Accumulation of Diplostomum spp. (Digenea: Diplostomatidae) metacercariae in the eyes of 0+ and 1+ roach (Rutilus...

Unlike other long-term studies of Diplostomum spp. metacercariae in fish eyes, this study investigated accumulation of the parasites in the eyes of very young roach for 3 consecutive years. Fish were caught by electro-fishing in the summers of 1990, 1991 and 1992 and the numbers of parasites located in the hosts eyes were recorded. Two distinct peaks of parasite abundance were observed, 1 in late June and the other in mid-September each year. Significantly, parasite abundance decreased after each of the peaks. Results obtained in a parallel investigation of the accumulation of the same parasites in rainbow trout in an adjacent fish farm showed the same 2 periods of accumulation each year. However, in the fish farm, no decrease in parasite abundance was observed at any time. The decreases in parasite abundance in the wild population can be attributed to mortality of the most heavily parasitized fish. As this did not occur in the farmed fish, in which similar levels of parasitism existed, it is likely that mortality in the wild population was an indirect result of the parasites.