Background. The two gyrodactylid species G. derjavini and G. salaris (a special Danish rainbow trout form which is non-pathogenic to salmon) show similar predilection for their host rainbow trout and express similar population dynamics (but different microhabitat-preference) when occurring in single species infections. The present study elucidates the colonisation ability and site selection of the two parasites when occurring in mixed populations on the same host. Materials and methods. Fry of rainbow trout, Oncorhynchus mykiss, infected either with a total of 459 G. derjavini (12 fish) or a total of 405 G. salaris (12 fish) (approximately 35 parasites per fish) were maintained with a total of 12 uninfected fish for two weeks. The infection dynamics and dispersal of parasites among the three groups were recorded during the period. For localization of specific parasites in various microhabitats (fins or skin) two fish from each group were preserved weekly and their parasites mounted for species determination. Results. Within the two-week study period, parasites spread from infected- to naïve rainbow trout, which obtained a parasite burden (293 parasites in 10 hosts) corresponding to the level of previously infected fish. Gyrodactylus derjavini demonstrated a higher colonisation ability especially on naïve fish but did also colonize previously G. salaris infected hosts. This species continued to exhibit predilection for both fins and body skin as microhabitats. In contrast, G. salaris was less mobile and less prone to colonise new hosts, especially infected ones. The parasite remained mostly in the preferred microhabitat (body surface) and colonised only fins to a much lower degree. Conclusion. The results suggest the existence of specific microhabitats on the body and fins of rainbow trout, which are occupied by specific parasites. This will to some extent minimize negative interactions between the two parasites on a short-term basis (two weeks). The different microhabitats of G. derjavini (both fins and body) and G. salaris (primarily body skin) did only change slightly during the two-week study period. The few interactions between parasites and rainbow trout may partially be mediated by local host response mechanisms. It is indicated that the predilection of G. salaris for body surface microhabitats must be taken into account when sampling for surveys are being performed.

Gyrodactylus salaris, Gyrodactylus derjavini, parasite, Monogenea, rainbow trout, Oncorhynchus mykiss, microhabitat, interaction