Many fish species migrate through the supply streams of fish cultivation facilities (ponds, farms and hatcheries). These streams are the sources of so-called exotic species which are undesirable in open waters. They appear in the Dobra River when ponds are drained and during other hatchery works conducted in spring (April-May) and autumn (October-November). As a result, the number of species varies seasonally from six to 15. The ichthyofauna density increases from 1.70 to 124.94 specimens 100 m⁻² , while the biomass grows from 27.86 to 962.95 g 100 m⁻² . The majority of escapee fish belong to limno- and stagnophilous species. Most of these fish remain in this foreign environment only for a short time of 2 to 4 months.
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