Aquaculture of extractive species, such as bivalves and macroalgae, already supplies a large amount of the production consumed worldwide, and further production is steadily increasing. Moving aquaculture operations off the coast as well as combining various uses at one site, commonly called multi-use aquaculture, is still in its infancy. Various projects worldwide, pioneered in Germany and later accompanied by other European projects, such as in Belgium, The Netherlands, Norway, as well as other international projects in the Republic of Korea and the USA, to name a few, started to invest in robust technologies and to investigate in system design needed that species can be farmed to market size in high energy environments. There are a few running enterprises with extractive species offshore, however, multi-use scenarios as well as offshore IMTA concepts are still on project scale. This will change soon as the demand is dramatically increasing and space is limited.

Blue mussel (Mytilus edulis) broodstock collected from the Irish Sea during wintertime (November) was conditioned with three different microalgae diets. Positive flagellates (PF) treatment consisted of Pavlova lutherii, Isochrysis galbana (T-Iso), and Chaetoceros calcitrans (1:1:1). Positive diatoms (PD) treatment consisted of Pavlova lutherii, Chaetoceros calcitrans, and Skeletonema costatum (1:1:1). Broodstock animals in the PF and PD treatments were fed a total of 2.4 × 10¹¹ algae cells per day. Animals in the negative flagellates (NF) treatment received only 1/8th of the total amount of algae of the PF diet. The conditioning diets had an impact on spawning success and broodstock fecundity but not on hatching rate, which was similar in all three groups. The best results were obtained with the PD diet where 84% of the conditioned animals spawned and females released 5.0 × 10⁶ eggs on average. Animals belonging to the PF and NF treatments released, on average, only 3.6 × 10⁶ and 1.6 × 10⁶ eggs, respectively. Although the amounts of algae provided to the broodstock animals had no effect on the hatching rate, the D-larvae resulting from the NF treatment were smaller in size than the larvae from the other treatments. Biochemical analysis of the different broodstock groups at the end of the experiment revealed higher carbohydrate levels in group NF than in PF and PD, supporting the theory that gametogenesis is mainly supported by the energy from the glycogen reserves. As far as we are aware this is the first study describing hatchery broodstock conditioning of blue mussels under fully controlled conditions.

Oyster reefs are among the most threatened marine habitats globally. In Europe, oyster reefs have been extirpated from most locations within their historical range. Active restoration of the native oyster (Ostrea edulis) in Europe has grown substantially in recent years. In sharing experiences between oyster restoration projects in Europe at the Native Oyster Restoration Alliance conference, NORA2, in Edinburgh in May 2019, it became apparent that a number of similar barriers are experienced. This study identified the top 40 questions, which, if answered, would have the greatest influence on the policy and practice of oyster restoration in Europe. Initially 71 people were consulted across 28 institutions and 11 European countries to generate 194 questions. An established process of one round of pre‐workshop voting followed by a one‐day online workshop and two post‐workshop rounds of voting resulted in the final 40 questions. Questions were broadly grouped into the following 10 themes: baselines, site selection, restoration methods, quantifying benefits, disease management, biosecurity, genetic diversity and population differentiation, policy and management, novel technologies, and current and future threats. We anticipate that this list will provide a starting point for developing collaborative projects across the NORA network, as well as assisting policy makers and funders with identifying key areas that need to be addressed in order to overcome existing barriers to scaling up oyster restoration in Europe.