Sea urchin outbreaks and nematode epizootics in Vestfjorden, northern Norway

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Abstract
Large-scale overgrazing of kelp beds by the green sea urchin, Strongylocentrotus droebachiensis O.F. MÜLLER, has recently occurred off the coast of northern Norway. Overgrazed ‘Isoyake’ bottoms dominated by crustose coralline algae and sea urchins have persisted for more than five years in the Vestfjord area, although decreased urchin populations and localized macrophyte recovery were observed at two nearby sites in September 1984. The adult stages of a recently discovered ovoviviparous nematode endoparasite, Echinomermella matsi JONES & HAGEN, often occupied a large portion of the perivisceral coelom of infected specimens of S. droebachiensis. No parasites were found in the co-occurring sea urchins Echinus esculentus L. and S. pallidus G.O. SARS, suggesting that E. matsi exhibits a high degree of host specificity. In 1983 parasite prevalence at seven sites in Vestfjorden ranged from 6 to 56 %. Densities of S. droebachiensis ranged from < 10 to > 100 individuals/m2, and were negatively correlated with parasite prevalence. There is evidence that high sea urchin mortality occurred in areas with a high prevalence of parasitism. This suggests that macroparasitic disease, rather than predation or microparasitic epizootics, is the critical factor terminating sea urchin outbreaks in Norway. Both intraspecific cannibalism among S. droebachiensis, and interspecific sea urchin predation of S. droebachiensis on E. esculentus, were common in recently overgrazed areas. The sea urchin S. pallidus occurred in low densities at all study sites. It had a cryptic appearance and was not active in kelp bed destruction.