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Studies on the occurrence of cyclostomes and fishes of the drainage area of the river Płociczna were carried out in 1996. The following parameters were defined: constancy of occurrence (C), domination index (D), and fractions of the biomass (W) represented by individual species. The results were confronted with the data from the river Płociczna published by the other authors within 1986-1994. A total of 1 cyclostome and 32 fish species was presently recorded to inhabit the drainage area of the river Płociczna. In this number there were 1 cyclostome and 7 fishes which are protected species. The part of the drainage area of the Płociczna under study, constitutes a section of the Drawieński National Park. The species occurring in that part, as well as in its buffer zone were discussed.
Part of the lower Drawa River between Dubie Lake and the Płociczna River mouth, stretching for a distance of 38.8 km, belongs to the Drawieński National Park, established in 1990. Electrofishing conducted on the 8 river stretches within the park limits yielded a total of 26 fish species and one cyclostomate species. The leading species appeared to be the roach, with the domination level reaching 56.16%. The other abundant species were: burbot (D = 7.81%), silver bream (D = 6.88%), and perch (D = 5.95%). The remaining species occurred in smaller numbers. The Drawa River within the Park limits is inhabited by 3 species of the family Cyprinidae: minnow-Phoxinus phoxinus, schneider-Alburnoides bipunctatus, and bitterling-Rhodeus sericeus amarus, all under strict protection and one cyclostomate species of the family Petromyzonidae-river lamprey-Lampetra fluviatilis, protected at the stage of larval development.
e size, length/weight relationship, sex ration and gonadosomatic index of the autumn and spring upstream run populations of the river lamprey have been studied. The size attained by the lampreys from the lower Vistula basin (Drwęca R.) is among the largest recorded for this species. In autumn males in the studied rivers outnumber females. In this period lampreys are bigger than during the spring run. The mass of gonads and the GSI in early spring are higher than in autumn.
The plastic and meristic characters of the river lamprey were studied in 145 specimens of the autumn and spring run populations from the north-Polish rivers (the Grabowa, Drwęca, Wda, Parsęta). Among the examined lamprey populations entering the Polish rivers in autumn no essential differences in body proportions are observed. In comparison with autumn populations, the lampreys that start their spawning migration as late as spring differ more considerably with respect body proportions. Besides, they are characterised by more marked sexual dimorphism, which can be related to more advanced development of gonads.
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