Comparison of Prevalence Abundance and Diversity of Helminth Parasites in 1950 and 1998

A comparison of Figures 10 and 19 reveals that the prevalence values for parasites of trout of various age groups have changed dramatically between 1951 and 1998. Although C. metoecus was one of the most prevalent trout parasites in the 1950s both C. metoecus and C. farionis were conspicuous by their absence in the 1998 fish sample. With the exception of N. rutili the prevalence values for trout parasites in the sample taken in 1998 were less than the corresponding ones in the 1950 sample.

The community structure analysis of brown trout parasites (Table 14) also reveals that major changes had occurred between 1950 and 1998. Thus, species richness had declined from 8 to 5 and at the component population level N. rutili had replaced C. metoecus as the dominant parasite. The various component population indices, including the Berger-Parker index, Simpson's diversity index, Brillouin's index, and the equitability index and the slope 'a' of the geometric series had all decreased. At the infrapopulation level (Table 14) the mean number of parasite species per fish and the maximum number of parasite species per fish had halved between 1950 and 1998. During 1998 the percentage values for fish with no parasite species or with one or two parasite species only were less than in 1950 whereas the converse was true for the percentage values for fish with three to six parasites only. These results were reflected in the mean Brillouin's indices for all fish and infected fish. The maximum Brillouin's index was also lower in 1998 than in 1950.

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