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Following the first record of red swamp crayfish (Procambarus clarkii, Girard, 1852) in Carpathian Basin in January 2015, the established populations were found in a tributary creek (Sulák creek, Érd-Ófalú) and the adjacent River Danube in June 2016. Thereafter, we focused to survey the populations of the red swamp crayfish at the national scale. The aim of this study was to summarise the current distribution of the red swamp crayfish in Hungary and evaluate its effects on the fish assemblages in selected tributary creeks of the River Danube (including comparison of sites with red swamp crayfish presence/absence). During our field surveys, the fishes and crayfishes were collected using electrofishing, various types of traps, and handling nets, involving several water habitats. We detected the species in a 144 rkm-long section of the River Danube, in several thermal and man-made ponds in area of Budapest and in three tributary streams of the River Danube. The occurrence of the species was recorded in the drainage area of River Tisza in thermal ponds, their outflows and natural habitats of Laskó creek near Egerszalók in November 2017. The results of our twoyear- long survey showed that the non-native red swamp crayfish is capable to substantially alter associated fish fauna. Thus, our results are in line with previous findings on red swamp crayfish elsewhere, confirming invasive habits of the species. Further monitoring of this species in Hungary is strongly recommended. [ABSTRACT FROM AUTHOR]

Thermal and urban waters are frequently subjected to the releases of aquatic pets, which often occur in unexpected assemblages of native and non-native species. To document this, we conducted a half-year-long (January - July 2018) field survey the crayfish and fish species present in Barát and Dera creeks (two sampling sites per each) and sections adjacent to their mouth in the River Danube. Sampling sites were inspected monthly using a combination of catching methods. Altogether, four non-native crayfish, ten nonnative and twelve native fish were recoded. Several individuals of spiny-cheek crayfish (Faxonius limosus), marbled crayfish (Procambarus virginalis), red swamp crayfish (Procambarus clarkii) and Australian redclaw (Cherax quadricarinatus) were sampled in the thermal water tributary of Barát creek. The Dera creek was not influenced with the thermal or industrial warm water, but the urbanised area affected the water quality, which was still acceptable for occurrence of spiny-cheek crayfish, marbled crayfish and red swamp crayfish. Besides already well established population of spiny-cheek crayfish, also marbled crayfish and red swamp crayfish inhabited the River Danube itself. Seven non-native decapods have been reported in the Hungarian wild so far. However, this is to our knowledge the first published report on co-occurrence of three North- American crayfish as well as a combination of North-American and Australasian crayfish species in Europe. The new faunistic records of exotic live-bearing fish species from the family Poeciliidae and their hybrids from Barát creek were also obtained. These findings highlight the significance of pet trade as an introduction pathway and thermal as well as urban waters as target sites for new introductions. Roles of established nonnative species and their possible spread are issues requiring further targeted research. [ABSTRACT FROM AUTHOR]