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.In 7 out of 10 localities the prevalence of infection was higher in the sample taken adjacent to the farm than in the control, and overall prevalence was 13.7 O/O in periwinkles near the farms and 6.1 % in snails from the control sites, a highly significant difference.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. llngua 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 '10 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. ljngua cysts were located only on the fins in relatively small numbers.