LONG-TERM AND SEASONAL-CHANGES IN COMPOSITION AND RICHNESS OF INTESTINAL HELMINTH COMMUNITIES IN EELS ANGUILLA-ANGUILLA OF AN ISOLATED ENGLISH RIVER

The need for more long-term studies on helminth communities was addres sed by examining changes in composition and diversity of the intestina l helminth component and infracommunities in eels of a small isolated river over 12 years. Examination of samples over one summer season ind icated that single samples were representative of community richness i n that year. In 1985 the community was species poor(1 species only) an d with zero diversity, but by 1996 it comprised six species and all pa rameters at both levels indicated that it was the richest community ye t reported from eels. This overall increase in diversity was not regul ar or uniform from year to year: rather, the general trend could be re versed in any year(s) and variation between years was considerable. Th e occurrence of many species was erratic and the identity of the domin ant species varied between years. The helminth species that colonised the river contributed to helminth species richness and had an impact o n community diversity. It is believed that the overall increase in div ersity reflects the changing conditions in the habitat and catchment, but the erratic fluctuations in parameters may reflect the size of the river and the absence of any rescue effects. The findings are compare d with the few other long-term studies and it is suggested that the ex tent of the variation over long periods makes it very difficult to use helminths as indicators of other than general environmental changes o r to detect short-term changes due to such specific sources as polluti on.