GYRODACTYLIDAE AND GYRODACTYLOSIS OF SALM ONIDAE

21 Gyrodactylus species, described from salmonids are presented, arran ged in six Gyrodactylus-groups (Table I, Figs. 6-8). Findings of the s pecies in fish farms and/or natural waters are summarized (Table II). With data from Gyrodactylus species in natural waters in Scandinavia a nd in the Baltic as a background, biological, ecological and behaviour al observations in natural waters of G. salaris Malmberg, 1957 and G. derjavini sensu MALMBERG et MALMBERG (1987) in Swedish and Norwegian r ivers with wild salmon are presented. The unique viviparity, asexual a nd sexual reproduction and the reproductivity in Gyrodactylus are emph asized. Gyrodactylosis on salmonids in natural waters and fish farms i s dealt with, e.g. the G. salaris gyrodactylosis in Norwegian rivers a nd in Swedish and Danish salmonid farms. The ultrastructure of wounds caused by G. salaris and results from Norwegian and Canadian experimen ts with Gyrodactylus species on salmonids are presented. The natural g eographical distribution of salmonids, distribution by man, and econom ics, including culturing of salmonid species are reviewed. With regard s to the geographical distribution of the salmonid host species, the p resence of the six Gyrodactylus species groups within North America an d Eurasia (Figs. 1, 2) is discussed. It is stressed that an interconti nental 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 environm ental demands of Gyrodactylus species in natural waters are discussed and the presence of host and parasite related seasonal variations, pre ferences and tolerances pointed out (Fig. 3). Influences upon Gyrodact ylus species by fish farm conditions are dicussed: reproductive and sp reading capacity, as well as a strict host specificity present in natu ral waters, maybe influenced; genetic drift may result in new pathogen etic forms. Thus fish farms may act as culturing and spreading centra for Gyrodactylus species (Fig. 5). The rainbow trout, O. mykiss, is st ressed as a unique, ''new'' host for a number of Gyrodactylus species. In conclusion it is recommended: further studies on salmonid stocks c oncerning differences in resistance to a pathogenetic Gyrodactylus spe cies, comparative studies of wounds (Fig. 4) caused by different Gyrod actylus species, studies of the unique viviparity and the complicated interactions between macro- and microenvironmental conditions in Gyrod actylus.