Adherence of kelp (Saccharina latissima) gametophytes on ropes with different binder treatments and flow regimes

The cultivation of kelp typically involves two stages, where an indoor hatchery phase preceedes the grow-out phase at-sea. The in situ adhesion of microscopic propagules onto specifically designed substrates using binders has been proposed as an alternative to conventional hatchery methods where juvenile seaweeds are cultured on seeded twine, aimed at saving resources while increasing productivity as it obviates the hatchery phase. Here we tested how well kelp (Saccharina latissima) gametophytes adhere to cultivation ropes using two binder types (agar and κ-carrageenan) and application treatment (separate or mixed application) under two ecologically relevant flow regimes (5 and 15 cm s⁻¹), and a control condition (0 cm s⁻¹) in a laboratory flume. Our findings indicate that the effectiveness of a binder to retain S. latissima gametophytes onto cultivation rope, measured by the sporophyte density, was comparable for all binder types in the high flow velocity (15 cm s⁻¹) treatments, including the non-binder control treatment. Sporophyte densities were highest in the low flow velocities (0 and 5 cm s⁻¹) in the absence of a binder compared to all other treatment combinations. In conclusion, our results highlight that the effectiveness of binder assisted seeding of kelp propagules did not differ between the binder and non-binder treatments and was unaffected by flow velocities for the binder treatments. These findings are important in the development of novel methods and further optimisation of existing binder-based methods aimed at retaining seaweed propagules onto cultivation rope.