OCCURRENCE OF CESTODE LARVAE IN BROWN TROUT AFTER STOCKING IN A LARGEREGULATED LAKE IN NORTHERN FINLAND

The aim of this study was to obtain relevant information on larval ces tode infection of brown trout Salmo trutta m. lacustris, for fish stoc k management purposes in the large, regulated Lake Inari in northern F inland. Compensatory stockings of brown trout have been carried out an nually since the mid-1970s. A total of 209 brown trout, which were sto cked at the age of 3 yr, were studied for larval cestodes in 1994 and 1995. Diphyllobothrium dendriticum was clearly dominant among the 4 ce stode larval species found. The other species were D. ditremum, Triaen ophorus crassus and T. nodulosus. After 1 summer in the lake the preva lence of D. dendriticum infection was 75 % (abundance 5.3) and 46% (ab undance 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 t he body cavity were found to be infected. In addition, capsules contai ning D. dendriticum were found in muscles after the second summer in t he 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 eq ual to 6+) in 1994 and 1995, respectively. This has decreased the comm ercial 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. Indi cations,of elimination of individuals most heavily parasitized with D. dendriticum were not obtained for the present material. D. dendriticu m was not found in the potential prey fishes of the brown trout studie d in Lake Inari: vendace Coregonus albula, whitefish Coregonus sp. and nine-spined stickleback Pungitius pungitius.