Mapping the intertidal seaweed resources of the Outer Hebrides

Executive Summary The Scottish Association for Marine Science (SAMS) in association with the Hebridean Seaweed Company (HSC) were commissioned by Scottish Enterprise and Highlands and Islands Enterprise in March 2010 to assess the intertidal seaweed resources of the Outer Hebrides, as comprised by the rockweed Ascophyllum nodosum. The project was divided into two areas: (1) Engagement and consultation with SEPA, SNH, Crown Estate and other key stakeholders involved in licensing seaweed harvesting in Scotland to ascertain the restriction and requirements for measuring seaweed stocks and sustainable harvest. The purpose of this engagement was to develop a robust field methodology agreed with key stakeholders. (2) A mapping exercise to deliver a step-based approach to levels/availability of harvest, accounting for economies of scale and identifying opportunities and new areas for harvest justifying investment in infrastructure and equipment. The agreed field methodology was used in shore surveys between April and October 2010 and combined with habitat modelling to deliver map-based estimates of the biomass of rockweed. The approach is flexible enough to allow estimates of biomass in other areas of Scotland with relatively little extra survey work. SAMS/HSC 2010 estimates for the Outer Hebrides coastline ranged from 154000 to 185000 metric tonnes (t) depending on the data and assessment method used. These values were close to the 1947 estimate of 123000t made by the Scottish Seaweed Research Association. 90% of rockweed biomass was in high yield harvestable areas (>60t/km). The largest percentage of the best estimate of total biomass (170000t) was predicted to be found on Lewis (69000t, 41%), followed by North Uist (38000t, 22%), South Uist (32000t, 19%) and Harris (27000t, 16%), Barra having very little (2%). On the basis of accessible resources, within 3km of landing sites, the present harvesting industry can access 61000 tonnes of Ascophyllum, 36% of the total. Most of this is in Lewis (33000t), with the remainder almost equally divided between Harris (12000t) and North Uist (13000t). With a larger range of landing sites, this could rise to 97000t or 59% of the total available. For a sustainable harvest, areas should be left for at least four years between cutting events to allow regrowth. Faster recovery is possible if longer plants are left by harvesting (>25cm), at a cost of reduced harvests (50%). The sustainable annual harvest should be no more than 25% of the total accessible biomass (15000t for the current industry, and 25000t if a larger network of landing sites were developed). Stakeholders did not raise any specific issues with the proposed and adopted protocol. Where interested parties expressed an opinion, this was that full details of the methods used should be made available. Stakeholders also made their views known on the further development of intertidal seaweed harvesting in the Outer Hebrides. Opinions ranged from a desire for less regulatory interference in the harvest through to a strong need to protect natural habitats and species from potential damage. An accurate assessment was welcomed as a good first step for clear and rational management of the resource.