Overgrazing of kelp beds along the coast of Norway

Abstract

The aim of this study was to better understand the down-grazing of kelp beds by sea urchins (Strongylocentrotus droebachiensis) along the coast of Norway. Barren grounds were first observed in sheltered areas along the coast of the counties of Trøndelag, Nordland and Troms in 1974. In the 1980s, the barren grounds spread to areas more heavily exposed to waves. In the 1990s, the kelp beds were re-established in some localities in southern Trøndelag, initially in wave-exposed areas. In the northernmost parts of Norway, i.e. the counties of Troms and Finnmark, the barren ground areas may still be increasing. Crabs (Cancer pagurus) and common eiders are the most common predators on urchins. Predation on sea urchins in kelp beds is probably not among the factors that limit the sea urchin populations. Along the coast of Nordland and further north, sea urchins are infected by nematodes, resulting in a low, but significant increase in their mortality. No re-growth of kelp beds has been found in the most infected areas. In the late 1960s and the early 1970s, a high occurrence of echinoderm larvae was observed in deeper waters. This was a period with cold water, which may have caused high recruitment of sea urchins. The bet-hedging life strategy of sea urchins may account for the sudden increase in the size of the populations. In the present paper I propose the hypothesis that higher individual growth rates and higher mortality rates in the south than in the north may explain the decrease in the populations, which may in turn account for the re-growth of kelp in the southern areas.