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, therefore growth can continue when the distal blade is damaged or removed. For this reason, cropping the blade above the meristem when harvesting natural kelp stands is thought to facilitate regrowth and minimise habitat disturbance. The aim of the study was to determine the regrowth potential of cultivated kelp in the Shetland Islands (UK) within one growing season. L. digitata and S. latissima grown on long lines at sea were harvested 10 cm above the stipe-blade transition zone in May – June and sampled monthly until the end of August. Each sample was photographed and the image analysed to estimate growth rates and levels of biofouling. The rate of growth remained stable for both species whether cropped or whole, S. latissima exhibited greater growth in length than L. digitata between May and August. In late summer, severe biofouling by ascidians made up 32% and 15% of the wet biomass at two sites, however, S. latissima appeared less severely affected than L. digitata. Consequently, S. latissima shows the greatest potential for the application of cropping to improve cultivated yields. However, the period for regrowth is limited by low yields in early spring and blade degradation in late summer. The results of this study also lend support to the use of cropping as a more sustainable harvesting method facilitating regrowth in natural kelp stands.