Factors affecting colonization and assemblage structure of fish in newly created ponds.

The construction of new ponds is a widely used mitigation measure in wetland restoration. While fish colonization can compromise their ecological function, knowledge of how frequently, how rapidly, and by which mechanisms fish colonize these habitats is severely limited. In our study of 123 newly created, originally fishless ponds in the Central European agricultural landscape, 58.5% were found to host fish. Fish were already present in 40% of one-year-old ponds and the proportion of non-colonized ponds did not depend on their age. The most common species were the invasive topmouth gudgeon (Pseudorasbora parva) and Prussian carp (Carassius gibelio), along with the native rudd (Scardinius erythrophthalmus). The likelihood of fish colonisation increased with pond area and depth but decreased with the width of littoral vegetation and distance from the nearest settlement, the latter suggesting a strong effect of anthropogenic dispersal. Ponds hosting invasive or stocked fish assemblages exhibited significantly reduced water transparency and submerged macrophyte cover, indicating potential negative impact on their biodiversity. Overall, the study highlights that without effective control of fish dispersal, newly created ponds in agricultural landscape risk losing much of their conservation value and, in some cases, may even facilitate the persistence and spread of invasive species.