New records of alien monkey goby, Neogobius fluviatilis (Pallas, 1814), in the waters of the great Masurian Lakes system (north-eastern Poland).
The invasion of aquatic environments by alien fish species is an important issue in ecological research. Alien fish species, such as Ponto-Caspian gobies, can impact native communities and ecosystems through predation, competition, parasite vectors, disturbance, and habitat modification. In this study, we present new locations of monkey goby Neogobius fluviatilis in the waters of the Great Masurian Lakes system (north-eastern Poland). Four methods (Nordic gillnets, electrofishing, fishing rod, and Vulkan crayfish trap) were used for fishing. The presence of N. fluviatilis was recorded in nine lakes of different morphometry and trophic status, one canal, and at one site of the Pisa River in 2018-2020. Total length and body weight of N. fluviatilis were rather small (42-113 mm; mean of 81 ± 19 mm and 0.3-13.7 g; mean of 6.4 ± 3.6 g, respectively). The most specimens were in the range of 80 to 100 mm. Our study documented that N. fluviatilis inhabits lakes, rivers and canals. The invasion of N. fluviatilis into the Great Masurian Lakes system occurred across the Pisa River from the Narew River. In the Great Masurian Lakes, the northernmost place in the system in which N. fluviatilis was recorded was Lake Niegocin. We may expect that N. fluviatilis can quickly spread farther, reaching lakes situated in the northern basin of the system in the near future.