Human impact on fish assemblages in the Nemunas River, Lithuania.

Abstract

The Nemunas River is one of the largest rivers of the eastern part of the Baltic Sea with a total length of 937 km and a drainage area of 97,860 km 2 . Historical data show that more than 50 fish species have been present in this river. Recent ichthyological investigations were carried out in the lower and middle Nemunas in 1996-2000. In total, 34 fish species were caught with a beach seine and multi-mesh size bottom and floating gill nets. When comparing these findings with available historical data, sharp changes have been observed in fish communities. Due to dramatic environmental changes reophilous stenobiontic fish species have decreased. Especially sharp changes have been observed in the abundance of migratory fish species. The main spawning grounds of salmon, sea trout and vimha were lost after the middle reaches of the Nemunas River had been dammed. This and other factors, such as pollution, land reclamation and deforestation, are the probable reasons for the decrease of migratory fish species. During the recent years, when wastewater treatment plants were built, pollution of the Nemunas River and the Curonian Lagoon (Baltic Sea) has decreased. Now, nitrogen and phosphorus levels are 2-3 times lower in comparison with the period 10-12 years ago. Migratory species have recently been observed to increase in number again. The main spawning grounds of numerous commercial fish species of the Curonian Lagoon (pikeperch, bream, wels), migratory fish and lampreys of the Baltic Sea (sea lamprey, river lamprey, smelt, twaite shad) and rare fish species have been found in the lower part of the Nemunas River. A programme for the rehabilitation of the Nemunas River Delta, including hydrotechnical, hydrological and nature protecting measures, is now being prepared in Lithuania.