Abstract
ULTFARMS is an EU Horizon project dedicated to unlocking the potential of sustainable offshore aquaculture by developing and testing cultivation systems for low-trophic species in co-location with offshore wind infrastructure. At the FINO3 test site in the North Sea, near the DanTysk wind farm, three systems were deployed for the cultivation of Saccharina latissima (kelp), Mytilus edulis (blue mussel), and Ostrea edulis (European oyster). Preliminary results demonstrate promising growth performance: mussels achieved flesh content up to 47%; kelp grew up to 1.3 mm/day despite a short cultivation period; and oysters are under continued observation (results expected by September 2025). These findings validate the technical feasibility and biological viability of offshore aquaculture and offer a foundation for further development of multi-use marine sites.
Introduction
The European Union’s blue economy strategy emphasizes climate-neutral food production, optimized marine spatial planning, and co-use of ocean infrastructure. ULTFARMS directly contributes to these goals by designing and validating aquaculture systems for low-trophic species that can coexist with offshore wind farms. The project aims to overcome technical, biological, and economic barriers to offshore farming while supporting circularity, biodiversity enhancement, and regional economic value chains , contributing to the Ocean Mission Baltic & North Sea Lighthouse. The FINO3 pilot site, located adjacent to the DanTysk wind farm in the North Sea, offers a challenging but strategic setting for testing aquaculture prototypes under real offshore conditions. The project’s goal is to assess biological viability, system resilience, and scalability of co-located farming of macroalgae and filter feeders, paving the way for environmentally and economically sustainable offshore aquaculture operations.
Materials and Methods
Three aquaculture systems were installed at the FINO3 offshore research site:
The study was conducted at the FINO3 research platform, located in the German Bight of the North Sea, adjacent to the DanTysk offshore wind farm. This location offered a high-energy offshore environment ideal for testing robust aquaculture infrastructure under real marine conditions. Within the ULTFARMS project, three cultivation systems were installed to evaluate the growth potential of Saccharina latissima (sugar kelp), Mytilus edulis (blue mussel), and Ostrea edulis (European oyster).
The algae cultivation system consisted of a net structure that extended from the sea surface down to a depth of two meters. It was anchored on both ends using delta flippers connected to chain links and reinforced by concrete blocks for additional stabilization. S. latissima specimens were sourced regionally and propagated in a laboratory setting. Two seeding approaches were employed: a direct seeding method, where spores were applied directly onto the net, and a twine-seeding method in which pre-seeded twine was attached to vertical lines flanking the net structure. The entire net system was assembled and seeded at the harbor shortly before being deployed at the offshore site. Due to unforeseen technical complications, the algae had to be harvested after only three months of growth.
The mussel cultivation system featured a submerged backbone composed of a polypropylene tube suspended approximately seven meters below the water surface. Attached to this backbone were nets. Buoyancy was provided by flotation buoys, which were evenly spaced and connected to the backbone . The structure was anchored similarly to the algae system, using delta flippers, chain, and concrete weights. D ifferent net materials were deployed in early spring to encourage natural mussel spatfall. The system remained at sea also during winter to allow a natural grow-out phase of up to two years. Subsamples were collected periodically during routine inspection and maintenance to monitor settlement and growth progression.
For oyster cultivation, a novel system was developed consisting of small, commercially available oyster baskets arranged within a steel cage frame. Each cage included a weighted base for stability on the seafloor. The entire unit was equipped with an acoustic remote control (ARC) retrieval mechanism mounted on the upper frame. This system could be triggered remotely, deploying a buoy attached to a retrieval line, which enabled surface access to the submerged cage. The baskets were filled with hatchery-reared European oysters verified to be free of Bonamia infection in order to protect the trial site from potential pathogen introduction. The oyster system was fully assembled and stocked before offshore deployment in May 2024 and shall remain on site until September 2025 . G rowth data is expected after the current cultivation cycle concludes.
This comprehensive setup allowed for simultaneous testing of different species, materials, and deployment strategies under the same environmental conditions, providing valuable early data for offshore multi-use aquaculture development.
Results
Algae (Saccharina latissima)
The kelp cultivation trial, despite being limited to a three-month period due to technical constraints, showed encouraging biological performance. Growth rates of up to 1.3 mm per day were observed, aligning with published benchmarks for early-stage development in offshore kelp farming. However, growth was uneven. The central and lower sections of the net exhibited moderate to good growth, while the uppermost zone showed little to no growth. This absence of biomass in the top section suggests adverse effects from high surface turbulence and wave-induced mechanical stress. No substantial difference in growth performance was observed between the direct seeding and pre-seeded twine methods.
Mussels (Mytilus edulis)
Mussel settlement and growth were patchy, with clear differences in performance among different tested net materials. The standard polypropelene mesh netting supported the highest spat density and showed the best results . In contrast, fuzzy rope nets had poor results . Subsamples taken during mid-season inspections and final autumn maintenance revealed well-developed mussels on the nets, with flesh content reaching up to 47%. This figure reflects excellent meat yield and is comparable to quality standards for commercial mussel production. However, variability within and between net types highlights the need for improved design.
Oysters (Ostrea edulis)
The oyster system is still deployed last visual inspection revealed system robustness under offshore conditions . Growth data was not available at Abstract submission time but is expected to be presented during the conference.
Discussion
These preliminary trials at the ULTFARMS FINO3 pilot site provide a foundational proof-of-concept for integrated, offshore aquaculture in co-location with wind energy infrastructure. Although the biological results were mixed in uniformity, each of the systems showed technical feasibility and species-specific success indicators that merit further investigation and upscaling.
The short duration of the kelp trial limited conclusions on seasonal yield, yet the observed growth rates suggest that S. latissima can develop efficiently in exposed sites . The absence of growth in the upper net layers likely results from physical abrasion and motion during heavy seas, indicating a need to adapt net design or focus seeding below the wave-affected zone. Future efforts should investigate longer grow-out durations.
Mussel cultivation showed high potential for biomass yield and quality. The high flesh content of up to 47% supports commercial interest, but patchy settlement highlights need for further research.
Oyster growth data remains pending . But the structural setup and ARC retrieval system look promising. This technology can drastically reduce offshore maintenance costs by removing the need for diver-based operations. Assuming acceptable growth and survival rates continue, this system may serve as a model for future offshore shellfish farming infrastructure.
These results represent a promising first step toward a scalable and environmentally compatible model for offshore aquaculture within Europe’s evolving blue economy framework.