Even well managed kelp harvesting puts substantial ecological pressure on natural kelp beds by increasing disturbance levels and removing resources from the ecosystem. Such activities are not compatible with the conservation objectives of, and should not be permitted in, NATURA 2000 sites.
In non-designated areas, well managed and controlled kelp harvesting could be envisaged but experience to date in the Irish inshore zone gives no grounds for optimism that “owned in common” resources can be managed rather than over exploited and then abandoned. Any kelp harvesting project must be carefully monitored throughout its lifetime. Possible unwanted side effects include the replacement of Laminaria hyperborea first by L. digitata and then Sacchoriza polyschides, removal of biomass necessary for other trophic levels and the elimination of older plants of all species. Despite economic limitations there is a reasonable case for encouraging research on the cultivation of kelp for harvesting rather than relying on wild populations. This case relies on the fact that kelp absorbs large quantities of nitrogen and phosphorus (1000 kg wet weight contains 15kg of nitrogen) and may be of use in removing excess nutrients. It is probable that the economic benefits of managing excess production of nitrogen in coastal waters will eventually far outweigh those of the limited commodity market for alginates that is possible using Irish wild kelp beds.
Several case studies conclude that no significant effects are apparent from routine mechanical and hand-harvesting practices in the long-term on the kelp standing stock and its associated flora (California and Macrocystis, Canada and Laminaria longricuris, Norway and L. hyperborea). Nevertheless, all studies showed a short- term effect that disappeared over time ranging from weeks (Macrocystis) to 5 years (L. hyperborea). Norwegian scientists suggested that harvesting should occur at 6 - 7 year intervals to suit the growth/recruitment cycle and to allow total re-establishment of the epiphytic communities before the next trawling season. Other studies concluded that there were insufficient data or a lack of knowledge, i.e., California and Tasmania. One study in Chile showed a shift in community structure from a kelp
dominated habitat to an Ulva dominated habitat, which was finally replaced by Gracilaria. However, this was a small-scale scientific handharvesting trial and performed in the absence of grazers and was dependent on season. Nevertheless, the same study demonstrated a positive effect on recruitment of the kelp Lessonia trabeculata. No case studies are known from Ireland except for a few investigations on Laminaria digitata and L. hyperborea conducted by the Irish Seaweed Centre. These studies provide some information about associated flora, kelp growth, biodiversity and regeneration of kelp forests after experimental harvesting. However, the trials were only conducted over a one-year period that was too short to monitor complete restoration of the kelp beds in experimentally harvested areas. Moreover, the small-scale trials cannot substitute for investigations on the impact of commercially used harvesting gear (e.g., scoubidou and seaweed dredge) and do not take geographical distribution into account and therefore a lack of knowledge remains.
It is self-evident that the harvesting of kelp has significant negative impacts on invertebrates even though recovery rates for some taxa are very short: removing the kelp by whatever means involves the removal of the epifaunal communities, more than likely some of the epibenthic species and obviously, the key stone species on which these communities depend. For this reason, introducing this sort of activity into areas that have been designated as conservation sites is regarded as inappropriate. The fact that a well-regulated and seemingly sustainable industry has developed in Norway using research data on re-colonization rates of both plants and invertebrates suggests that this sort of model may be appropriate for trial in other locations. It has to be noted, however, that these experiments were carried out to support an industry to harvest kelp. Constant removal of kelp will never allow the macrofaunal community to re-establish itself fully and it must be accepted that the licensing of such activities effectively sacrifices that area of seabed of ever becoming a natural community again, that is, until the activity stops permanently. This situation is somewhat analogous to farming practices on land whereby management practices do not allow the natural vegetation to re-colonise cleared ground. References to the ephemeral nature of kelp forests in locations outside Ireland and the role that sea urchins and top predators can have in regulating the kelp beds have not been observed in Irish waters to date. The longer cycles recorded for north west Atlantic waters need to be assessed for Irish
waters and with the postulated rise in sea water temperatures due to climate change, the extent and character of the kelp forests may be affected.
Emphasis in fisheries research has been focused on the biology of commercial species due to the obvious need to manage these resources in a sustainable manner. The development of a mechanical kelp industry in Ireland would present a challenge for fisheries management in terms of marine biodiversity as it could impact on the habitat of species that have not previously received much attention scientifically. In addition, the knowledge required for determining the impacts of seaweed harvesting on fish populations is unlike the information that has been required for fisheries stock assessment, in that the habitat of the fish population is targeted rather than the individuals themselves. This undoubtedly presents a challenge as fish populations have generally been managed in the context of a reproductively active standing stock being able to sustain itself in a healthy habitat. The removal of healthy habitat may have much wider implications for the marine ecosystem, requiring a real consideration of the ecosystem approach to management and research. The current paucity of information in relation to fish habitat associations in Ireland and Europe needs to be addressed further so that well informed decisions can be made with regard to conservation management of sensitive fish species and essential fish habitat.
Birds occupy an important yet littled studied trophic level within the kelp forest food chain. What is clear however, is that any negative ecological impacts of mechanical kelp harvesting will have direct and in-direct knock-on effects for birds. Important areas for birds are protected through the designation of Special Protection Areas (SPAs) under the EU Directive 79/409/EEC on the Conservation of Wild Birds. In general, mechanical kelp harvesting should not be conducted near important breeding bird colonies, near important sites for wintering shorebirds and waterfowl or near important staging areas for migratory birds. The ecosystem effects of mechanical kelp harvesting are likely to depend on harvesting frequency, harvesting intensity and biomass removal rates. However, given an ecosystem approach (Ugarte & Sharpe, 2001) in terms of careful management and regulation for a sustainable kelp harvesting industry, there is great potential for impacts upon birds to be minimised.