Destructive grazing of kelp beds by sea urchins in Vestfjorden, northern Norway

Abstract

Expansive overgrazing of beds of Laminaria hyperborea resulting in barren ‘Isoyake’ bottoms dominated by sea urchins and crustose coralline algae were observed in the fjord during the summers of 1980 and 1981. The overgrazing process was studied by estimating the densities of kelp and sea urchins, by measuring sea urchin diameters, and by examining the contents of sea urchins in undisturbed kelp beds, in kelp beds undergoing destruction, and in overgrazed areas. Strongylceentrotus droebachiensis was the dominant sea urchin.capable of destroying kelp beds and preventing kelp reestablishment for an unknown number of years. Echinus esculentus participated in kelp-bed destruction but disappeared after the kelp beds were destroyed. It took about one year from S. droebachiensis infiltration to complete kelp-bed destruction. Many S. droebachiensis in the ‘Isoyake’ areas were heavily infested with an unidentified endoparasite.