Saccharina latissima cultivation in the Belgian part of the North Sea – challenges and lessons learned in preparation to move offshore

Jessica Knoop¹, Koen Allewerelt², Peter Bossier³, Stefanie Debels², Annelies M. Declercq³, Steven Degraer⁴, Daan Delbare⁵, Sander Devriese⁶, Bert Groenendaal², Francis Kerckhof⁴, Thomas R.H. Kerkhove⁴, Evert Lataire⁷, Frank Leroy², Simon Petit⁶, Laura Pilgrim⁸, Elisabete Pinto da Silva⁹, Ajie B.K. Pribadi⁷, Brecht Stechele³, Jan Vanaverbeke⁴, Nathalie Van Caster⁶, Loes Vandecasteele¹, Dirk Vandercammen⁹, Nancy Nevejan³ and Olivier De Clerck¹

¹Phycology Group, Ghent University, Krijgslaan 281, Sterre S8, 9000 Ghent, Belgium ²Brevisco, Hendrik Baelskaai 38, 8400 Oostende, Belgium ³Laboratory of Aquaculture and Artemia Reference Center, Ghent University, Coupure Links 653, 9000 Ghent, Belgium ⁴Marine Ecology and Management, Operational Directory Natural Environment, Royal Belgian Institute of Natural Sciences (RBINS), Vautierstraat 29, 1000 Brussel, Belgium ⁵Research Institute for Agriculture, Fisheries and Food, Jacobsenstraat 1, 8400 Oostende, Belgium ⁶Jan De Nul NV, Tragel 60, 9308 Hofstade-Aalst, Belgium ⁷Maritime Technology Division, Ghent University, Ghent, Belgium ⁸Colruyt Group, Edingensesteenweg 196, 1500 Halle, Belgium ⁹Parkwind, Esplanadestraat 10, 8400 Oostende, Belgium.

Interest is high in using available offshore sites within wind parks for seaweed cultivation in European waters due to increased competition between different sectors for marine space. One aspect of the Belgian Pilot within the Horizon 2020 project UNITED (Multi-Use offshore platforms demoNstrators for boostIng cost-effecTive and Eco-friendly proDuction in sustainable marine activities) focuses on developing feasible cultivation techniques for the brown alga Saccharina latissima in these high energy environments. Six seaweed nets (6x3m and 4x2m) were installed during trials at the pre-operational test site – 5 km off the Belgian coast – over two growth seasons (2020/2021 and 2021/2022). Different substrates (nearshore/offshore and rope net type), seeding techniques (nursery versus direct seeding) and strains (originating from North France and The Netherlands) were tested. We identified seeding technique as a major factor determining yields. Over the first cultivation season, direct seeding resulted in very low density (max. 60 individuals m^-1) and yield (max. 0.5 kg m^-1) compared to net sections kept in a nursery for four weeks (max. 200 individuals m^-1 and 1.1 kg m^-1). No differences were observed between different seaweed strains. Optimization of the nursery and adjustment of the direct seeding method resulted in higher yields in the second growth season. However, nets kept in the nursery before deployment still showed higher yields (max. 2.8 kg m^-1) compared to the adjusted direct seeding (max. 1.3 kg m^-1). Results indicate the need for optimized seeding techniques allowing for successful large-scale cultivation of S. latissima in highly exposed offshore conditions.