The richness of ichtiological fauna in Warta Mouth National Park is influenced by a very diversified local hydrological net composed of rivers, canals, melioration ditches, oxbow lakes and floodlands. Big variety of habitats, abundant diversed vegetation, ground and physical and chemical parameters of the water as well the connection of Warta with other watercourses (e.g. Odra and Drawa) make the waters of the Park a convenient place for many fishes. So far 35 species have been recorded including these that probably come to the Park for their spawing season.
The most numerous are fishes from the carp family profusely represented by common bream, silver bream, roach, crucian carp and the only predator from the carp family i.e. asp.
The shallow waters, abundant in food, are an ideal place for reproduction and feeding of pike, a predatory fish numerous in the floodlands of Warta. Other predatory fishes of the Park are: perch, pikeperch and the biggest domestic fish species, catfish. The waters of the Park are numerously settled by burbot, a fish from the gadidae family which has convenient feeding conditions on the sandy and stony bed of Warta. Maszówek Canal (Stara Warta) is a place where dwells a numerous population of perch and roach, canal’s slow current creates equally good conditions for other fishes from carp family, tench and crucian carp.
In Postomia River presence of bitterling has been confirmed. It is an ostracophile fish i.e. it is using shells for reproduction.
Among the fishes under strict species protection in the waters of Warta Mouth National Park dwell: bitterling, spined loach, stone loach, Moderlieschen, and weatherfish which is listed in the Polish Red Data Book of Animals.
Among rarities, in the Park dwell: vimba bream and sturgeon. A curiosity is a fact that to the Park during the spawing season come Atlantic salmon and bulltrout which spawing nests are yearly observed in Drawa River.