Introduction of fish species into European fresh waters: Economic successes or ecological disasters?

The introduction of fish species into fresh waters is commonplace around the world. There are many examples of well-defined successes (e.g. kapenta, Limnothrissa moidon, into Lake Kariba or Lake Kivu), questionable economic successes (e.g. Nile perch, Lates niloticus, into Lake Victoria or tilapia into Sri Lanka) and abject failures (ruffe, Gymnocephalus cernuus, into the Great Lakes of North America or carp, Cyprinus carpio, into Australia). Similar conclusions can be drawn about the introductions of fish, whether they are accidental or deliberate, into fresh waters in Europe. This paper presents case studies which highlight the problems faced with assessing the environmental, economic and socio-political successes of fish introductions in Europe. The introduction of Chinese carps, particularly the silver carp, Hypophthalmichthys molitrix, into the Danube Delta in Romania to control algal blooms resulting from eutrophication processes or escapees from fish farms, highlights many of the issues. They now contribute to some 30 % of the commercial catches, and the landings of the preferred indigenous fish species have declined dramatically. The Chinese carps are now considered to be breeding in the Delta and may lead to an ecological catastrophe. By contrast, the introduction of rainbow trout, Oncorhynchus mykiss, is considered by many as a success story. The species now supports major sport fisheries throughout Europe, which provide major economic and recreational benefits, but in other cases it has caused considerable loss of native species. There are many other examples to illustrate the positive and negative impacts of introductions. The introduction of pikeperch, Stizostedion lucioperca, into many Western European lakes has resulted in major commercial fisheries, but its release into the eastern rivers of England was heralded as an ecological disaster because it coincided with a collapse in the cyprinid fisheries. Although evidence does not altogether substantiate this argument, the introduction had profound effects on the endemic fish populations. These positive and negative impacts are highlighted, and suggestions for a code of practice to minimise the potential impact of introducing fish species and alternative mechanisms for stock enhancement are discussed.