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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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Summary: A fungal swimbladder infection caused by Verticillium lecanii was diagnosed in one‐year‐old Baltic salmon. The fungus was easily isolated on nutritionally poor basal medium and Sabouraud glucose (2%) medium at room temperature. The colonies were white to cream‐coloured and cottony. The phialides were solitary and rose erect from the conidiophores. Verticillium conidiophores were also seen. Conidia of size 2.5–5.0 × 1.0–25 μm and mainly ellipsoidal were found in slimy groups on top of the phialides. Haemorrhagic swimbladder inflammation with sloughing of the epithelium was seen histologically. Hyphae were evident in the tissue and secondary bacterial infection was also found. The infection occurred in a period of extremely low water temperature (<1°C) when primary bacterial infections are seldom seen. Zusammenfassung: An der Schwimmblase von einjährigen Zucht‐Lachsen wurde eine Pilzinfektion durch Verticillium lecanii diagnostiziert. Der Pilz ließ sich leicht bei Zimmertemperatur auf einem nährstoffarmen Basal‐medium wie auf Sabouraud‐Glucose‐Agar (2%) anzüchten. Die Kolonien waren weiß bis cremefarben und flockig. Solitäre Phialiden wuchsen aufrecht auf den Konidiophoren. Wirtelige Konidiophoren warden ebenfalls beobachtet. Vornehmlich ellipsoidische Konidien der Größe 2,5–5,0 × 1,0–2,5 μm waren in Schleimhäufchen an der Spitze der Phialiden angeordnet. Histologisch wurde eine hämorrhagische Schwimmblasenentzündung mit Epithelablösung gefunden. Im Gewebe zeigten sich Hyphen wie auch eine sekundäre Bakterieninfektion. Die Infektion trat bei extrem niedriger Wassertemperatur auf (< 1°C), bei der primäre Bakterieninfektionen selten beobachtet werden.