Aspects of the biology of diplostomid metacercarial (Digenea) populations occurring in fishes in different localities of northern Finland.
Diplostomid metacercariae were found at a high prevalence and in large numbers in the eyes of a wide variety of fish species from different freshwater and brackish water environments in northern Finland. The parasites were found in 21 of 25 fish species, some of which were marine, from the oligohaline brackish waters of the Bothnian Bay, Baltic Sea; in all 9 fish species from a large oligotrophic lake in northeast Finland (Yli-Kitka); and in all 5 fish species from a small hypereutrophic lake (Kuivasjärvi). Diplostomids in the lens of fishes from the Bothnian Bay, studied during 2 periods extending over 7 years, suggest the presence of a stable and predictable system despite the extremely narrow transmission window between the first (snail) and second (fish) intermediate hosts. Diplostomids in the lens of fishes from the Bay and Lake Kuivasjärvi predominated (>50% infection in 14 of 21 species) over those from the vitreous body of the eye in most cases, but were much lower (<18%) in Lake Yli-Kitka. The situation in the vitreous body was the reverse since diplostomids from this site predominated over those in the lens in fishes from Lake Yli-Kitka (>80% prevalences in 5 of 9 fish species). This difference is thought to be related to differences in the piscivorous bird fauna. The accumulation of diplostomid metacercariae in the fish, together with the lack of marked seasonal variation, confirms the longevity of these larvae in their fish intermediate hosts. These metacercariae form a reservoir which is mainly responsible for the maintenance of diplostomid suprapopulations in northern Finland.