A total of 654 char, Salvelinus alpinus (L.) and 33 brown trout, Salmo trutta L., were examined for the plerocercoids of Diphyllobothrium dendriticum (Nitzsch, 1824) and D. ditremum (Creplin, 1825). Only char were found to be infected. Both parasitic species were abundant. The incidence and mean intensity of infection for D. dendriticum were 83.2% and 8.8, respectively; for D. dirrernurn 93.7% and 45.0. There was no variation in the intensity of infection between different parts of the lake. The incidence and intensity of infection increased with the age of the fish to age 8+ for both species. From this age the intensity of infection leveled off or decreased. No differences in intensity of infection between the sexes were found. The frequency distribution of parasite counts were overdispersed and fitted the negative binomial distribution even within single age-groups. The two parasite species showed a high positive correlation in intensity of infection, D. ditremum was concentrated in the stomach and pyloric caeca. D. dendriticum was more widely distributed within the host and was supposed to kill heavily infected fish.

A total of 654 char, Salvelinus alpinus (L.), from Lake Bjellojaure, Sweden, were examined for plerocercoids of Diphyllobothrium dendriticum (Nitzsch, 1824) and D. ditrernurn (Creplin, 1825) from September 1969 to December 1974. Plankton samples showed that the dominating copepod species in the lake were Eudiaptomus graciloides (Lilljeborg), Cyclops scutifer Sars and Heterocope appendiculata Sars. E. graciloides passed the winter as adults and C. scutifer as copepodids while H . appendiculata was restricted to a short period in the summer. Copepods occurred in the stomachs in September-October and to a certain extent in April-May, but only cyclopids and Heterocope sp. were found. C.scutifer is proposed to be the main first intermediate host of both D. dendriticum and D. ditremum. The potential final hosts were thecommon gull, Larus canus L., for D. dendriticurn and the black-throated diver, Gavia arctica (L.), for D. ditremum. One common gull was experimentally successfully infected with plerocercoids of D. dendriticum. The parasites showed no seasonal cycle in incidence of infection.

No abstract

Growth dynamics of five stocks of Arctic charr, were compared from the fry stage in June 1986 until December 1987. Each stock originated from a different lake in Sweden: Hornavan; Nfickten; Torr/Sn; Rensj6n and Visj6n. Hornavan charr reached the highest final mean weight (237 g), but they were not significantly heavier than VisjSn charr (206 g). Growth of fish of the three remaining stocks was similar but less rapid. The superior growth performance of Hornavan charr was most pronounced during autumn. Coefficients of variation for individual size within stocks was between 30 and 54%, with Hornavan and N~ckten charr displaying least variability. Charr from Lake Visj6n had a significantly lower proportion of mature males (6%) than the other stocks (19-29%) at the end of the experiment. The results demonstrate that differences in growth potential exist between stocks of charr, and these differences may be related to differential seasonal rhythms in growth. Variations in early growth, size distributions and sexual maturation seemed to be subordinate factors affecting overall growth performance.

The growth dynamics of six different Swedish stocks (Arjeplog, Arevattnet, Bergnäs, Båthälla, Granbo. Gullspång) of landlocked brown trout were compared. The fish were kept in 1‐m2 troughs from start of feeding in spring 1988 until September 1989. Thereafter the stocks were reared together in 4‐m2 troughs until the termination of the experiment in October 1990. After the first summer of growth, Bergnäs trout had the highest mean weight and Granbo and Båthälla trout the lowest. In September 1989 the ranking of stocks with respect to mean weight had not changed. During the rest of the experiment, i.e. when stocks were reared together, Arevattnet trout increased in weight most rapidly and reached the highest final mean weight. Bergnäs trout grew very slowly after the stocks had been merged. Granbo and Båthälla trout ended up with the lowest mean weights. Sexual maturation in 1 + males was very rare. Gullspång trout had the highest proportion (42%) of mature 2+ males, while Granbo trout had the lowest proportion (4%). With the exception of Gullspång trout, mature males generally had a higher mean weight than immature fish. The experiment revealed considerable differences in overall growth, even between stocks with similar life histories in nature. Hatching time, frequency of sexual maturation and hatchery selection were identified as potential mechanisms behind the observed differences. In all, stock differences in important traits for commercial aquaculture are considerable, which makes further stock comparisons necessary before the start of breeding programmes.