Abstract
Macroalgae are seen as a sustainable resource for industries including agriculture, food, and nutraceuticals. Aquaculture is the dominant form of global seaweed biomass production, yet wild-harvesting remains important in Europe. Fucoid species such as Fucus vesiculosus commonly colonise intertidal shores and artificial structures, including oyster farms in Ireland, where they represent a largely unexploited biomass resource. However, the amount of biomass produced annually on oyster farms is unknown, making it unclear whether this resource could be harvested sustainably and potentially provide a new opportunity for shellfish farmers. Hence, this study investigated the potential for sustainable harvesting by management of F. vesiculosus fouling on an oyster aquaculture site, quantifying post-harvest seaweed biomass yield and regeneration. F. vesiculosus thalli were harvested under three treatments, a control (no cut), a longer cut to ~6 cm (Cut L) and a shorter cut to ~3.5 cm (Cut S) and regrowth was quantified over a 12-month period from April 2025 to 2026. The harvested F. vesiculosus thalli regenerated up to 87% of pre-harvest lengths within 12 months. The thalli showed non-linear seasonal growth patterns, with thallus lengths increasing over time. The Cut S treatment resulted in a higher relative growth rate among treatments and a greater initial biomass yield of 0.54 kg per m of trestle. Having determined the total number of oyster trestles in a 1 km stretch of shoreline, we estimated that ~11 tonnes of biomass could be harvested using the Cut S treatment. Our results suggest that low intensity partial harvesting of settled F. vesiculosus on oyster farms could be sustainable, and our results provide proof-of-concept for future co-location of Fucus crops and oysters within a shellfish aquaculture site.