ABSTRACT
For more than 20 years grazing by the green sea urchin (Strongylocentrotus droebachiensis) have reduced former luxuriant kelp (Laminaria hyperborea) forests to barren grounds along the mid-Norwegian coastline. Following observations of unexpected severe population reductions of S. droebachiensis at Vega, we questioned if such mortalities could be related to the presence of the endoparasitic nematode Echinomermella matsi. Our survey from Frøya to Harstad (700 km aerial line) exceeded the known distribution range of E. matsi. In the south we registered a dramatic reduction of S. droebachiensis compared to two years earlier, and observed expansion of kelp into former barren ground areas. In central areas we found a notable decline in the density of green sea urchins and signs of macroalgal regrowth. In the north, however, high densities of S. droebachiensis occurred at all sites. E. matsi was not recorded in the southern area, whilst S. droebachiensis was heavily infested by E. matsi in central areas. In northern areas E. matsi was only found at a few sites. Green sea urchin mortality in the absence of E. matsi in the south suggests that other insofar unidentified agents were involved. The pattern of parasite occurrence elsewhere indicate that the unidentified agents strongly contribute to urchin mortality and that the parasite probably not might terminate mass occurrences of green sea urchins.
