POPULATION REDUCTIONS OF STRONGYLOCENTROTUS-DROEBACHIENSIS (ECHINODERMATA) IN NORWAY AND THE DISTRIBUTION OF ITS ENDOPARASITE ECHINOMERMELLA-MATSI (NEMATODA)

Observations of sea urchin Strongylocentrotus droebachiensis (O. F. Mu ller) mass mortality at Vega island in mid Norway, and reduced sea urc hin density in another area 280 km (aerial line) to the south, initiat ed a more extensive survey of sea urchin and kelp Laminaria hyperborea (Gunn.) Foslie forest distribution along 700 km of coastline. Records of the distribution and prevalence of the nematode Echinomermella mat si Jones and Hagen, 1987, an endoparasite of S. droebachiensis, were a lso included as the parasite has been postulated to cause sea urchin m ass mortality. Sampling was concentrated in 5 areas of coastline along mid and northern Norway. Each area was studied along a wave exposure gradient. Old kelp forests have persisted along the outermost exposed parts of the coastline despite the 20 yr population outbreak of sea ur chins. Inshore of the exposed kelp zone and towards the mainland, sea urchin dominated barren grounds still persisted. The distribution and abundance of S. droebachiensis in the southernmost area (Froya) was gr eatly reduced, and expansion of kelp into former barren grounds result ed. Reduced numbers of sea urchins and some macroalgal regrowth also o ccurred in the 2 central areas 140 and 280 km north of Froya, whilst h igh population densities of small sized S. droebachiensis on 'barrens' still dominated the inner zone in the 2 northern areas. The study ind icated that S. droebachiensis populations, and hence the barren ground state, may be in a labile phase along the southern 300 km of the coas t, whilst its slate in the northern areas seems more persistent thus f ar. E. matsi was not found in the southernmost area. S. droebachiensis was heavily infested in the 2 central areas. In northern areas E. mat si was found only at a few sites. The pattern of parasite occurrence a nd sea urchin regression makes the role of E. matsi in this process qu estionable and indicates that 1 or more unidentified waterborne agents have been involved.