Environmental footprint of farmed blue mussels compared to common protein sources The environmental footprint of a number of protein-rich food sources were evaluated from literature: Farmed blue mussels and Pacific oysters, wild-caught Atlantic cod and herring, northern shrimp, farmed Atlantic salmon, vegetarian protein sources as well as beef, pork and poultry. Studies were selected and major methodological choices aligned to render a reasonably comparable set of products. Due to lack of data and methodological consistency, the only environmental impact category that was possible to quantify across all products was greenhouse gas emissions, other environmental impacts are discussed qualitatively. Emissions were compared both per kg of edible product, per kg of protein and per recommended daily intake (RDI) of a number of key nutrients. The nutrients selected are nutrients for which seafood plays or could play an important role for intake: iodine, selenium, vitamin D, omega-3 fatty acids and iron. Results show that both bivalves - blue mussels and Pacific oysters – together with herring are the lowest emission food sources included per kg of edible food. Vegetarian protein sources and cod are close, but somewhat higher. Poultry, pork, shrimp and salmon are intermediate, while beef caused the highest emission, the latter due to its high methane emissions from digestive processes. For fed species, i.e. the three landbased animal foods and salmon, production of feed represents a major part of emissions, in particular when consisting of a large proportion of soy from countries with expanding agricultural land. Bivalves have a lower protein content than the other foods (10-13% vs 18-22% for other animal foods and 15-17% for vegetarian foods) and therefore emissions per kg of protein are slightly higher than per kg edible food. When instead the RDI is used as the basis of comparison, the amount of each food needed to supply the RDI and associated emissions are presented. Mussels and oysters were again most low-carbon source for four out of the five nutrients analysed. For vitamin D, which mussels do not contain, oysters was the most carbon-efficient source, as a result of the combination of high content and low emissions. Mussels had the highest content of iron of all foods included. For other nutrients, content was high but not the highest and it is due to low emissions that mussels are the most climate efficient way of covering the RDI. Obviously, no one covers their RDI with only one type of food and this calculation was only done in order to compare across foods. The RDI represents an average value and there is variability between individuals and groups in the population with regard to specific needs. A given content of a nutrient does not necessarily have the same effect, as their bioavailability can differ between foods, but in general animal foods such as mussels have high bioavailability especially for minerals. The independence of feed is a major advantage of farmed bivalves and improvement options focus on increasing the edible yield as much as possible through farming and harvesting practices. All in all, mussels are shown to have a major potential as low-emission nutritious foods of the future, especially since worst-case assumptions for edible yield were made and if yield was higher, the difference would be even higher and mussel emissions even lower.