Abstract
The responses of ruffe ( Gymnocephalus cernuus (L.)) abundance to eutrophication were studied in the brackish water of the Baltic Sea off Helsinki. 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 but less abundant in the most eutrophic areas. Due to the better purification of municipal sewage and the relocation of effluent discharge sites, primary production has declined in the most eutrophicated inner bays during recent decades. The responses of ruffe reproduction to these changes were studied on the basis of catches taken with small beach seines in 1982 to 1996. The number of zero-catches of 0+ ruffe increased significantly after oligotrophication, but decreased again in 1996 after the accidental reintroduction of waste waters.
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