RESPONSES OF RUFFE (GYMNOCEPHALUS-CERNUUS (L.)) ABUNDANCE TO EUTROPHICATION

The responses of ruffe (Gymnocephalus cernuus (L.)) abundance to eutro phication were studied in the brackish water of the Baltic Sea off Hel sinki. The productivity gradients in the area permitted an analysis of the dependence of ruffe catches taken with gill nets in 1969 to 1972 on spatial differences in eutrophication level. There appeared to be a relationship between the level of primary productivity and the catch per unit effort of ruffe. The gill net catches showed that ruffe were more abundant in eutrophic than oligotrophic areas bur less abundant i n the most eutrophic areas. Due to the better purification of municipa l sewage and the relocation of effluent discharge sites, primary produ ction has declined in the most eutrophicated inner bays during recent decades. The responses of ruffe reproduction to these changes were stu died on the basis of catches taken with small bench seines in 1982 to 1996. The number of zero-catches of 0+ ruffe increased significantly a fter oligotrophication, bur decreased again in 1996 after the accident al reintroduction of waste waters.