Environmental factors and commercial harvesting: exploring possible links behind the decline of the kelp Laminaria digitata in Brittany, France
Résumé
Over the last decade, the commercially harvested quantities of the kelp Laminaria digitata have decreased along the coast of Brittany, falling from an annual production of around 60,000 t to the current production of 50,000 t Environmental parameters and the intensity of harvesting are the main potential sources of this decline. To analyse the impact of the environment, several environmental factors such as temperature, light intensity and storm intensity were included in multivariate analyses to explore their potential effects on standing stock variables (density and biomass of L. digitata populations in north Finistere). Results show relatively limited temporal fluctuations in biomass and density, although density has generally increased since 2000. Biomass appeared to follow a periodic cycle of 12 years, but could not be linked to any environmental factor. Temperature seemed to have the largest effect (negative when it increases), although this effect was masked during the last years of the study period. Analyses on harvest data revealed that the lower landings of L. digitata were not correlated to a decrease in the available algal stock but to insufficient harvesting effort. This may be due to a decrease in the number of days spent harvesting kelp and in the number of kelp-harvesting vessels. While the production per vessel has increased since 2000, this increase was insufficient to increase or even maintain total production.