Protected and alien fish species in Polish lakes in 2014-2021

The occurrence, abundance, contribution, and size structure of protected and alien fish species were studied in 535 lakes situated in the northern and central parts of Poland in 2014-2021. We hypothesised that protected fish species, due to their status, would be rare in terms of abundance within certain lakes and/or less distributed. Four protected (Cottus poecilopus, Cobitis taenia, Misgurnus fossilis, and Rhodeus amarus) and eight alien species of fish (Ctenopharyngodon idella, Neogobius fluviatilis, Pseudorasbora parva, Acipenser baerii, Carassius gibelio, Ameiurus nebulosus, Hypophthalmichthys molitrix, and Cyprinus carpio) were identified in the studied lakes. Among protected species, the most common and frequent were R. amarus (65% of the studied lakes) and C. taenia (47% of the studied lakes). Other two species (C. poecilopus and M. fossilis) were recorded in one (0.2%) and 18 lakes (3.4%), respectively. Alien species were most often represented by C. gibelio, which occurred in 96 lakes (18% of the studied lakes). The presence of the remaining seven alien species was found in not many lakes (0.2-5.2% of the studied lakes). The abundance of C. gibelio was positively, while A. nebulosus was negatively related to the trophic state of lakes. Cobitis taenia was positively correlated with morphometric parameters of lakes (area and maximum depth). It came as surprise to us that some protected species were quite frequent and sometimes numerous, in contrast to the majority of alien species.