ABSTRACT
Ornamental tropical fish are kept in home aquaria for many decades. It is often that fish that grown too big or are unwanted by the owner are released into natural environment. Most tropical fish have zero chance for survival in polish natural water bodies not only through the winter, but in many cases even through summer months. There are several reports of non-native fauna species in our waters. Successful introductions took places in few artificial canals with thermally polluted waters namely pumpkinseed Lepomis gibbosus (Linnaeus, 1758) in warm canal of Odra river, dramatically changing local ecosystems. Object of this paper is Silver arowana (Osteoglossum bicirrhosum) caught in hand aquarium net in Powsinkowskie Lake. The lake is one of the most important water reservoir in most urbanized part of Poland โ its capital Warsaw. Because of its location in highly urbanized area it is the reservoir of intense uncontrolled introduction of unwanted fish. Most tropical alien species may not be able to survive in our local waters but may cause severe local ecosystem imbalances for example they may possibly be hosts to pathogenic organism that our fish have no immune system tuned to. Therefore places like Powsinkowskie Lake is a good place for scientist to perform periodic inventory to determine the species composition, fish numbers and condition.
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