The influence of parasitism on fish population success

P>The influence of parasitism on first-year growth and recruitment success of two cyprinid species in the Yorkshire Ouse catchment, England, was investigated using a 14-year dataset. This study demonstrated a significant role of parasitism in growth and recruitment success of roach, Rutilus rutilus (L.), and European chub, Squalius cephalus (L.) populations. Muscle infections by Bucephalus polymorphus Baer (Digenea), Myxobolus pseudodispar Gorbunova (Myxozoa) and Myxobolus pfeifferi Thelohan were considered important, with significant relationships between these parasites and year-class strength and age-0 fish length. Other parasites, such as Phyllodistomum sp. and Goussia sp., were implicated in host success to a lesser extent. Parasitism may be a major factor in recruitment and account for a high proportion of the variation in year-class strength, although this varied among locations.