Dactylogyrus (Monogenea) species were studied on the gills of roach (Rutilus rutilus) from a total of 293 fish in 3 interconnected lakes in Central Finland. One of the lakes is eutrophic and polluted by a paper and pulp mill, one is eutrophic and one is oligotrophic. Nine Dactylogyrus species were found. The structure of the monogenean communities was analysed at the component and infracommunity level and the Dactylogyrus fauna of the lakes was compared. Although it was found that the component communities tended to be very similar there were also differences. The abundance and diversity were highest in the polluted lake; the infracommunities of this lake were considerably more often dominated by Dactylogyrus micracanthus and the similarities between the infracommunities were higher within the lakes than between the lakes. It is suggested that these differences are due to the differing water quality of the lakes. The nature of the monogenean community, its possible interactivity, is also discussed.

The Dactylogyrus fauna was studied from the gills of 293 roach between February 1988 and April 1989. Roach were caught from three interconnected lakes in Central Finland. Nine Dactylogyrus species were found. Of these the seven most abundant were used for studying the abundance of species on different gill-arches, niche breadth and niche overlap between the species. It was found that all species had a species-specific preference for certain gill-arches. The total abundances of all the species went through distinct seasonal changes, peaking during the spring and early summer. Some seasonal variation was also exhibited in gill-arch preferences. Niche breadth and species overlap were mostly higher during the period of peak occurrence of the various species, but in the case of some species they appeared to be unrelated to abundance, indicating possible niche restriction by other species. Most significant was the restriction of D. micracanthus to the fourth gill-arch during the period of peak occurrence of D. similis. Experimental work is needed to verify the reasons for variations in the microhabitat distribution of certain Dactylogyrus species.

The formation and development of monogenean communities on the gills of roach fry was followed in 1992 from early June to October (size range 9 to 47 mm). Roach fry (n=291) were sampled weekly from the small, humic River Rutajoki in central Finland. A further 209 roach fry were reared in a fish farm supplied by water from the river. Four Dactylogyrus species were found: D. nanus, D. crucifer, D. micracanthus and D. suecicus. Other species found on the gills were Gyrodactylus sp. and Paradiplozoon homoion. The first Dactylogyrus juvenile occurred on a 12 mm long fish fry in late June and the first adult (D. nanus) 1 week later in Tank 1. D. nanus was also the most common parasite in the river. Young fry had high numbers of Gyrodactylus sp. on the gills compared with adult roach in previous studies. Signs of site preference were found; D. nanus and D. micracanthus preferred the 2nd gill-arch and Dactylogyrus juveniles the outmost one. Abundances of monogenean infracommunities increased until the middle of August when, on average, 1·9 species and 3·7 worms per fry were noted and decreased then as the water temperatures fell. A lack of competition between species is suggested; for example, D. crucifer appeared on the gills at that time when abundance of D. nanus was at its highest. The roach fry hatched and reared in the fish farm revealed that larvae of the most common dactylogyrid species were able to disseminate and colonize over 120 m from the river to the farm. Influence of change colonization events was shown by the appearance and development of Gyrodactylus sp. population on roach fry in only 1 of the 2 tanks at the farm.

Experimental exposure to bleached kraft pulp and paper mill effluent (BKME) at a concentration of 10% significantly reduced the abundance and mean number of species of Dactylogyrus on the gills of naturally infected roach, Rutilus rutilus, over a 3 week period. Seven Dactylogyrus species were recorded which differed widely in their susceptibility to the effluent. The experiment coincided with a natural spring peak in dactylogyrid infections on roach. BMKE exposure did not prevent parasite reproduction but post-larval abundance was significantly higher on control than effluent-exposed hosts. There was no evidence of a BKME-induced shift in microhabitat distribution of dactylogyrids. Elevated levels of infection with Dactylogyrus spp. have been recorded on roach from a Finnish lake containing relatively low concentrations of BMKE. The high exposure concentration in this experiment produced a similar reduction in dactylogyrid infections to that reported in separate studies in close proximity to Swedish pulp and paper mills. The discrepancy between the results of the Finnish and Swedish field investigations is therefore considered to be due to differences in BKME concentration between the study areas.