Splice seeding as an alternative to conventional twine-wrapping for Saccharina latissima cultivation: biomass yield, substrate efficiency, and commercial implications

As the emerging temperate kelp industry seeks to scale in the west, established seeding methodologies, principally twine and direct seeding, present persistent bottlenecks in hatchery costs, space requirements, and operational reliability. This study evaluates splice seeding, a precision-spacing technique common in East Asian kelp aquaculture, as an alternative approach for Saccharina latissima cultivation. Seeded twine was cut into 3 cm sections and manually inserted into 12 mm twisted cultivation ropes at 5, 10, and 20 cm intervals; conventional twine-wrapping served as the control. Sea trials in central Norway (November 2024 – June 2025) demonstrated a stepwise decline in absolute biomass yield with increasing splice interval: 2.9, 2.4, and 1.8 kg m⁻ 1 , compared to 3.8 kg m⁻ 1 for conventional twine. However, evaluated as biomass per meter of seeded twine (a metric we term "seeding substrate efficiency") splice seeding yielded 4.7, 8.0, and 12.0 kg m⁻ 1 compared to 3.5 kg m⁻ 1 for the control, representing material efficiency gains of 36–248%. The density of large sporophytes (> 40 cm) was the primary driver of total yield (Pearson's r = 0.80, p < 0.001), and their lower absolute numbers in splice treatments explains the observed yield gap. Increasing large sporophyte density within splice sections through earlier deployment, higher initial seeding densities, or improved substrates represents the most direct pathway to yield parity and would enable splice seeding to deliver its full economic and operational advantages for Western kelp aquaculture.