Reduksjon av tareskogen på Helgelandskysten

Abstract

Reduced densities of Laminaria have been reported from coastal waters in Sør-Trøndelag, Nord-Trøndelag and Nordland counties. A provisional study of algal vegetation and benthic fauna was made by diving west of Vega in Helgeland, Nordland county (position 65°41'N 11°43 'E), 17-19 October 1979. The study revealed a significant difference in sublittoral density of sea urchins between three locations selected for abundant, scattered and no Laminaria growth. Strongylocentrotus droebachiensis dominated over Echinus esculentus at all three locations. The lowest sea urchin density was found at the location with abundant Laminaria, and a high density of sea urchins at the location with no or scattered Laminaria. It is therefore presumed that grazing by increasing numbers of sea urchins is a major factor causing a reduction of Laminaria. The catfish (Anarhichas lupus) may be the most important predator on sea urchins in these waters, and the catfish is preyed upon by seals. It is therefore suggested that increasing local stocks of grey seals (Halichoerus grypus), may have influenced the catfish abundance, and thus allowed the sea urchins to increase with a subsequant detrimental effect on the vegetation of Laminaria.