Regrowth and biofouling in two species of cultivated kelp in the Shetland Islands, UK

The meristem of the kelps Laminaria digitata and Saccharina latissima is located at the base of the blade, growth can therefore continue when the distal blade is lost due to erosion or harvesting. The aim of the study was to determine the regrowth potential of cultivated kelp in the Shetland Islands (UK) to assess feasibility of harvesting twice in one growing season. Laminaria digitata and S. latissima grown on longlines at sea were sampled between March and August, and harvested either at the stipe (whole) or 10 cm above the stipe-blade transition zone (partial) May–August. Image analysis was used to estimate blade length, width, area and fouled area. S. latissima increased in length more than L. digitata between March and August, and in August, while whole S. latissima experienced a net loss in length, partially cut blades had a net increase. Whole blades significantly increased in length for both species but only partially cut S. latissima significantly regrew. In late summer, severe biofouling by tunicates made up 12 and 27% of the biomass at two sites. Interestingly, S. latissima was less fouled than L. digitata. Consequently, S. latissima shows the greatest potential for the application of partial cutting to improve cultivated biomass yields. However, the period for regrowth is limited by low yields in early spring and blade degradation in late summer. In order to optimise biomass yields, further understanding is needed on the abiotic and biotic factors that control growth and biofouling on natural and farmed seaweed.