On Wednesday 8 August 2026, Aarhus University sailed out on the vessel Helle from Seasight Group to inspect the seaweed lines located in Vattenfall’s Vesterhav Syd Offshore Wind Farm off Hvide Sande.

The weather was perfect, with calm water and bright sunshine after storm Dave had passed over Denmark.

The seaweed lines were deployed in October 2025 as part of the WIN@sea and OLAMUR projects. They were attached to ten buoys and anchored to the seabed with concrete blocks. Each line is attached to 5 metres of cultivation line, on which sugar kelp of local origin was seeded.

Since October, Vattenfall’s Crew Transfer Vessel (CTV) had been regularly monitoring the buoys, but the seaweed had not been inspected since deployment. It was therefore exciting to see how the growth had developed.

The seaweed was clean and in good condition but had not grown as much as expected based on the yield in 2025. The yield was significantly lower than expected at this time, also compared to last year’s growth. A tube-forming amphipod had established itself on the lines, and the grey mud tubes had shaded the seaweed. Some seaweed plants also showed clear signs of wear and tear – possibly after the storm.

Our temperature loggers showed that it had been a cold winter for the small seaweed plants, with temperatures below 5 degrees for a longer period. For this reason, we are lowering our expectations for the final harvest, which is planned for mid-May 2026.

The results clearly show that there is a need for more knowledge about seaweed cultivation in the North Sea to achieve optimal growth in the area. We need more knowledge about timing of deployment and harvesting, and about the design of cultivation systems. Cultivation practices must be adapted to the harsh environment and interaction with other organisms. The North Sea is a challenging place for seaweed cultivation, but also an area with great potential. The high salinity and generally favourable conditions make it a very promising location for seaweed farming.

Multi-use between seaweed cultivation and wind farms can improve safety for personnel and equipment, enable better monitoring of cultivation systems, and create opportunities for shared use of marine space.

Aarhus University downloaded data from salinity, temperature, and light loggers and redeployed the loggers and seaweed lines in the North Sea for further growth until harvest in May.

Thanks to Kåre, Helle, and Vattenfall’s Marine Coordination Center (MCC) and the CTV team!