Seasonal studies of the biology of Achtheres percarum in perch, Perca fluviatilis, from four Finnish lakes over a 3-year period

Achtheres percarum infection on the gills of perch Perca fluviatilis was studied from four lakes in central Finland in monthly or bimonthly samples of fish during a 3-year period. The highest prevalence and intensity of infection (30%, 0.7 copepods/fish) were found in the oligotrophic, unpolluted Lake Peurunka and the lowest (6%, 0.1 copepods/fish) in the eutrophic and polluted Lake Vatia. According to hierarchical loglinear models the prevalence of A. percarum infection was related to the lake, but not to the year. However, when logit models were constructed such that two 'seasons' ('cold' and 'warm' periods) were included, it was apparent that the year also influenced the prevalence of A. percarum , but only during the 'warm' period of the year. Achtheres percarum had an age dependent relationship only in Lake Peurunka, where the infection increased with increasing age of the fish. Some developmental stages of A. percarum parasitizing the gills of perch are described. Data on the seasonal occurrence of A. percarum developmental stages are given and it is suggested that at least two generations are produced annually.