We hereby provide the first documented data on the occurrence of a viable Procambarus virginalis population in a nature reserve in Poland and the steps that were taken to prevent further introductions of the species in the country. To date, Poland represents the most north-eastward distribution area in Europe, where the species occurs in ecosystems with natural water temperature regimes. The ecological plasticity of P. virginalis and its obligate parthenogenetic reproduction make this crayfish an exceptional invader. The crayfish may have detrimental effects on the native invertebrates, amphibians, and fish, and alter the state of the entire ecosystem. Therefore, we investigated the presence of the species in the nature reserve of Pojezierze Łęczyńskie Landscape Park after a suspected P. virginalis individual was found by a local citizen. The nature reserve also includes an area designated to protect native amphibians and turtles. Our study revealed the presence of a thriving population of P. virginalis in the protected area and clear indications of its impact on native and invasive species, both in aquatic and terrestrial environments. If no action is taken, the species will likely expand to other water bodies across the country. An extensive sensibilization campaign allowed us to find additional invaded areas and significantly contribute to the effective ban of this and other invasive crayfish species from private and commercial online trade. Raising public awareness and banning invasive crayfish species trade is essential for successfully detecting and preventing further introductions.

Invasive alien species are currently a major ecological problem. One such species is stone moroko, Pseudorasbora parva (Gobionidae), which has only been found to date in natural or agricultural water bodies (lakes, rivers and streams, carp ponds). Therefore, the current study is the first description of stone moroko in urban green areas (Warsaw, Poland). The fish were caught with a hydrobiological net. Four catches were conducted in ponds located in Morskie Oko Park (pond 1) and the Saxon Garden (pond 2) – two in fall and two in the spring, while two spring catches were conducted in Dolinka Służewska Park (stream). The population in pond 2 apparently survived the winter, and the fish caught in June displayed visible signs of sexual dimorphism and were significantly larger (average body mass: 1648 ± 1123 mg; range: 460–6090 mg) than those caught in the pond in the two fall samplings (404 ± 122 mg; 154–729 mg). After recording substantial numbers of specimens in the fall, no fish were caught in pond 1 during either of the spring catches, while only a few moroko were caught in the stream in April (1212 ± 869 mg; 454–2901 mg) and none were caught in June. Throughout the study, 59 specimens were caught in pond 1, 74 in pond 2, and 7 in the stream.

The aquarium trade is one of the main pathways for the introduction of non-native freshwater species. Such species include crayfish, which are valued ornamental animals commonly kept in aquaria. Some crayfish have been released into the environment, becoming invasive alien species. Due to the threat they pose to biodiversity and related ecosystem services, they have been subject to legal restrictions as invasive alien species of Union concern. In Poland, examples of species that have entered aquatic ecosystems this way include red swamp crayfish Procambarus clarkii and the marbled crayfish Procambarus virginalis. Given the highly developed aquarium pet trade in Poland, a detailed analysis of the availability of crayfish in the pet trade was conducted. This study examines the presence and sale of crayfish at locations at zoological trade fairs, shops, and online marketplaces in Poland. Additionally, pricing, the volume of imported crayfish, and their welfare in the year 2024 are presented. Crayfish were recorded in all surveyed locations. In shops and zoological trade fairs, five crayfish species were recorded, including the invasive alien species P. clarkii and C. destructor. Online advertising platforms featured 15 crayfish species, of which four were invasive alien species. Cambarellus patcuarensis was the most commonly sold species in all examined places. Crayfish were often kept in poor condition. Furthermore, many sellers probably disguised the species of crayfish being sold illegally. This work shows that trade in invasive alien crayfish species is still widespread in Poland.

Invasive alien species are one of the major problems in Polish fauna and flora. They can displace native species and cause economic losses. Such animals as red swamp crayfish have a wide tolerance to changing living conditions, which means that their range of occurrence increases over time. The current area of red swamp crayfish in Poland includes several locations, but reports indicate that this species inhabits many other reservoirs. It is very important that the red swamp crayfish population is regularly monitored, which may limit the number of this crustacean in Polish waters.

Invasive alien species are currently a major ecological problem. One such species is stone moroko, Pseudorasbora parva (Gobionidae), which has only been found to date in natural or agricultural water bodies (lakes, rivers and streams, carp ponds). Therefore, the current study is the first description of stone moroko in urban green areas (Warsaw, Poland). The fish were caught with a hydrobiological net. Four catches were conducted in ponds located in Morskie Oko Park (pond 1) and the Saxon Garden (pond 2) – two in fall and two in the spring, while two spring catches were conducted in Dolinka Służewska Park (stream). The population in pond 2 apparently survived the winter, and the fish caught in June displayed visible signs of sexual dimorphism and were significantly larger (average body mass: 1648 ± 1123 mg; range: 460–6090 mg) than those caught in the pond in the two fall samplings (404 ± 122 mg; 154–729 mg). After recording substantial numbers of specimens in the fall, no fish were caught in pond 1 during either of the spring catches, while only a few moroko were caught in the stream in April (1212 ± 869 mg; 454–2901 mg) and none were caught in June. Throughout the study, 59 specimens were caught in pond 1, 74 in pond 2, and 7 in the stream.