THREATS TO THE LAKE FISH COMMUNITIES OF THE U.K. ARISING FROM EUTROPHICATION AND SPECIES INTRODUCTIONS by IAN J. WINFIELD (NERC Institute of FreshwaterEcology, The WindermereLaboratory, Far Sawrey,Ambleside, CumbriaLA22 OLP, U.K) ABSTRACT Eutrophication and species introductions are recognized as two of the major threats to lake fish communities in many parts of the world. Examples of the direct and indirect ways in which eutrophication threatens salmonid, coregonid, percid and cyprinid populations of U.K. lakes are briefly reviewed. In addition, the threats posed by the introduction of new species to these relatively species-poor communities are also illus- trated and discussed by reference to several case histories. KEY WORDS:conservation, threats, community eutrophication, species-introductions, U.K. INTRODUCTION In an introduction to a recent international symposium on rare fishes and their conservation, Le Cren (1990) noted that fish conservation has often been concerned largely with single species and suggested that in the future more attention should be given to fish communities. This philosophy is particularly appropriate to the conservation of U.K. lake fishes because, while no endemic species are present, the relatively low species richness has resulted in fish communities markedly different from those found in similar habitats in mainland Europe. In addition, these communities are also of conservation value because in contrast to those elsewhere in Europe, their populations are largely free from the effects of commercial fisheries. Such locally unexploited species include Arctic charr Salvelinus alpinus (L.), vendace Coregonus albula (L.) and populations of C. lavaretus (L.) known as schelly, powan and gwyniad in England, Scotland and Wales respectively. A review of the current status of threatened fish, freshwater and marine, in the British Isles is provided by MAITLAND &LYLE (1991). In general, fish communities may face threats arising from overfish- ing, physical habitat changes, pollution and species introductions 234 (LOWE-MCCONNELL, 1990), although global warming may become an important additional factor in the future. For U.K. lake communities, the greatest threats probably originate from pollution, particularly acidification and eutrophication, and species introductions. Indeed, LOWE-MCCONNELL (1990) considers that the introduction of exotic species poses the major threat to lacustrine communities worldwide. As the problem of U.K. lake acidification has been widely addressed in recent years, see for example several papers and references therein in MAITLAND al. (1987), the present review is restricted to the effects of eutrophication and species introductions. EUTROPHICATION The process of lake eutrophication may have both direct and indirect effects on fish communities, the latter of which may be particularly dramatic when combined with species introductions. Salmonids and coregonids A thorough account of the direct eutrophication threat to a lake salmonid, the Arctic charr, in Windermere, England (fig. 1), is pro- vided by MILLS et al. (1990), while for coregonids such enrichment was reported to be responsible for at least one of the losses of the only two vendace populations in Scotland (MAITLAND, 1979). Eutrophication threats to the U.K.'s last two vendace populations in Bassenthwaite Lake and Derwentwater in the English Lake District (fig. 1) are cur- rently being investigated by the author. Encouragingly, the only core- gonid of Northern Ireland, the Irish pollan Coregonus autumnalis pollan Thompson, has survived the very eutrophic conditions of Lough Neagh (fig. 1) during the 1970s, probably owing to the well-mixed nature of this large but shallow lake which deters the development of low oxygen levels typically associated with advanced eutrophication (WINFIELD & WOOD, 1990). Percids and cyprinids Eutrophication has also had considerable effects on the percid and cyprinid populations of lakes, although in such situations indirect effects appear to be more important (see review by PERSSON, 1991). The pattern of change in the U.K. has been similar to that observed elsewhere in Europe, with a shift from percid to cyprinid dominance with increasing eutrophication, although with typically fewer species being involved. Thus, the cyprinids roach Rutilus rutilus (L.) and bream Abramis brama (L.) often become dominant with increasing eutro- .