Changes in Abundance of Two Percids, Perca fluviatilis and Gymnocephalus cernuus, along a Productivity Gradient: Relations to Feeding Strategies and Competitive Abilities

Relative abundances of perch (Perca fluviatilis) and ruffe (Gymnocephalus cernuus) were studied in eight lakes ranked along a gradient of increasing productivity. The diets of perch and ruffe in relation to food resources were quantified over a 5-mo period in one lake of moderate and one lake of high productivity. The abundance of perch decreased and that of ruffe increased along the productivity gradient. Large perch ate fish over the whole season in the more productive lake, but rarely ate fish in May and June in the less productive lake. All size classes of ruffe fed mainly on macrobenthos in both lakes. The diversity of macrobenthos was higher in the lake of low productivity, as was the diet breadth of ruffe. The abundance patterns of perch and ruffe are likely related to the simultaneous effect of increasing prey abundance and decreasing light penetration with increasing productivity. Furthermore, the abundance of perch is likely affected by increasing interspecific competition and that of ruffe by decreasing predation pressure as productivity increases.