Monogeneans and other parasites on the gills of roach (Rutilus rutilus) in Central Finland : differences between four lakes and the nature of dactylogyrid communities

Abstract

1173 roach were studied between August 1985 and April 1989 for gill parasites. The entire material was used to analyse the occurrence of Paradiplozoon homoion on the gills, while gyrodactylid monogeneans on the gills and fins were studied only during the years 1986-87 (767 fish). The dactylogyrid monogenean fauna was studied during 1986 and 1988 (660 roach) and in relation to the community level, during 1988 and 1989 (293 roach). For the material from 1986 the occurrence of other gill parasites was also analysed. The material was collected from four interconnected lakes, one of which is eutrophic and polluted due to a paper and pulp mill, one of the lakes is oligotrophic and in a natural state and two are eutrophic. The prevalence of P. homoion infection was found to be lowest (1.9%) in the polluted and highest (8.5%) in one of the eutrophic lakes. The abundance was low and no clear seasonal variation was found. Worms with eggs were found from June to September. The proportion of larvae of P. homoion was largest on the fourth gill arch, although adults of this parasite was significantly more common on the first arch. The co-occurrence of gill parasites of roach was found to be random, except for P. homoion and Ichthyophthirius multifiliis, which were significantly positively associated. Four Gyrodactylus species were found, of which G. prostae was the most common in all of the lakes. The total Gyrodactylus infection was 8.9-17% on the fins and 6.5-13.3% on the gills of roach. These monogeneans showed a clear seasonality, with peaks in both prevalence and abundance at the beginning of June. The differing water quality of the lakes is suspected to influence the abundance and site of infection of the gyrodactylids. Dactylogyrid monogeneans also had a clear seasonal occurrence, but because the prevalence of infection was high throughout period of the study, the seasonality was seen in only the abundance and diversity. The abundance and diversity were found to be significantly higher in the polluted lake. Nine Dactylogyrus species were found. D. crucifer and D. nanus were found to be the core species in all of the component communities. D. suecicus, D. micracanthus, D. similis, D. fallax and D. caballeroi were secondary species with intermediate prevalences, and D. sphyrna and D. vistulae were rare satellite species. There were differences in the composition of the Dactylogyrus fauna of roach in the lakes studied. The species composition of the polluted lake changed between the two years studied to be more like that of the eutrophic lakes. When the Dactylogyrus fauna was studied at the community level it was found that the component communities tended to be very similar, but some differences were found. Infracommunities on roach in the polluted lake were more often dominated by D. micracanthus. The similarities between the infracommunities were higher within the lakes than between the lakes. The differences in dactylogyrid communities are suggested to be due to the differing water quality of the lakes. All seven of the most abundant dactylogyrid species had species-specific preferences for certain gill-arches. These preferences showed some seasonal variation. For some Dactylogyrus species niche breadth and species overlap appeared to be unrelated to abundance, indicating possible niche restriction by other species. It is suggested that Dactylogyrus communities may be interactive in nature during the peak occurrence of the species.