Eutrophication, pollution and fragmentation: effects on the parasite communities in roach and perch in four lakes in central Finland.

Parasite communities in the four study lakes reflected the influences of habitat fragmentation, pollution and eutrophication. Discriminant analysis of communities at the individual host level reveal two major axes. One, characterized by reduced numbers of digeneans and myxosporeans and increased numbers of acanthocephalans and monogeneans, contrasts communities in a lake affected by chemical pollution from a pulp mill with two eutrophic, less polluted lakes. Changes in the density of intermediate hosts, direct effects on ectoparasites and impaired immune systems were regarded as important mechanisms. The second contrasts communities in an oligotrophic, unpolluted lake with the two eutrophic lakes, and was more complex, reflecting habitat fragmentation, and pollution or eutrophication, probably mediated by the same mechanisms as above. Monitoring easily seen discriminating parasites following 8 years of reduced pollutant loading showed some, but not all, of the effects of pollution could be reserved in a relatively short time.

Relationships between the parasites of some wild and cultured fishes in two lakes and a fish farm in central Finland.

A total of 526 Atlantic salmon and 500 brown trout from a fish farm were studied for parasites, as were 272 roach, 251 perch and 150 whitefish from the lake which formed the water source, and 196 roach and 136 perch from the effluent recipient lake. The cultured fish harboured 14 parasite species, of which most were protozoan generalist parasites. Nine species were found on salmon and 12 on brown trout. Epizootically the most important species were Ichthyobodo necator, Chilodonella cyprini and Ichthyophthirius multifiliis. The mean number of species per studied fish was 0.71 for salmon and 0.61 for brown trout. The seasonal occurrences of the ectoparasites were found to be irregular due to preventative bathing of the fish. The wild whitefish harboured 19 parasite species, roach 41 and perch 33. The proportions of the parasite species common to both wild and cultured fishes were 17% of the species from roach, 36% of those from perch and 47% of the species from whitefish. Jaccard's similarity index, indicating similarities between wild and farmed fish, was highest between whitefish and brown trout. It was concluded that the source of parasites in the fish farm is the water supplying lake, but the farm itself was unlikely to effect the fish parasite fauna of the water recipient lake, although some ectoparasites, which had high prevalences in this lake, could originate from the farm. The high standard of maintenance and hygiene in this farm kept problems caused by parasites under control.

Environmental factors affecting monogeneans parasitic on freshwater fishes.

The environmental factors that act on fish parasites have received increasing attention in recent years. The effects of changing environmental conditions on parasite communities can provide valuable information on the effects of pollution on the ecosystem as a whole. This, of course, demands that we have a detailed knowledge o f parasite communities and the mechanisms acting upon them in a variety of natural conditions. Here, Mari Koskivaaro looks at those environmental factors that affect monogeneans parasitic on freshwater fishes.

Dactylogyrids on the gills of roach in central Finland: features of infection and species composition.

Monogenean parasites were examined from the gills of 660 roach (Rutilus rutilus) in four interconnected lakes in Central Finland between February and November 1986 and in three of the same lakes between February and December 1988. One of the lakes is eutrophic and polluted due to a paper and pulp mill, one is oligotrophic and in a natural state, and the other two lakes are eutrophic. The prevalence of Dactylogyrus infection was always high. Differences between the lakes and the years were observed in the intensity of infection, which was significantly higher in the polluted lake. The intensity was also higher in older fish. Nine Dactylogyrus species were found, and of these D. crucifer and D. nanus were numerically dominant in all of the lakes studied, especially in the oligotrophic lake. In 1986 D. fallax, D. similis and D. suecicus constituted significant proportions of the fauna in the polluted lake. D. micracanthus was most common in the eutrophic lakes. In 1988 the species composition in the polluted lake was most similar to that in the eutrophic lake.