Notes on the Distribution of “Black Spot” Shell Disease in Crustacean Fisheries

“Black Spot” a necrotic shell disease in crustacea, is a naturally occurring condition in crabs (Cancer pagurus) and is more common in older animals, where the moult frequency is reduced. The results of field observations in various commercial crab fisheries suggest that the incidence of “Black Spot” shell disease is higher in lightly fished populations (e.g. S. W. Ireland) than in established fisheries, where intensive exploitation results in the removal of larger (older) crabs from the stock. These observations are used to explain suggestions by other workers that the incidence of shell necrosis is high in areas used for dumping sewage sludge where fishing intensity is low.