Introduction
Despite mixed market signals over the past years, it is expected that the European seaweed market will experience steep growth (Jueterbock et al., 2025, Mendes et al., 2022), and the scarcity of suitable near-shore areas will require the expansion of the matured industry towards exposed offshore cultivation. In Portugal, practically no suitable near-shore areas exist for the early phase of offshore seaweed farming, but the potential of exposed offshore sites is vast. The co-location of seaweed with established fish farms in an Integrated Multi-Trophic Aquaculture (IMTA) approach has been widely discussed over the last years (Buck et al., 2028), but in practice it has not been implemented in existing near-shore farms. Lack of short-term economic benefits and space restrictions around existing near-shore farms – in particular in Northern Europe – are the likely main reasons for this. The move towards offshore can attenuate these obstacles, allowing for promising operational synergies and shared infrastructure that the IMTA concept can bring along. Both the potential for IMTA operation in an offshore farm and the performance of native species in these highly dynamic environments in Portugal remains poorly understood. This study evaluates a two-season pilot project in the Algarve (southern Portugal), detailing the technical, logistical, and biological results, challenges and opportunities for scaling offshore cultivation of native macroalgae.
Materials and Methods
A 1-hectare offshore mooring grid was installed adjacent to existing fish cages at a depth of 25–30 m. The system was designed with 50 m transverse longlines suspended at a depth of 3 m to support the seeded substrate. Target species focus was on native kelps (Saccharina latissima, Laminaria ochroleuca), Saccorhiza polyschides and tentatively the chlorophytes and Ulva sp., Codium spp. and the rodophyte Porphyra umbilicalis. Seedlings were cultivated in onshore hatcheries, transported in controlled thermal conditions, and deployed at sea between October and March, partly over 2 seasons. Operations were intentionally adapted to rely on existing infrastructure, vessels and operational schedule of the existing finfish operations, and seaweed farm operations like deployment, monitoring were conducted with a small vessels (5–6 m) to validate the feasibility of light logistical maintenance without disrupting the primary heavy-machinery needs of the fish farm.
Results and Discussion
In particular for the larger brown seaweeds, the growth trials showed promising results, despite the short and constrained match of operation windows for hatchery and sea operations. The six macroalgae had different levels of hatchery know-how and former experience both in lab- and sea-scale, and in particular the results for the rodophyte and chlorophytes were less conclusive. The offshore environment presented significant biological and technical constraints. Heavy biofouling proved to be a primary challenge; small epiphytes and mussel encrustations rapidly colonized the substrate lines, outcompeting and suffocating the seaweed seedlings before they could establish significant biomass, particularly in the case of Codium spp.. Hervibory signs were also observed, most noticeably in L. ochroleuca blades. Mechanically, the structure sustained minor but impactful damages, such as broken seeded threads and friction wear caused by crossing ropes and hydrodynamic tension. Furthermore, logistical execution was heavily constrained by strict biological deployment windows colliding with winter weather instability and the simultaneous operational demands of the fish farm.
Despite the high biological pressure from biofouling and hervibory, this pilot demonstrates that offshore IMTA presents viable opportunities for spatial expansion and the reduction of operational costs through shared maritime logistics. Managing the narrow deployment windows requires a delicate balance between optimal seaweed transfer times and the daily priorities of fish husbandry. Overcoming the observed encrustation bottlenecks will require further optimization of deployment timing, depth adjustments, and the establishment of routine offshore maintenance techniques and protocols. Ultimately, these initial trials establish a crucial technical foundation for developing competitive, large-scale offshore macroalgae aquaculture and advancing the Blue Bioeconomy in Portugal.
Acknowledgment
This work was financially supported by the "Pacto da Bioeconomia Azul" (Project n�� C644915664-00000026) under WP5 Algae Vertical, funded by the Next Generation EU and the Recovery and Resilience Plan (PRR) of Portugal, under the "Agendas para a Inova����o Empresarial" incentive.
References
Jueterbock, A., Hoarau-Heemstra, H., Wigger, K. et al. Roadmap to sustainably develop the European seaweed industry. npj Ocean Sustain 4, 22 (2025). https://doi.org/10.1038/s44183-025-00122-9
Mendes, M.C.; Navalho, S.; Ferreira, A.; Paulino, C.; Figueiredo, D.; Silva, D.; Gao, F.; Gama, F.; Bombo, G.; Jacinto, R.; et al. Algae as Food in Europe: An Overview of Species Diversity and Their Application. Foods 2022, 11, 1871. https://doi.org/10.3390/foods11131871
Buck BH, Troell MF, Krause G, Angel DL, Grote B and Chopin T (2018) State of the Art and Challenges for Offshore Integrated Multi-Trophic Aquaculture (IMTA). Front. Mar. Sci. 5:165. doi: 10.3389/fmars.2018.00165