THE RUFFE POPULATION OF LOCH-LOMOND, SCOTLAND - ITS INTRODUCTION, POPULATION EXPANSION, AND INTERACTION WITH NATIVE SPECIES

The ruffe (Gymnocephalus cernuus), never before recorded in Scotland, was discovered in Loch Lomond in 1982. During the 1980s ruffe became w ell established and expanded its range throughout the loch and through the slow-flowing influent and effluent tributaries, only being exclud ed from tributaries with high flow rates. Recoveries of fish entrained at a pumping station, abstracting water for domestic supply, provide a measure of the rate of population expansion between 1982 and 1996 an d the current status of ruffe in Loch Lomond Between 1982 and 1992 the ruffe population grew exponentially. Since 1992 the population appear s to have stabilized to some extent but at a very high level. Gill net ting surveys conducted throughout Loch Lomond have shown that the high abundance of ruffe recorded at this site is representative of a large population throughout the loch. The diet of ruffe feeding in Loch Lom ond differed both spatially and seasonally. Although prey choice was d iverse, it was primarily composed of benthic macro-invertebrates, but at times included underyearling fish (including ruffe) and eggs of oth er fish species. The most important of these is the powan (Coregonus l avaretus), a broadcast spawner of national conservation value. To exam ine the possibility of dietary overlap with perch (Perca fluviatilis) and brown trout (Salmo trutta), the diets of these three species were compared. The data showed very little evidence of overlap in diet, sug gesting that feeding resource competition between ruffe and trout and ruffe and perch does not occur between adults of these species. Eviden ce for the disruption of predator-prey relations by introduced ruffe i s reviewed. Ruffe are now the primary prey species for cormorants (Pha lacrocorax carbo), herons (Ardea cinerea), and northern pike (Esox luc ius) feeding in Loch Lomond. Calculations based on predator population sizes and consumption rates suggest that potentially up to 17 metric tons of ruffe, representing over 2 million individuals, may be being c onsumed annually by these predator species. The effects on native fish of this shift in diet in these predator species is discussed, as is t he ability of ecologists to predict the impact of invasions of new spe cies in aquatic communities.