Size, age, growth and mortality of the green sea urchin Strongylocentrotus droebachiensis in Vega and Hammerfest, Norway.

Since the late 1960’s the green sea urchin Strongylocentrotus droebachiensis (O.F. Müller)has grazed down large areas of kelp forest from the county of Trøndelag to the Russian border. This comprises a coastal area of more than 2000 km2 with a large proportion of barren areas which are characterized by bare rock, gravel and other earthen material and sparse or no vegetation present. Recent investigations indicate that in the southern areas the kelp forest is returning. This means that the numbers of urchins are likely declining (such as in Vega). On the other hand, the grazing is unaffected in the more northern part of Norway (such as in Hammerfest). The main goal of this project was to study the size, age, growth and mortality of sea urchins in a region where the population is declining (Vega) and compare this with a region with no evident population reduction (Hammerfest). Urchins were sampled from three barren stations and three kelp forest stations (200 individuals from each station) in the heavily grazed northern part of the barren ground area (Hammerfest at 71ºN), and from three barren ground stations from the southern part of the barren ground area (Vega 65ºN). In the Vega area, no sea urchins were found inside kelp forests. These samples were shipped alive to the laboratory where size and age (growth rings) of the sea urchins were determined. Further analyses were performed to determine growth and mortality. The results indicated that the populations on kelp beds reached larger sizes, were younger and had a higher rate of growth than on the barren habitats within Hammerfest. There was also found that between barren grounds S. droebachiensis in Hammerfest reached larger sizes, were younger and had a higher rate of growth than on the barren habitats in Vega. However, there was no evidence for mortality to be different between habitats and areas. It was concluded that: 1) size and growth for different populations varied significantly between stations in Hammerfest, 2) age structure was similar between stations in Hammerfest but was significantly different within barren grounds in Hammerfest and Vega, 3) no differences in mortality were observed between habitats and areas