Species of Gyrodactylus (Platyhelminthes, Monogenea) on salmonids in Sweden

Abstract

Judging by our investigations hitherto of Swedish natural waters, the only Gyrodactylus species infecting salmonids are: Gyrodactylus salaris Malmberg, 1957, Gyrodactylus derjavini Mikailov, 1975, Gyrodactylus lavareti Malmberg, 1957 and one other unidentified Gyrodactylus species. G. lavareti was found on Coregonus lavaretus (L.) in Lake Ottsjön (Jämtland, 1955) and the unidentified species (one specimen only) on Thymallus thymallus (L.) in Lake Satisjaure (Lappland, 1971). Gyrodactylus 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 Baltic region (nine rivers investigated), and in two river systems in the Atlantic region (eight rivers investigated). G. derjavini was found on Salmo trutta in three rivers in each region. Our results from Swedish 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. derjavini was found in two of 12 farms in the Baltic region and one of six farms 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, macroenvironment (type of waters, other physical parameters), seasonal variations, or life cycle dependent variations of infestation intensity. The absence of G. derjavini in rivers north of the Dalälven may depend on the geographical distribution. Migrating smolts of S. salar (three rivers) and precocious males (one river) seem to be infected more than smaller salmon. Fish farm (or ‘culturing’) dependent changes in Gyrodactylus 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 problems in fish farms. Taxonomically, G. salaris and probably G. derjavini also, cause problems. For example, are G. salaris and Gyrodactylus thymalli Zitnan, 1960 really different species, and how large is the range of variation in G. salaris and G. derjavini? With regard to similar species, complementary morphometric, experimental infection investigations and electrophoretic (DNA) analysis are recommended.