Some observations on the mass mortality of the freshwater fishRutilus rutilus(L.)

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Abstract
From 24 May to 4 July 1961 about 12,000 roach, Rutilus rutilus (L.), died in a lake at Brynmill Park, Swansea, South Wales. Even this high figure may be an underestimate of the actual mortality rate as it is based entirely on extractions of dead fish from the surface waters by members of the Brynmill and District Angling Club, who used landing nets from the shore only, although at fairly regular intervals. The daily counts of dead roach are represented on a histogram (Text-fig. 1). It is also of interest that in the early summer of 1961 large numbers of roach succumbed in other regions of the British Isles; about 30,000 died in the River Trent, Derbyshire, and 3000 in the lake in St James's Park, London. Newspaper reports in both localities attributed the cause to a deficiency of oxygen. For the outbreak in South Wales, however, similar reports suggested that ‘white spot disease’ caused by the ciliated protozoan Ichthyophthirius was responsible for the mortality.