A total of 1255 roach Rutilus rutilus (L.) and 866 perch Perca fluviatilis (L.) from four interconnected lakes in Central Finland differing in trophic status and pollution level were studied for parasitic ergasilid copepods between August 1985 and December 1988. In addition, 109 whitefish (Coregonus sp.) were studied from one of the lakes.
Four ergasilid species were found: (the prevalence and intensity/fish, respectively, for the whole material are given in parentheses) Ergasilus briani (16·9%, 0·5), Neoergasilus japonicus (15·6%, 0·4) and Paraergasilus longidigitus (2·1%, 0·02) on the roach and Ergasilus sieboldi (9·9%, 0·1) and P. longidigitus (4·9%, 0·05) on the perch. Logit analysis was used to assess the significance of the year, the lake and the season for the prevalence of infection. Generally, the prevalence of E. briani on roach depended on the season only, with higher prevalences in summer. That of E. sieboldi on perch was mainly dependent on the lake, being most frequent in the oligotrophic lake. Both lake, year and season affected the prevalence of N. japonicus in roach. The more complicated system for N. japonicus was explained by the long duration of its free-living stage, its loose attachment and different location on the host, being found on the fins whereas E. briani and E. sieboldi were on the gills.
All species were found throughout the year, and age had no clear influence on infection. The occurrence of egg-sacs and length distributions of the copepods suggest that there are two generations a year, e.g. an overwintering E. briani generation which produces offspring in June to attack the fish in July, and a second generation which attacks the fish from August onwards. Whitefish were not infected by these parasites, only one fish being found which carried a single individual of E. sieboldi.