Reasons for the Decline in Parasitic Fauna between 1950 and 1998

Several reasons can be advanced to explain why the helminth parasites of trout had declined in prevalence, abundance and diversity from 1950 to 1998. Firstly, the decline is partly due to the catastrophic decline in brown trout density and to a change in the age structure since 1950 (Figure 20). Thus, the trout population in 1998 consisted mainly of juvenile fish in the first and second year and older, mature fish were uncommon. It is possible that the change in age structure is due to a shift from what was predominantly a brown trout life history strategy to that of the migratory sea trout. This change may be regarded as an adaptation to the River Teifi and its tributaries becoming harsher and more erosive in character as a result of changes in land use. Many of the juvenile fish in the 1998 sample may therefore have been sea trout parr and it is noteworthy that trout in the year classes 4-6 were absent from the sample (Figure 20). As the prevalence, abundance and diversity of helminth parasites in fish are positively correlated with age it is possible that the lower values in the 1998 sample may be partly due to the preponderance of juvenile fish.

Another possible reason for the decline in prevalence, abundance and diversity of parasites from 1950 to 1998 is that the intermediate hosts have become less abundant. Preliminary sampling in 1997 had revealed that S. corneum, the intermediate hosts of P. simile (Thomas, 1957, 1958a, b), which were abundant

Figure 20 The percentage of brown trout in various age groups in 1950 and 1998.

0 0

Post a comment