International Council ICES CM 2003/R:07 for the Exploration of the Sea Theme Session: Freshwater and Diadromous Fishes in the Baltic Sea Long-term (1976-2002) changes in freshwater and diadromous fishes occurrence in the bottom layer of the southern Baltic Sea (on the basis of the Polish Young Fish Surveys) :áRG]LPLHU]*U\JLHODQG.RUGLDQ7UHOOD Sea Fisheries Institute, ul. Kollataja 1, 81-332 Gdynia, POLAND fax: +48 58 6202831, e-mail:
[email protected] ABSTRACT. The main purpose of the Polish Young Fish Surveys programme was to estimate both the spatial distribution and year-classes abundance of cod, flounder, herring and sprat in the southern Baltic Sea. Freshwater and diadromous fishes occur as by-catch in these research catches. The basic aim of this paper is to assess the long-term changes in the occurrence of freshwater and diadromous fishes in hauls and in their CPUE, in the relation to nine trawling profiles at 10-m strata between depths of 10-110 m and time periods. The results of 2953 bottom research catches carried out from 1976 to 2002 were analysed. Nine freshwater fish taxa (pikeperch, perch, ruffe, bream, roach, pike, sabrefish, rainbow trout, three-spined stickleback) were recorded in the catch composition. Moreover, nine diadromous fish species (eel, whitefish, shad, allis shad, vimba, sea trout, salmon, smelt, river lamprey) were also noted. Pikeperch was the dominant species occurring in 8.4% of the hauls followed by shad and eel at 4.3 and 3.0%, respectively. Keywords: by-catch, freshwater and diadromous fishes, geographical and bathymetrical distribution, southern Baltic.