The influence of seeding method and water depth on the morphology and biomass yield of farmed sugar kelp (Saccharina latissima) at a small-scale cultivation site in the northeast Atlantic

Abstract Seaweed farming is a rapidly growing global industry, driven by increasing demand for biomass with a range of commercial applications. A major barrier limiting expansion of the industry is the need for cost-effective approaches to production. Established twine seeding methods are reliable, but recently developed binder methods offer potential scalability while minimising hatchery costs. Here, we compared growth of the farmed kelp Saccharina latissima using these two seeding methods. We also examined the influence of water depth on biomass production within a vertical cultivation system. Twine consistently achieved greater S. latissima biomass yield, with mean biomass almost four times higher than from binder seeding, and sporophytes reaching significantly greater density and total length. The biomass, length and density of S. latissima decreased with increasing water depth, a pattern which was more pronounced with twine seeding. We also observed morphological variability, with larger individuals on twine compared with binder treatments at all depths. Natural settlement of the non-target macroalga Sacchoriza polyschides was also recorded, at significantly greater biomass on binder treatments and at greater depths. Further work is needed to examine the predictability and extent of natural settlement of S. polyschides , and its potential as a commercially-viable species. Overall, twine seeding methods out-performed binder at a relatively dynamic, open coast, small-scale cultivation site. Moreover, while vertical cultivation systems can maximise yield relative to the spatial footprint of a seaweed farm, the marked reduction in cultivated biomass with increasing water depth should be considered within the local environmental context.