Infection of brown trout with Diphyllobothrium dendriticum procercoids.

The aims of this experimental study were to develop a practical method of controlling the number of Diphyllobothrium dendriticum procercoids delivered to a fish host; to examine the effect of different procercoid doses (3, 7 and 15) on the plerocercoid level in fish; and to examine the potential mortality caused by plerocercoids. The experiment was terminated after 2 months. The prevalence of infection at the lowest dose level, 17.2%, was statistically significantly lower than at medium and high levels, 63.3% and 56.7%, respectively. The mean intensity increased slightly along with the dosage. The contribution of D. dendriticum to the death of a few fish could not be proved. It is concluded that the present method is practical for use in larger experiments as well.

Seasonality of two gill monogeneans from two freshwater fish from an oligotrophic lake in northeast Finland.

The seasonal occurrence of Dactylogyrus amphibothrium and Discocotyle sagittata from the gills of ruffe (Gymnocephalus cernuus) and whitefish (Coregonus acronius), respectively was studied in Lake Yli-Kitka, a large oligotrophic lake in Northeast Finland. The lake, located near the Arctic Circle, is ice-covered for 7-8 months of the year. The prevalence of D. amphibothrium infection was 70.7% and remained high throughout all size-classes of fish. The length distribution and developmental stages of the worms indicated two generations per year. The overwintering generation produces a summer generation which lives for only a few weeks and matures in July. Discocotyle sagittata has only one generation yearly. Its worms mature and produce eggs in July-August and recruitment of the new generation starts in the autumn. The prevalence of D. sagittata infection varied between 40% (in January) and 6.7% (in September); smaller fish were more heavily infected than larger fish.

Occurrence of Phyllodistomum umblae (Fabricius, 1780) in the ureters of coregonids of Lake Yli-Kitka in northeastern Finland.

380 whitefish (Coregonus sp.) and 260 vendace (Coregonus albula L.) were studied from Lake Yli-Kitka in northeastern Finland during 1980-1982. 33% of the whitefish and 38.5% of the vendace were infected with Phyllodistomum umblae (Fabricius, 1780), the average intensity of infection being 4.2 and 2.3 per infected fish, respectively. No clear seasonality was observed in either the prevalence and intensity of infection or in the level of maturation and the length of P. umblae in both fish species, which points to a continuous invasion and maturation of these trematodes. The prevalence and intensity of P. umblae infection was highest in the middle-sized whitefish. Similar results were obtained when the mean number of worms per fish and the level of overdispersion (S2/x) are plotted against the length of the fish. With regard to vendace the prevalence and intensity of infection per infected fish and per fish studied increased as the fish length increased: only the variance-to-mean ratio remained constantly low. Overdispersion indices (S2/x) revealed that in the whitefish P. umblae is more overdispersed than in vendace when studied using data from seasonal periods with size-groups as homogenously infected as possible. The negative binomial distribution gave a good fit in the case of vendace and with one exception also for whitefish, and the values of parameter k are well within the limits most often found in parasitological studies.

The ecology of Crepidostomum species (Allocreadiidae: Digenea) in the salmonoids of Lake Yli-Kitka in northeastern Finland.

Crepidostomum spp. were studied from 417 whitefish (Coregonus acronius Rapp, 1854), 220 vendace (Coregonus albula L.) and 38 brown trout (Salmo trutta m. lacustris (L.)) from Lake Yli-Kitka in northeastern Finland during 1980-1983.12.7% of the whitefish were infected with Crepidostomum with a mean intensity of 4.6 worms per infected fish. The species involvedare C. farionis (Müller, 1784) and Crepidostomum wikgreni Gibson et Valtonen, 1988, which differed from C. farionis in the number and size of the eggs. Both types were encountered in the gall-bladder and intestine, but were unable to be distinguished as immature stages. 21% of the brown trout harboured C. farionis in the intestine with a mean number of 1.5 worms per infected fish. Two C. farionis specimens occurred also in the intestine of one vendace. Whitefish smaller than 150 mm did not harbour Crepidostomum specimens, but in bigger fish the prevalence and the mean intensity of infection increased to a certain limit as the fish got larger. The prevalence of Crepidostomum infection also clearly increased during the three seasons in the three size-groups studied. The seasonal pattern of Crepidostomum in whitefish of the size greater than or equal to 150 mm was variable in that the lowest values, both in the prevalence and mean intensity of infection, occurred between September and December in 1980 and between April and August in 1982. Reinvasion was found throughout the year, although the main recruitment occurred during autumn and early winter. Eggs matured between late spring and early autumn. The smallest worms occurred mainly in the gall-bladder, and most of the gravid specimens were located in the intestine. Both the variance-to-mean ratio and the low values of the parameter k of the fitted negative binomial distribution reveal the Crepidostomum specimens are overdispersed in the whitefish of Lake Yli-Kitka.