A 6-year study of Thelohania contejeani parasitism of the noble crayfish, Astacus astacus L., in Lake Steinsfjorden, S. E. Norway

Abstract. During 1981-1986, the mean prevalence of Thelohania contejeani parasitism of the noble crayfish, Astacus astacus L., in Lake Steinsfjorden, S. E. Norway, was 0.29%. The crayfish population is heavily exploited. Thelohania contejeani infection levels increased significantly during the fishing season, and variations between years were more pronounced in catches after the fishing season (0.38-2.11%) compared to catches before the fishing season (0.05-0-19%). Sixty-eight per cent of the variation between years could be explained from variations in trap effort. Scuba-diving yielded significantly higher T. contejeani infection levels than simultaneous trapping with baited traps. Thus, in years with heavy exploitation, a larger fraction of non-infected crayfish are removed from the population as these are more susceptible to capture. The findings do not support the hypothesis that density and cannibalism influence T. contejeani infection.