Population dynamics of Gyrodactylus salaris (Monogenea) infecting Atlantic salmon, Salmo salar, parr in the river Batnfjordselva, Norway

Population dynamics of Gyrodactylus salaris on 5 year classes of Atlantic salmon, Salmo salar, in the river Batnfjordselva in Norway was studied from April 1991 to June 1994. Prevalence was 100% on all year classes throughout the study period, except for a marked decline in winter and spring of 1992. Median abundance generally increased during summer and peaked in September/October at levels between 400 and 1,300 parasites. The lowest number of parasites was observed during winter and spring. The seasonal dynamics in the first year of the study differed markedly from the remainder of the study; abundance peaked already in June/July on 1+ and 2+ fish the first year, but decreased throughout summer and peaked again in autumn. A collapse in the G. safaris population was observed in the winter of 1991/1992, with prevalence being 0-40% on all year classes. Parr aged 2 yr harbored significantly fewer parasites than younger parr during the winter months. Parasite intensity on precocious males was significantly higher than on sexually immature males and females. Gyrodactylus salaris was found on the fins (76%), gill filaments (15%), and head/body (9%). During the last 2 yr of the study the gills were increasingly infected, with 40% of the parasites found on gills in some samples.