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During the summer months of 1991 and 1992 an increased mortality of sea trout Salmo trutta m. trutta (L.) and brown trout Salmo trutta m. lacustris (L.) occurred at a freshwdter fish farm in northwestern Finland. Mortality began when the water temperature rose above 12°C. From 1 to 6 DiphylloDothr~um dendriticun? (Nitzsch, 1824) larvae per fish were found in the atrium and in some cases in the ventricle of the heart in 63 to 86Yo of the dead l + , 2+ and 3+ sea trout and brown trout in July 1991 and from June to September 1992. The fish that died showing the typical symptoms usually had this cestode larva blocking the atrioventricular orifice. In histological sections, a massive, chronic myocarditis was evident around the larva in the atrial wall. Another typical symptom of the dead fish was a ruptured atrium of the heart. Mortality caused by D. dendriticum was of great importance to the total mortality at the farm. Low prevalences of D. dendriticum larvae were also found in the hearts of fish caught live. A few encapsulated D. dendriticum and D. ditremum (Creplin, 1825) larvae were found within the visceral cavity of 10% of these fish. The probable first intermediate hosts for D. dendrjticum in the inlet water are Eudiaptomus graciloides [Lilleborg) and Cyclops scutifer Sars. Metacercaria of Ichthyocotylurus sp. (Trematocla) were frequently present in small numbers on the ventricle wall of sea trout and brown trout and epicarditis of different grades was observed.

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The aim of this study was to obtain relevant information on larval cestode infection of brown trout Salmo trutta m. lacustris, for fish stock management purposes in the large, regulated Lake Inari in northern Finland. Compensatory stockings of brown trout have been carried out annually since the mid-1970s. A total of 209 brown trout, which were stocked at the age of 3 yr. were studied for larval cestodes in 1994 and 1995. Diphyllobothrium dendriticum was clearly dominant among the 4 cestode larval species found. The other species were D. ditremum, Triaenophorus crassus and T nodulosus. After 1 summer in the lake the prevalence of D, dendriticum infection was 75% (abundance 5.3) and 46% (abundance 0.9) m 1994 and 1995, respectively. After 3 or more years in the lake (>6+) every brown trout was infected with a mean number of about 130 larvae for both years. All organs in the body cavity were found to be infected. In addition, capsules containing D. dendriticum were found in muscles after the second summer in the lake and the prevalence of muscle infection was 73% (abundance 3.2) and 95% (abundance 7.1) in the oldest age group (26+) in 1994 and 1995, respectively. This has decreased the commercial value of the brown trout. A slight positive correlation between the number of D. dendriticum and the condition index (Fulton) of fish was found in each age group, although histological stud~es of heavily lnfected fish revealed severe chronic granulomatous peritonitis. Indications of el~mlnation of individuals most heavily parasitized with D. dendriticum were not obtained for the present material. D. dendriticum was not found in the potential prey fishes of the brown trout studied in Lake Inari: vendace Coregonus albula, whitefish Coregonus sp. and nine-spined stickleback Pungitius pungitius.

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