Kelp Forest

Scotlands Living Landscapes

• Foreword
• Introduction
• What is kelp and where is it found?
• Why kelp forests are important in Scottish coastal waters
• Scottish kelp plants
• Light – the critical factor
• Oarweed forests
• Cuvie forests
• Kelp parks
• Beyond the kelp zone
• Kelp productivity
• Uses of Kelp
• Kelp harvesting
• Afterword

Kelp harvesting

Large-scale cuvie harvesting (over 150,000 tonnes per year) takes place in Norwegian coastal waters, where it is undertaken by tug-sized custom-built vessels with open holds the size of large caravans. Kelp harvesting is managed on a rotational basis, ensuring that each area is harvested only once every 4 years to allow regrowth of the plants.

The kelp dredge itself resembles a huge, iron garden rake, 3m wide and with elongated spiked teeth, which is carried along the sea bed on raised skis. The weight of the dredge as it is pulled forward rips dozens of kelp plants from the rock, holdfast and all, which are then caught between the teeth. At the end of a harvesting run, the dredge may have been drawn 50-250m along the sea bed, and have over a tonne of plants hanging in a compact mass from its teeth.

Kelp harvesting also occurs in other areas of the world. Different harvesting methods are used for different kelp species. For example, in California the giant kelp is the basis of a huge alginate extraction industry. However, in this case only the tips of the 30m plus plants are harvested, by being cropped at the surface of the sea. The bulk of the plant remains attached to the seabed after harvesting, in contrast to cuvie harvesting, and the fast growth rate ensures rapid replacement of harvestable plant material.

Kelp harvesting has been considered in Scottish waters, using Norwegian-style kelp dredges, and pilot studies have been carried out. Freshly harvested weed provides higher quality raw material for alginate production than tangle, due to differing amounts of decay in the latter before drying. Harvested material is also more readily accessible, and available in larger volumes, than tangle from the isles.

There are a number of important factors which should be addressed in the management of any harvesting operation should it happen in the future. For example, in Norway the 4-year reharvesting period results in growth of smaller plants with fewer associated animals and seaweed than a mature kelp forest. This means that the kelp has fewer contaminants to remove prior to alginate processing, but also that the forest itself is impoverished in comparison to an unharvested forest. A 6 or 7 year reharvesting cycle has recently been suggested in Norway, not only to allow better development of the kelp-dependant communities, but also to fit in with natural cycles of population changes in kelps.

Other factors which may have a bearing on management of kelp harvesting activity include the role of urchin grazing in recovery of the kelp forest after harvesting, and the importance of kelp forests as a habitat for lobsters and pelagic fish species.

Anecdotal evidence from Norway and California links kelp removal with coastal erosion. Kelp forests may play a part in reducing coastal erosion by damping down the force of incoming waves before they hit the shore, and by reducing current-induced sand movement around their holdfasts. Although this is only speculation at present, this factor may require further investigation before large-scale kelp removal takes place at potentially vulnerable sites around Scotland.