MACROPARASITIC EPIZOOTIC DISEASE - A POTENTIAL MECHANISM FOR THE TERMINATION OF SEA-URCHIN OUTBREAKS IN NORTHERN NORWAY

Outbreak populations of the green sea urchin, Strongylocentrotus droebachiensis Muller, in Vestfjorden, Northern Norway, are infected by the endoparasitic nematode, Echinomermella matsi Jones and Hagen, 1987. The prevalence of E. matsi has increased from 5.5% in 1983 to 65.4% in 1991 at a study site in Godoystraumen. Infected sea urchins had a lower mean gonad index than non-infected sea urchins, and two thirds of the infected sea urchins were reduced to virtual castrates of unknown sex. The mean density of the S. droebachiensis-population in Godoystraumen appeared unchanged, but the mean test diameter had decreased by 14.6%, from 36.4 mm in 1983 to 31.1 mm in 1991. As a result of these changes the reproductive capacity of the sea urchin population appears to have been reduced by approximately 58%. The data analysis was augmented by the use of a PEPLOM (PErcentile PLOt Matrix), which appears to be a novel combination of two powerful methods of graphical data analysis, i.e., the percentile comparison graph and the scatterplot matrix (SPLOM). Changes in the size-frequency distribution, and in the size-dependent pattern of parasite infection in the S. droebachiensis-population in Godoystraumen, are consistent with a hypothesis of increased parasite-related mortality. The larger hypothesis, that E. matsi may function as a macroparasitic terminator of sea urchin outbreaks in northern Norway, could not be rejected.