Regulatory processes in the monogenean Gyrodactylus salaris Malmberg—Atlantic salmon ( Salmo salar L.) association. I. Field studies in southeast Norway

Abstract

The seasonal population dynamics of Gyrodactylus salaris on wild Atlantic salmon ( Salmo salar) under natural conditions were studied over a period of 19 months in the River Glitra, a tributary to the River Lierelva in southeast Norway. The abundance of the parasite on 1 + and older salmon parr was positively related to water temperature, i.e. the abundance was highest in summer and lowest at the end of winter. In the 0 + cohort, numbers of G. salaris increased steadily through summer and autumn, while in the 1 + and older cohorts of salmon parr numbers of G. salaris decreased in the same period. Generally there was a negative relationship between parasite intensity and host size in the 1 + and older salmon parr, demonstrating heterogeneity in susceptibility and/or resistance to G. salaris among host individuals. There is also evidence in the data of parasite-induced host mortality, from the distributional pattern of G. salaris with respect to host age.