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

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 <i>Saccharina latissima</i> 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 <i>S. latissima</i> 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 <i>S. latissima</i> 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 <i>Sacchoriza polyschides</i> 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 <i>S. polyschides</i>, 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.