The Borssele windfarm is situated 24 km off shore the coast in the Southwest of The Netherlands. The government designated several areas for multi use: mariculture, passive fishing, nature development and other types of renewable energy. OOS SMF developed a Semi-Submersible Mussel Farm (SMF). The test version of the SMF is located in the Borssele windfarm. It consists of a 10 x 10 x 10 m platform (moored stationary) that functions as the anchoring point on one side for 5 longlines of 200 m while the other end of the longlines is fixed to buoys. In addition, four vertical columns can be filled with seawater to submerge the platform during adverse weather conditions. The SMF has a self-supporting power supply consisting of four robust solar panels of 110Wh and a wind turbine to feed batteries.
As part of the ULTFARMS project technical performance of the system and the growth performance of mussels was monitored. The platform and longlines were monitored remotely daily using two onboard cameras. ROV (remotely operated vehicle) underwater images were obtained for automatic identification of species on the mussel lines to determine mussel presence, size and biodiversity. In addition, mussel samples were collected for analysis of contaminants.
The SMF was deployed in a highly exposed environment, which resulted in repeated longline failure. These issues proved challenging to address due to the continuously changing currents (direction and velocity) at the site, where no slack tide occurs. The main challenge encountered relates to the system's structural robustness. Following each component failure, the affected part was analysed, strengthened or redesigned. However, this often resulted in subsequent failures of other components. For instance, after the initial diabolo malfunctioned, a reinforced version was developed. When the longline subsequently broke, it was replaced with a Dyneema longline. Consequently, the system is still in an iterative phase of assessment and improvement. Upgrades were also required to the monitoring system. The existing power supply was enhanced with an additional battery to ensure sufficient energy storage. The onboard computer was replaced with a more robust model. These improvements have resulted in a stable and reliable monitoring and power supply system. Another challenge is procuring suitable support vessels. All vessels must be registered within the windfarm's operational system, and the windfarm operator and the Dutch Coastguard both require a five-day notice period.
As the longlines entangled before harvesting no culture cycle was completed. However, mussels from a test rope were collected 7 November 2024. At 6 months old, their average shell length was 44 mm and meat content 26%. The mussel samples for contaminant analysis were collected during two monitoring periods (7 November 2024 and 6 March 2025). No exceedances of the maximum levels for contaminants in were were observed. All concentrations remained well below the limits established in Regulation (EU) 2023/915 on maximum levels for certain contaminants in food.
Underwater videos were made of the droppers to determine mussel presence, size and biodiversity showed that the dropper ropes on the longlines did not catch a lot of mussel seed. The type of rope used for the droppers was different from the test ropes. The dropper ropes did not have the loops that the test rope had. This may have caused the difference in mussel yield. A test with the ROV on an inshore longline farm demonstrated automatic identification of species on the mussel lines.
Acknowledgment
This project was funded by the European Union's Horizon Europe research and innovation programme under Grant Agreement No 101093888.