GYRODACTYLIDAE AND GYRODACTYLOSIS OF SALMONIDAE

21 Gyrodactylus species, described from salmonids are presented, arranged in six Gyrodactylus-groups (Table I, Figs. 6-8). Findings of the species in fish farms and/or natural waters are summarized (Table II). With data from Gyrodactylus species in natural waters in Scandinavia and in the Baltic as a background, biological, ecological and behavioural observations in natural waters of G. salaris Malmberg, 1957 and G. derjavini sensu MALMBERG et MALMBERG (1987) in Swedish and Norwegian rivers with wild salmon are presented. The unique viviparity, asexual and sexual reproduction and the reproductivity in Gyrodactylus are emphasized. Gyrodactylosis on salmonids in natural waters and fish farms is dealt with, e.g. the G. salaris gyrodactylosis in Norwegian rivers and in Swedish and Danish salmonid farms. The ultrastructure of wounds caused by G. salaris and results from Norwegian and Canadian experiments with Gyrodactylus species on salmonids are presented. The natural geographical distribution of salmonids, distribution by man, and economics, including culturing of salmonid species are reviewed. With regards to the geographical distribution of the salmonid host species, the presence of the six Gyrodactylus species groups within North America and Eurasia (Figs. 1, 2) is discussed. It is stressed that an intercontinental spreading of Gyrodactylus species on salmonids might have been impossible, because of their limnique origin and the high salinity of the Atlantic and the Pacific Oceans. Macro-, micro- and mixed environmental demands of Gyrodactylus species in natural waters are discussed and the presence of host and parasite related seasonal variations, preferences and tolerances pointed out (Fig. 3). Influences upon Gyrodactylus species by fish farm conditions are dicussed: reproductive and spreading capacity, as well as a strict host specificity present in natural waters, maybe influenced; genetic drift may result in new pathogenetic forms. Thus fish farms may act as culturing and spreading centra for Gyrodactylus species (Fig. 5). The rainbow trout, O. mykiss, is stressed as a unique, ''new'' host for a number of Gyrodactylus species. In conclusion it is recommended: further studies on salmonid stocks concerning differences in resistance to a pathogenetic Gyrodactylus species, comparative studies of wounds (Fig. 4) caused by different Gyrodactylus species, studies of the unique viviparity and the complicated interactions between macro- and microenvironmental conditions in Gyrodactylus.