels of up to 15%, without adverse effects on growth. Higher inclusion levels (eg, 20-50 %) have been explored with variable, species-specific and dose-dependent results.
Atlantic salmon: Inclusion of a product derived from Laminaria species (AquaArom) at up to 10% increased feed intake and promoted growth. A proprietary blend of seaweeds (OceanFeed™) at 15% also appeared to favourably change tissue lipid composition.
Rainbow trout: Studies have shown that including IMTA (integrated multitrophic aquaculture)-cultivated red seaweed Gracilaria vermiculophylla at 5% is feasible without reducing growth. Similarly, Saccharina latissima at 1-2% has been added without detrimental effects while at higher inclusion rates of 10-15% there is evidence of growth suppression. Interestingly, research suggests that low-level dietary supplementation (5-10%) of Ulva could reduce weight loss during short-term fasting periods, potentially offering economic advantages for pre-harvest management.
Enhancing product quality
Seaweeds offer natural solutions for improving the quality of salmonid flesh, particularly concerning fatty acid profiles and pigmentation - which are key quality market preferences for consumers.
Omega-3: While salmonids can be efficient at utilising dietary protein and lipids for energy, they still require the long-chain omega-3 EPA and DHA that are essential for maximum growth and optimum health. These are also crucial nutrients for human consumers. Studies have demonstrated that dietary inclusion of the brown seaweed Macrocystis pyrifera at 3-6% significantly elevated flesh levels of EPA and DHA in rainbow trout fillets, while Saccharina latissima had a lipid-lowering effect without compromising fillet EPA and DHA levels.
Pigmentation: Farmed Atlantic salmon rely on dietary supplements for flesh colouration. The red seaweed Palmaria palmata at 5% inclusion enhanced Atlantic salmon fillets with a desirable yellow/orange colour, suggesting its potential as a natural pigment alternative to synthetic carotenoids. Porphyra dioica also showed a positive effect on the flesh pigmentation of rainbow trout.
This potentially beneficial pigmentation effect could be particularly relevant for organic salmonid farming.
Iodine enrichment: Seaweeds, especially brown species like Laminaria digitata, are rich sources of iodine. Supplementation of Arctic char feeds with Laminaria digitata at 0.8% led to a four-fold increase in fillet iodine content without negatively impacting flesh quality.
Immune modulation and health management
Seaweeds can be rich sources of bioactive compounds including, in particular, various polysaccharides and sulphated polysaccharides, which can act as natural immunostimulants, enhancing non-specific (innate) immune responses.
Atlantic salmon: Alginate, derived from the brown seaweed Ascophyllum nodosum and used as a binder in wet feeds, has been linked to significantly higher lysozyme levels in salmon, indicating an immunostimulatory effect with the potential to support health and increase profit margins (lysozyme is an antimicrobial enzyme that forms part of an animal’s innate immune system). Extracts from Laminaria hyperborea demonstrated immunomodulatory effects, enhancing phagocytic and bactericidal activities, highlighting their potential as immunostimulators against microbial infections. Other red seaweeds, Pyropia columbina and Gracilaria chilensis, have shown potential as immunomodulators, with G. chilensis increasing antiviral activity.
Rainbow trout: Gracilaria vermiculophylla at up to 5% enhanced the innate immune response without compromising growth or nutrient utilisation. Macroalgal-based products containing alginic acid from Laminaria digitata have been shown to act as immunostimulants, modulating fish phagocyte activity. In addition, sulphated polysaccharides extracted from Gracilariopsis persica and Ulva intestinalis successfully improved the immune system and stress tolerance in rainbow trout.
Practical considerations and future directions
While the potential of seaweeds in salmonid aquaculture is significant, successful development would require careful management.
The effectiveness of seaweed inclusion is highly dependent on both the seaweed species and the fish species, as well as the dietary inclusion level. Furthermore, the composition of seaweed changes with the season and so efforts would be needed to produce a standardised ingredient for inclusion.
Seaweeds can be included as fresh biomass, dried meals or as extracts, with the form of delivery influencing success. Processing methods, including enzymatic digestion, can significantly improve the digestibility of proteins from certain red seaweeds.
Some seaweed polysaccharides can be anti-nutritional factors, potentially limiting protein digestibility. Further research is required to mitigate these effects through processing or targeted extraction.
For seaweeds to become a major protein source, they must also be price-competitive with existing plant proteins. However, their attributes as functional additives (eg, for immune health or pigmentation) may provide significant added value to farmers.
Research into optimising culture conditions for selected seaweed species is crucial to improve their nutritional composition and ensure a consistent product for aquaculture feeds. Developing concentrated protein products from seaweeds, similar to soybean protein concentrates, is an important goal.
As such, seaweeds represent a promising and sustainable addition to the catalogue of potential alternative feed ingredients for the salmon and trout industry. They have a proven efficacy to improve growth, enhance flesh quality, increase beneficial fatty acid content, provide natural pigmentation and boost fish immunity in specific instances, and this offers a multi-faceted approach to addressing industry challenges. Continued research into optimising seaweed utilisation will contribute significantly to the future sustainability and profitability of salmon and trout farming.
This article was written by Prof Adam D Hughes from the Scottish Association for Marine Science, Scotland, and Prof Douglas R Tocher from the Guangdong Provincial Key Laboratory of Marine Biotechnology, Shantou University, China, based on the article "Hughes AD, Twigg GC, Msuya FE, Padmakumar KP, Tocher DR. The use of macroalgae in feeds for finfish aquaculture." Frontiers in Aquaculture 4:1570842’ published in June 2025.
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