Transmission of the monogenean Gyrodactylus salaris

The present study is focusing on the transmission of the monogenean ectopar asite Gyrodactylus salaris Malmberg, 1957, a major pathogen on natural popu lations of No:Norwegian Atlantic salmon, Salmo salar L. In laboratory exper iments the transmission rate of G. salaris after direct host to host contac t was positively correlated with water temperature (1.2, 4.7 and 12.2 degre es C). The transmission of detached G. salaris in the planktonic drift was studied in field experiments where salmon parr were individually isolated f or 24 hours in small wire mesh cages suspended in the water column. Ten out of 157 salmon parr (prevalence 6.4%, mean intensity 1.0) contracted G. sal aris infections after this exposure. Furthermore, 200 uninfected marked sal mon parr were released into the same area of the river. After 24 and 48 hou rs; respectively 18 and 19 marked parr were caught by electro-fishing. The prevalence of G. salaris was 44.4% (mean intensity 1.9) after 24 hours, ris ing to 57.9% (mean intensity 2.3) after 48 hours. Gyrodactylids have no spe cific transmission stage or swimming ability, but detached G. salaris drift ing in the water column were found to infect salmon parr. However, the tran smission rate was markedly higher to free-living fish, suggesting that tran smission routes such as indirect transmission from the substrate or direct contact transmission from infected live and/or dead fish, are relatively mo re important than transmission by drifting detached parasites.