Arctic charr (Salvelinus alpinus (L.)), an adequate host to Gyrodactylus salaris (Monogenea)

Abstract

Arctic charr is a suitable host to Gyrodactylus salaris, and G. salaris is a non-pathogen to this host species. This parasite is highly pathogenic to Norwegian Atlantic salmon and it has driven most affected salmon populations to extinction. In Troms County in northern Norway, two rivers are infected. These two rivers, Skibotnelva and Signaldalselva, are the northernmost G. salaris infected rivers in Norway and the natural salmon populations are regarded extinct or close to extinction due to the parasite. Skibotnelva has been treated twice with the plant poison rotenone. Both treatments failed, and an ongoing hypothesis is that the occurrence of Arctic charr is the main cause of this. The overall aim of this study was to elucidate the role of Arctic charr as a long-term host to G. salaris. Both field and experimental studies have been undertaken. It was found that the abundance of G. salaris varies significantly throughout the year in both Skibotnelva and Signaldalselva, and is highest in the autumn and lowest in the spring. These seasonal fluctuations were repeated between years, and are mainly explained as temperature dependent constraints upon parasite reproduction in combination with host immune responses. <br>Further, it was documented that Arctic charr in allopatry can sustain a G. salaris population during the five winter months, and also that newly hatched charr fry are highly susceptible to the parasite. The newly hatched charr fry experienced additive parasite induced mortality. Furthermore, experimental studies indicated that parasite transmission occurs frequently within the charr fry population at all times during the colonization process. Hence, transmission probably is an important process in determining the infrapopulation structure by reducing the over-dispersion of G. salaris between hosts, and also may contribute to a prolonged period of growth of the parasite metapopulation. <br>In conclusion, this study clearly indicates that Arctic charr has to be valued as an important and adequate host to G. salaris which can sustain the parasite population within watercourses independent of the presence of infected salmon.