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 well established and expanded its range throughout the loch and through the slow-flowing influent and effluent tributaries, only being excluded from tributaries with high flow rates. Recoveries offish entrained at a pumping station, abstracting water for domestic supply, provide a measure of the rate of population expansion between 1982 and 1996 and the current status of ruffe in Loch Lomond. Between 1982 and 1992 the ruffe population grew exponentially. Since 1992 the population appears to have stabilized to some extent but at a very high level. Gill-netting 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 Lomond differed both spatially and seasonally. Although prey choice was diverse, it was primarily composed of benthic macro-invertebrates, but at times included underyearling fish (including ruffe) and eggs of other fish species. The most important of these is the powan ( Coregonus lavaretus), a broadcast spawner of national conservation value. To examine 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, suggesting that feeding resource competition between ruffe and trout and ruffe and perch does not occur between adults of these species. Evidence for the disruption of predator-prey relations by introduced ruffe is reviewed. Ruffe are now the primary prey species for cormorants ( Phalacrocorax carbo), herons ( Ardea cinerea), and northern pike ( Esox lucius) 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 consumed annually by these predator species. The effects on native fish of this shift in diet in these predator species is discussed, as is the ability of ecologists to predict the impact of invasions of new species in aquatic communities.