Number of species (S), density (N) and standing crop (B) were investigated in 331 sites in the catchment of the Narew River, the biggest tributary of the Vistula, against 4 categories of riparian ecotone intensity in various-width rivers. All populations were first analyzed together, then within the scope of three most abundant ecological spawning groups. The image obtained is to some extent blurred by anthropogenic alterations as well as humancreated ecotones, yet it is evident that a total lack of bankside trees had a negative impact on the values of S, N and B of all investigated populations. Exceptions are only small streams of the Bialostocka Upland, where at a high percentage of submerged vegetation and occasionally stony bottom (washed away moraine bars), the lack of riparian trees did not cause any decrease in any of the three populational parameters. The uninterrupted compact forestation of the banks was positively correlated with S, N and B only in the lower Narew River, where the river bed was more than 100 m wide, hence where the access of light was not already limited. For lithophilous species a river's naturalness was a more important factor conditioning their abundance than the development of riparian ecotones. Indifferent species displayed a high, positive dependence on the development of ecotones, while phytophils (whose development is vegetation-dependent) formed the most qualitatively and quantitatively abundant populations at a weak, and in some rivers, even medium forestation of the banks.
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