Laminaria hyperborea harvesting

Early assessments of impact

More recent impact studies of harvesting on kelp populations

The impact of harvesting on associated flora and fauna

The impact of harvesting on other species and beyond the kelp beds

The stipes of kelp plants cast ashore after storms have been collected commercially for many years as part of a mixed coastal economy in France, Ireland, Scotland and Norway. More recently, methods of dredging L. hyperborea on a commercially viable scale have led to this kelp being harvested under strict regulatory conditions in Norway and in Brittany (Arzel, 1996), and harvesting trials were carried out in Scottish waters in 1991 (H.T. Powell, pers.comm.). The commercial harvesting of L. hyperborea in French coastal waters was proposed only in 1995-6, as the local industry had previously concentrated on the collection of L. digitata (Arzel, 1996) and consequently, no impact assessments of harvesting have yet been published.

Early assessments of impact

Norway

The earliest assessment of the effect of kelp dredging in Norway was that of Svendsen (1972). He studied kelp beds at depths of 4 to 10 m over periods of up to 3 years after harvesting. These areas were quickly overgrown with new plants of L. hyperborea. Within 1 year the population was dense and well-developed. Although he regarded the beds as completely regenerated after 3-4 years in terms of Laminaria biomass, the individual plants were only half the height (about 1 m.) of the former mature plants (about 2 m tall). The re-grown biomass was made up by the greater density of smaller plants. From an industrial point of view, the stipes of these new plants were of better quality for alginate extraction because they were less contaminated by epiphytes. From an ecological point of view, even after 3 years, the disturbed biotope was species-poor in comparison to an undisturbed habitat. As in the manipulative experiments described earlier, the forest may regenerate sufficiently after 3-4 years to be harvestable again but it is certainly different in structure, both as regards the kelp plants and the subsidiary flora and fauna. A system of rotation of harvested areas was introduced by the Norwegian government to ensure that each area of kelp forest was harvested only once in 4 years to allow for regrowth of the Laminaria plants. It has since been recommended that this time scale be extended to 7-10 years to allow for the partial recovery of populations of non-kelp species.

More recent impact studies of harvesting on kelp populations

Norway

Sivertsen (1991) has compared the regrowth of kelp in areas trawled 1 - 5 years previously with areas freshly trawled and control areas. Large canopy-forming plants were absent until the fourth year after harvesting, but the structure of the kelp population was beginning to stabilise with little change in plant density between years 4 and 5. The age structure of the re-grown areas showed downwardly skewed age distributions in comparison to control areas. At 4 years after harvesting, kelp plants had only reached 2/3 of full before-harvesting canopy height in the re-grown areas. Sivertsen suggested that harvesting should occur at 6 or 7 year intervals to match the natural growth and recruitment cycles in the kelp population. In addition, the post-harvesting growth data showed that the re-establishment of kelp in harvested areas was primarily dependent on the growth of viable individuals remaining after harvesting. Climax kelp communities, dominated by canopy individuals, provide poor conditions (e.g. light) for new recruitment of sporophytes to the population, so presumably the new sporophytes that grow into the population were present prior to harvesting but were small enough to escape damage. The harvesting dredge used in Norway is designed to leave behind the small kelp plants, only collecting those of canopy height (H.T. Powell, pers. comm.).

A further interesting observation in Sivertsen's report is the temporary post-harvesting replacement (for one year only) of the L. hyperborea dominated forest with a population of S. polyschides as in the clearance experiments by Kain (1975).

The impact of harvesting on associated flora and fauna

Norway

In a separate survey that was primarily directed at the effects of kelp harvesting on other common organisms in the kelp biotopes (Rinde et al., 1992), the forest structure seemed to recover to something approaching normal after 3-4 years, but with persistent differences from an undisturbed forest. Rinde et al. (1992) argue that the forest may be re-harvestable after 4 years but that it does not provide the same physical environment for the other organisms which it shelters. They concentrated on the species thought to be most closely linked to the kelp and considered epiphytes, holdfast fauna and bottom fauna and flora separately. Their report contains a wealth of detail of the species present and the numbers within different animal groups.

The epiphyte community developed in complexity with the increased age of the host stipe. The plants in control areas, at about 10 years old, have a much richer and more extensive epiflora than the younger, replacement plants found in previously harvested areas. The development of a diverse epiflora was deemed to be a desirable feature of kelp biotopes because the physical structure of the epiphytes provides a habitat niche for species of amphipods, isopods, gastropods and small fish.

The epifaunal species present on the kelp stipes were not as diverse as the epiflora, usually consisting of several species of crust-forming bryozoans. Other animals were found only on the older plants; shrubby erect bryozoans and the sponge Halichondria sp. on 10-year-old plants, and tunicates on the 6 years post-harvest population. The full development of the epiphyte community of plants and animals seems to need much longer than the advised 4-year interval between trawling.

The holdfast fauna is richer in both species and numbers of individuals for 10-year-old plants from the control area than for younger plants from previously harvested areas. A physically obvious difference between the younger and older plants was the development of large numbers of the large tubeworm, Filograna, which forms a visually obvious feature only on the holdfasts of the older plants. As the kelp plants become older the area and the volume occupied by the holdfast increase, with an apparently related increase in the numbers of individual animals, and also in both the biomass and biodiversity of the holdfast fauna. Various larger species were found associated with the holdfasts: shrimps, lobsters, Hyas sp., Cancer sp., hermit crabs, Echinus esculentus and Strongylocentrotus droebachiensis. These species were absent from recently dredged areas and well established populations appeared only in the undisturbed kelp forest, suggesting that full biological restoration after harvesting may take at least 10 years.

Benthic macrofauna and macroflora were more diverse in the control area (51 species) than the recently dredged area (21 species). The dredged areas tended to have growth of other kelps on the bottom, e.g. Alaria esculenta, and also Desmarestia spp., while the bottom between the young L. hyperborea plants was uniformly covered with coralline algae after 3 years. In the control areas, there was a more diverse bottom community. The coralline algae were still a significant part of the bottom cover but were joined by species of cnidarians, bryozoans and sponges.

The impact of harvesting on other species and beyond the kelp beds

There may be consequences for kelp ecosystem components other than those directly associated with the kelp, including fish and lobsters, but these have not yet been investigated in Europe. Sivertsen (1991) suspected that kelp harvesting could have been the cause of dune erosion from adjacent areas. The removal of the kelps reduced the drag caused by the kelp beds, which had had a wave-damping effect. Higher energy waves reaching the adjacent sandy shores and their dune hinterland resulted in increased rates of sediment removal. In the United States, beds of artificial kelp have been used to prevent sand erosion from beaches.

Thrush (1986) refers to the importance of accumulations of laminarial detritus on the seabed of Lough Hyne, County Cork. The POM (particulate organic matter) had a significant input to the energy web of the benthic macrofauna of soft bottoms. The export of both POM and DOM (dissolved organic matter) would be significantly affected by kelp harvesting but the effects of this reduction have not been investigated.

Next section                     References