Regrowth after kelp harvesting in Nord-Trøndelag, Norway

Abstract

In 2010, harvesting of kelp (Laminaria hyperborea) was performedfor the first time in the county of Nord-Trøndelag, Norway.Underwater video surveys were performed just prior to harvestingand then on a yearly basis over a 4-year period to document thedevelopment of the kelp vegetation and sea urchin abundances. Inaddition, kelp plants were collected before harvesting and thenagain 4 years later for measurements of plant morphology, age,cortical growth and epiphytes. After 4 years, L. hyperborea hadregained its dominance at the harvested sites, however, plantage, sizes and epiphytes were still below pre-harvesting levels.However, the kelp biomass appeared restored 4 years afterharvesting, because of the high density of the recovering kelpvegetation. Furthermore, measurements of the cortical growthzones in transverse section of the stipes of the recovering kelpplants showed a higher growth rate during the post-harvestingperiod, than in plants from pristine kelp beds. Abundance of seaurchins was low during the entire monitoring period and grazingeffects on the recovering kelp vegetation, at the harvestedsites, appeared to be small. Interestingly, the age structure ofplants collected 4 years after harvesting showed that kelprecruits present as understory vegetation prior to harvesting,must have contributed substantially to the restocking of kelp.However, the density of understory kelp recruits 4 years afterharvesting was significantly lower than it had been prior toharvesting, and this may lead to a slower recovery if futureharvests occur before the stocks of understory kelp recruits arerestored.