PREVALENCE AND ABUNDANCE OF 2 TREMATODE PARASITES, DIPLOSTOMUM-PHOXINI AND MACROLECITHUS-PAPILLIGER IN EUROPEAN MINNOWS (PHOXINUS-PHOXINUS)IN AN ARTIFICIAL SWISS ALPINE LAKE

The influence of parasites on the breeding biology of the European min now (Phoxinus phoxinus) was studied by examining 268 adult and 59 youn g-of-the-year minnows from an artificial Swiss Alpine lake for parasit es during the breeding seasons in 1990 and 1991. Two trematode parasit es; Diplostomum phoxini, in their brains, and Mncrolecithus papilliger , in their guts, were found. The prevalence of D. phoxini was 100% in both years. The mean abundance was 231 in 1990 and 448 in 1991; the di fference was significant. Prevalence (and abundance) of M. papilliger was 37.5% (1.04+/-0.39) in 1990 and 59.5% (2.68+/-0.55) in 1991; also significantly higher in 1991 than in 1990. No differences in prevalenc e or abundance between males and females were found. Numbers of both p arasites were significantly correlated with fish length. There was sea sonal variation in the abundance of D. phoxini metacercariae, with a p eak in June/July in both years. The prevalence of M. papiller decrease d after July in both years. Distribution of the two trematodes was ove rdispersed as indicated by the variance to mean (abundance) ratio. The observed seasonal variation in D. phoxini abundance also suggests tha t sublethal parasites could influence the population dynamics of their hosts.