Advancing plastic-free kelp farming: Wool twine as a natural substrate alternative in longline cultivation

Optimising seaweed aquaculture to enhance sustainability and profitability whilst minimising environmental impacts is important as the seaweed industry grows. Traditional seeding methods utilise synthetic seedstring and/or rope, but there is an increasing need to transition to plastic-free seaweed farming to reduce microplastics and plastic pollution in marine environments. This study assesses the cultivation performance of S. latissima on biodegradable, untreated sheep's pure wool compared to a synthetic substrate (Dyneema UHMWP) to determine the feasibility of wool as a natural replacement. To test the feasibility of wool as a seedstring, two field trials were conducted utilising separate seeding methods, nursery timings, field deployment periods, and various diameters of wool: Year 1 (spore seeding; 3 months nursery; 4 months field) and Year 2 (gametophyte spraying; 1 month nursery; 6 months field) in Strangford Lough, Northern Ireland. Year 1 Dyneema achieved the greatest yield (4.4 kg · m −1 ), while pure wool substrata yielded 45–60% in comparison. Year 2 showed an increase in biomass with wool diameter: 2 mm: 0.84 kg · m −1 ; 4 mm: 1.72 kg · m −1 ; 6 mm: 2.63 kg · m −1 . The wool seedstring showed varying degrees of biodegradation at harvest. The 2 mm wool completely degraded, with kelp attaching directly to the plastic longline, while 4 mm and 6 mm wool exhibited fragmented degradation with sporophytes attached to both seedstring and longlines. Our findings demonstrate that pure wool seedstring substrates successfully supported sporophyte growth during both nursery and field deployment phases, representing a promising step toward reducing marine plastic pollution in aquaculture operations.