Seasonal variations of the opisthaptoral hard parts of Gyrodactylus derjavini Mikailov, 1975 (Monogenea: Gyrodactylidae) on brown trout Salmo trutta L. parr and Atlantic salmon S. salar L. parr in the River Sandvikselva,Norway

Seasonal variations in the size and shape of the marginal hooks, anchors and ventral bar of the opisthaptor of Gyrodactylus derjavini Mikailov, 1975 were studied. The G. derjavini specimens were collected from brown trout Salmo trutta L. parr and Atlantic salmon S. salar L. parr in the River Sandvikselva, southeastern Norway. Samples were taken at roughly monthly intervals during a 13-month period. The marginal hooks, anchors and ventral bars showed considerable seasonal variation in size, but varied very little in shape. The size increased when the water temperature decreased and vice versa.