In the present study we evaluated the musselspat feed MySpat formulated by INVE Technologies (Dendermonde, Belgium) in combination with small quantities of microalgae as a complete diet for young mussel seed Mytilus galloprovincialis (Lamarck 1819). Three different food levels were tested: a continuous algae supply over a period of 24 h of 150 cells ml-1 (Control diet 1, C 1), 75 ml-1 (C 2) and 24 cells ml-1 (C 3). In three additional treatments C 2 was supplemented with 2.8% and C 3 with 2.8% and 4.3% MySpat respectively. Percentage was calculated on life weight (LW). Mussel spat belonging to treatments C 3 + 2.8%MySpat and C 3 + 4.3%MySpat gained almost twice as much weight as the mussels fed the nonsupplemented algae diet C 3. There was no significant difference between the two supplementation levels, indicating that a level of 2.8% was sufficient. The mussel spat that received the supplement MySpat grew as fast as the animals that received 75 cells ml-1 being 702% increase in wet weight (WW) in 3 wk, so the same result was obtained with only 1/3 of the algae. This is interesting when one considers that the mussel spat in the last week of the experiment received 95% dry weight (DW) formulated feed and only 5% DW algae. The growth was well balanced between shell growth and increase of tissue weight, because the organic matter content of the animals was equal to or even higher than the positive control animals. Mussel seed on the C 3 diet had a fatty acid methyl ester (FAME) content of 6.6 mg gDW-1 , whereas this content quadrupled to 28.1 mg gDW-1 when 2.8% MySpat was given in addition to the algae diet, reaching levels even higher than for the positive control treatment. The fatty acid composition reflected the diet-composition, hereby proving the ingestion and assimilation of the diet. It is suggested that mussel seed regulate arachidonic acid (ARA) levels and keep the absolute amount in their tissues at 0.4 mg gDW-1 .

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.

Bacterioplankton is an important component of marine as well as fresh waters and accounts for a large fraction of the production of particulate matter in those ecosystems. The concept of bacteria as providers of essential nutrients in aquatic food webs has been raised most recently in relation to the utilization of bacteria as a food source in aquaculture food chains. This article represents a comprehensive review on the potential role of bacteria as a direct food source for aquaculture organisms. After studying shortly the analytical tools that can be applied to determine the uptake and assimilation of bacteria by higher trophic levels, it describes the bacterial loads that are found in closed and open aquaculture systems and elaborates on the composition of bacteria from a nutritional point of view. Next this study elaborates in more detail on the role of bacteria as food for the zooplankton groups used as live food in aquaculture hatcheries and for various groups of aquaculture target organisms, such as fish, crustaceans and especially bivalves. This includes an assessment of the specific uptake mechanisms of the different taxonomic groups of interest in relation to their efficiency to ingest bacteria. Finally, future research lines are suggested to elucidate the nutritional impact of the bacterial community in aquaculture systems and how to steer this in order to optimize the production.

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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 9 10 11 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 9 10 6 eggs on average. Animals belonging to the PF and NF treatments released, on average, only 3.6 9 10 6 and 1.6 9 10 6 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.

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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.

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