Population biology of the helminth parasites of Phoxinus phoxinus (L.), the minnow, in a Cardiganshire lake

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Abstract
Specimens of Phoxinus phoxinus (L.) were collected over a period of a year from September 1967 to September 1968 from Frongoch Lake, Cardiganshire, Wales. Of a total of 650 fish examined for helminth parasites, 32 % were definitive hosts for Macrolecithus papilliger, and 16·5% for Neoechinorhynchus rutili (Müller), both parasites of the alimentary canal. All the fish examined were second intermediate host for Diplostomum phoxini (Faust) in the brain and 1·67% for Ligula intestinalis (L.) in the coelom. An increase in incidence of infestation with M. papilliger in spring and autumn and with N. rutili in summer were evident. The sex of the fish had no apparent effect on worm burden whereas size was more significant. There was a decrease in incidence of infestation with M. papilliger and an increase with N. rutili together with an increase in intensity of infestation with D. phoxini as the fish became larger. The latter species accumulated in the brain as the host increased in age.