Edulis: offshore mussel culture in wind farms

The Edulis project wanted to investigate the feasibility of cultivating mussels in offshore windfarms, using longlines (50 m) supporting several dropper lines. One of the main objectives was to assess the technical requirements and optimize a mussel cultivation system suited for the harsh conditions of the Belgian North Sea, taking into account the physical forces acting on the system, environmental factors and the growing biomass. For this purpose, a first test line (power line) was installed to measure the impact of currents, waves and tides on the anchors, the longline and the dropper lines using several force measuring devices. This line was installed just before winter to test the most extreme water conditions, and showed that currents (continuous) and waves (peaks) caused the main impact on the infrastructure. These measurements were compared to a numerical model that was developed, which allowed to calculate the optimal position of the system relative to the main current direction. Also the optimal depth under the water surface was calculated to minimize the effect of waves on the system. A second test line (bio-line) focused on evaluating different systems (horizontal line vs. dropper lines) and rope materials to catch mussel spat. This experiment was repeated several years to exclude any coincidences from the observations. Several sampling campaigns allowed to monitor the number of mussel seeds per meter of dropper line, differences in function of water depth and how the density evolved through time. Also predation was monitored, but no issues were flagged during the project. The main problem that was observed was biofouling by Jassa, a small amphipod, leading to a significant increase in weight and also to a reduced feeding intake by the mussels during summer. Two full growth seasons over 3 years learned that it takes on average 15 months to grow a commercial size mussel of 6 cm in offshore conditions with a meat content of 25-30%, which is comparable to nearshore cultivation. An average yield of 10 kg mussels per meter was obtained, with little differences between the depth of the line. Both tests allowed to suggest the best suited cultivation systems with maximum mussel production and optimizing the space in windfarms. The data of this project were also used to optimize a mussel growth prediction model, allowing to calculate the expected mussel yield at any specific location of the Belgian North Sea, based on environmental factors. Finally, the sustainability of offshore mussel cultivation was calculated applying a Life Cycle Analysis approach and several recommendations were proposed for future designs (e.g. multiple use of anchoring points, using longlines with a longer lifespan). Although technical risks need to be further addressed and reduced before a commercial scale mussel production can start in the Belgian offshore windfarms, Colruyt, a partner in the project, recently installed a first mussel farm 5 km off the coast, using the results of this project. The Edulis project was the first project worldwide to demonstrate that is is feasible to cultivate mussels offshore, within a windfarm, and hence contributes strongly to the multiple use of our oceans which is considered a priority for the European Union to realize a Blue Economy. The project results contribute to the design of new offshore windfarms, taking into account the needs of both aquaculture and energy producers.