DAO 25:15-22 (1996)  -  doi:10.3354/dao025015

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 freshwater fish farm in northwestern Finland. Mortality began when the water temperature rose above 12*C. From 1 to 6 Diphyllobothrium dendriticum (Nitzsch, 1824) larvae per fish were found in the atrium and in some cases in the ventricle of the heart in 63 to 86% of the dead 1+, 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. dendriticum in the inlet water are Eudiaptomus graciloides (Lilleborg) and Cyclops scutifer Sars. Metacercaria of Ichthyocotylurus sp. (Trematoda) were frequently present in small numbers on the ventricle wall of sea trout and brown trout and epicarditis of different grades was observed.

Diphyllobothrium dendriticum . Diphyllobothrium ditremum . Ichthyocotylurus sp. . Sea trout . Brown trout . Heart . Hatchery . Mortality