Infection of estuarine fish populations by Cryptocotyle lingua (Creplin)

Fish populations in the Medway Estuary become infected with Cryptocotyle lingua (Creplin) shortly after their inshore migrations. The present study examines infection levels between 1973 and 1975 in four fish species; whiting, Merlangius merlangus (L.), pouting, Trisopterus luscus (L.), plaice, Pleuronectes platessa L. and flounder, Platichthys flesus (L.). Frequency of infection was highest in flounder, followed by pouting, plaice and whiting. Unusually, parasite density was lowest in plaice even though gastropod molluscs (the parasite's first intermediate host) were most common in their diet. Seasonal levels of infection are discussed and prove useful indicators to the migratory movements of individual fish populations. In whiting and pouting, incidence of infection increased with length of host while no relationship between infection and age of host was observed in either flatfish. Metacercariae were not distributed randomly over the body surface; in gadoids they were concentrated above the lateral line while in pleuronectids they were most intense in the fins and gills.