The pathology and seawater performance of farmed Atlantic salmon infected with glochidia of Margaritifera margaritifera

The pathology of glochidial infection of the freshwater mussel Margaritifer a margaritifera was examined in farmed Atlantic salmon Salmo salar in fresh water and for 150 days after transfer of salmon to sea water. Prevalence o f infection in fresh water was 95%, mean abundance 134 per fish and mean in fection intensity 140. Prevalence in sea water was 80-94% in the first 7 we eks after transfer but glochidia were absent, apart from remains, after 50 days in sea water. Glochidia on salmon in fresh water were associated with localized hyperplasia and fusion of secondary gill lamellae. Focally extens ive epithelial hyperplasia and fusion of secondary lamellae were present 4- 10 days after transfer to sea water. Twenty-three days after transfer, smal l nodules with a more discrete appearance were present suggesting partial r esolution of tissue response; hyperplastic responses associated with glochi dia were much reduced after 50 days. Plasma chloride in infected fish 10 da ys after transfer was 153 mmol.l(-1), significantly higher than fish withou t infection, suggesting poorer adaptation to sea water. No mortalities due to glochidial infection in sea water were recorded and there was no signifi cant difference in mean weight between infected and control fish. (C) 2000 The Fisheries Society of the British Isles.