Population dynamics of Strongylocentrotus droebachiensis in kelp forests and barren grounds in Norway

A northward trend of declining abundance of sea urchins (Strongylocentrotus droebachiensis) and increasing kelp recovery along the coast of Norway has provided an opportunity to explore whether the decline of sea urchins can be explained by systematic variations in demographic or fitness-related traits. The population density of sea urchins in mid-Norway (65A degrees 70'N) close to the boundary between barren grounds and recently recovered kelp forests was only 26 % of that on barren grounds in northern Norway (70A degrees 70'N). Populations were dominated by young (2-4 years) individuals and age-specific numbers decreased nearly exponentially with age. The variation in most demographic variables was high among replicate sampling sites and test size. Mean age and growth rates did not differ consistently among sites in mid- and northern Norway nor between barren ground and kelp sites. In contrast, mortality rates in mid-Norway were 50 % higher than in northern Norway. We suggest that increasing predation pressure on sea urchins partly explains the higher mortality in mid-Norway. Recent studies have shown that recruitment of sea urchins is significantly lower in mid-Norway than in northern Norway. The lower mean density of sea urchins, and hence the progressing recovery of the kelp forests in mid-Norway, may thus result from a combination of recruitment failure and increasing mortality.