SPECIES OF GYRODACTYLUS (PLATYHELMINTHES, MONOGENEA) ON SALMONIDS IN SWEDEN

Judging by our investigations hitherto of Swedish natural waters, the only Gyrodactylus species infecting salmonids are: Gyrodactylus salari s Malmberg, 1957, Gyrodactylus derjavini Mikailov, 1975, Gyrodactylus lavareti Malmberg, 1957 and one other unidentified Gyrodactylus specie s. G. lavareti was found on Coregonus lavaretus (L.) in Lake Ottsjon ( Jamtland, 1955) and the unidentified species (one specimen only) on Th ymallus thymallus (L.) in Lake Satisjaure (Lappland, 1971). Gyrodactyl us salaris was found on Salmo salar L. in two northern rivers (in one of them only on migrating smolts) and in one southern river in the Bal tic region (nine rivers investigated), and in two river systems in the Atlantic region (eight rivers investigated). G. derjavini was found o n Salmo trutta in three rivers in each region. Our results from Swedis h fish farms are: G. salaris was found in five out of 12 farms in the Baltic region and two of six farms in the Atlantic region; G. derjavin i was found in two of 12 farms in the Baltic region and one of six far ms in the Atlantic region. Thus, in natural waters as well as in fish farms, G. salaris and G. derjavini seem to have an uneven distribution . Apart from the limited number of investigations, this may be due to geographic distribution of the species, anthropochoric spreading, macr oenvironment (type of waters, other physical parameters), seasonal var iations, or life cycle dependent variations of infestation intensity. The absence of G. derjavini in rivers north of the Dalalven may depend on the geographical distribution. Migrating smolts of S. salar (three rivers) and precocious males (one river) seem to be infected more tha n smaller salmon. Fish farm (or 'culturing') dependent changes in Gyro dactylus species populations (genetic drift) are possible. G. salaris and G. derjavini are secondarily adapted to parasitism on Oncorhynchus mykiss (Walbaum), which may lead to a spread to new areas and problem s in fish farms. Taxonomically, G. salaris and.probably G. derjavini a lso, cause problems. For example, are G. salaris and Gyrodactylus thym alli Zitnan, 1960 really different species, and how large is the range of variation in G. salaris and G. derjavini? With regard to similar s pecies, complementary morphometric, experimental infection investigati ons and electrophoretic (DNA) analysis are recommended.