Long term studies on the population biology of two specie of eyefluke, Diplostomum gasterostei and Tylodelphys clavata (Digenea: Diplostomatidae), concurrently infecting the eyes of perch, Perca fluviatilis

Changes in the size of the infrapopulations of metacercariae of Diplostomum gasterostei and Tylodelphys clavata were studied in a small lake for a period of seven years in perch Perca fluviatilis . Tylodelphys clavata appeared in the lake at the commencement of the study and its subsequent population increase and changes were typical of those of a species recently established in a new habitat. Population size increased slowly at first, then rapidly, then declined again as population density approached equilibrium level. By contrast, the population size of D. gasterostei declined throughout the study. Two processes were responsible for the decline: a decrease in the proportion of fish carrying heavy infections which was continuous throughout the seven years and a dramatic decline in recruitment rate in young fish during the last two years. In earlier years recruitment rate had changed annually and changes in overall infrapopulation size had largely reflected the recruitment rate two years previously. It was concluded that these two processes were independent of each other and related to changes in the population density of other species of eyeflukes. The decline in the proportion of heavily infected fish was significantly negatively correlated with the increase in population density of T. clavata and the decline in recruitment rate in young fish was coincident with the appearance and infrapopulation increase of T. podicipina metacercariae in the eyes of young perch. It is suggested that these two species interact negatively with D. gasterostei and are currently partitioning the perch habitat between themselves.