Occurrence of the digenean Cryptocotyle lingua in farmed Arctic charr Salvelinus alpinus and periwinkles Littorina littorea sampled close to charr farms in northern Norway

Occurrence of Cryptocotyle lingua rediae was recorded in periwinkle samples collected adjacent to 10 charr farms and at control sites 1 to 5 km from the farms. Prevalences in periwinkles close to farms tended to increase with duration of farming at the site. The role of the final host, piscivorous birds, is considered. Samples of charr from 11 farms were investigated for visible black spots caused by encysted C. lingua metacercariae. No infected charr were recorded in the 2 land-based farms where seawater exposed to UV-light (photozone) was pumped to the tanks, whilst 83.2% of the fish exhibited black spots in the 9 farms where the charr were stocked in floating net cages in the sea. In most infected fish the C. lingua cysts were located only on the fins in relatively small numbers.