DISEASES OF WILD SEA-TROUT IN SCOTLAND IN RELATION TO FISH POPULATIONDECLINE

Sea trout catches have been unusually low in recent years in some area s 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 be ing investigated but climatic as well as biological influences are lik ely to be involved. A wide variety of pathologies has been detected in sampled sea trout, particularly in the kidney, liver, and heart but w ith no overall consistency in occurrence between areas or within indiv idual rivers. In some fish the lesions were sufficiently extensive to be considered to be possibly threatening to organ function, but no cor relation 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 northwe stern Scotland during early summer had cranial lesions and dorsal fin damage associated with sea lice. The prevalence and intensity of infec tion varied between and within sites with high intensities being found on both west and east coast rivers. There was no clear relationship b etween lice burden and the extent of fin damage on individual fish. Sa mples obtained in the late summer and autumn were clear of lice and sh owed no lesions. A wide range of other metazoan parasites was recorded , with no indication of correlation with host condition. No significan t bacterial infection was detected, but infectious pancreatic necrosis (IPN) virus was detected in fish from two west coast rivers. The sign ificance of the pathologies and occurrence of metazoan parasite and ot her 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.