Occurrence of Proteocephalus percae and P. cernuae in the perch and ruff in northern Finland.

The occurrence of two palearctic Proteocephalus species, P. percae and P. cernuae, in the perch and ruff, respectively, was studied in two areas of northern Finland in 1977-1983. In the Bothnian Bay, 243 perch (Perca fluviatilis) and 406 ruff (Gymnocephalus cernuus) were studied, of which 60% and 40%, respectively, were infected, while in Lake Yli-Kitka the prevalences were 10% in 213 perch and 24% in 307 ruff. Higher mean intensities of infection were found in the Bothnian Bay fish than in those from the lake, but no infected fish had as many as 10 worms. Proteocephalus infection in the perch and ruff did not vary significantly according to the length of the fish in either area, except that no P. percae were found in perch smaller than 70 mm in the lake. There was a prominent seasonal variation in the occurrence of P. cernuae in the ruff in both areas, but especially in the lake, where no proteocephalids were found in the ruff in July-October. Overlapping of generations was evident in P. percae in the sea perch, although only by virtue of the lack of mature worms at the end of the summer. Some mature proteocephalids were also found in winter in both fish species in the sea and in the ruff of the lake. The higher mean intensity values in winter, especially in P. cernuae infection in the sea ruff, may be explained partly by the active feeding of the ruff at this time in spite of very low water temperatures. Feeding activity in the perch is lower in the winter, and hence a lower mean intensity of P. percae infection was seen especially in the lake material at that time.