Occurrence of cestode larvae in brown trout after stocking in a large regulated lake in northern Finland

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) in 1994 and 1995, respectively. After 3 or more years in the lake ( greater than or equal to 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 ( greater than or equal to 6+) 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 studies of heavily infected fish revealed severe chronic granulomatous peritonitis. Indications of elimination 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.