Diseases of wild sea trout in Scotland in relation to fish population decline

Abstract

Sea trout cathceshave been unusually low in recent years in some areas of Scotland, particularly parts of the west coast, coincident with a decline in sea trout catches and stocks in parts of western Ireland. The causes of the widespread decline are at present unknown and are being investigated but climatic as well as biological influences are likely to be involved. A wide variety of pathologies has been detected in sampled sea trout, particularly in the kidney, liver, and heart but with no overall consistency in occurrence between areas or within individual rivers. In some fish the lesions were sufficiently extensive to be considered to be possibly threatening to organ function, but no correlation was established between severity and loss of fish condition. Juvenile trout sampled in seawater, in estuaries and lower rivers were mainly in good condition. However, some obtained in rivers in northwestern Scotland during early summer had cranial lesions and dorsal fin damage associated with sea lice. The prevalence and intensity of infection varied between and within sites with high intensities being found on both west and east coast rivers. There was no clear relationship between lice burden and the extent of fin damage on individual fish. Samples obtained in the late summer and autumn were clear of lice and showed no lesions. A wide range of other metazoan parasites was recorded, with no indication of correlation with host condition. No significant bacterial infection was detected, but infectious pancreatic necrosis (IPN) virus was detected in fish from two west coast rivers. The significance of the pathologies and occurrence of metazoan parasite and other infections is being studied but with the data currently available no cause-effect relationship between disease and decline of sea trout populations could be established.