A COMPARATIVE STUDY ON THE FEEDING COMPETITION OF THE EUROPEAN PERCH PERCA FLUVIATILIS L. AND THE RUFFE GYMNOCEPHALUS CERNUUS (L.) IN LAKE PIEDILUCO (UMBRIA, ITALY).

The abundance of European perch in Lake Piediluco has significantly dwindled in the last few years. The present study on diet overlap between perch and ruffe was prompted by the rapid expansion of the ruffe stock in the lake. This species was first found in the lake in 1996 and has since become one of the most abundant. The degree of diet overlap between the two species was analyzed by using data on the stomach contents of 275 European perch and 328 ruffe. Results are expressed as abundance (%N), occurrence (%S), weight percentage (%W) and index of predominance (Ip). The diet overlap index (α) was calculated by means of Schoener’s formula using the %W of each food item. As an estimate of the diet width of both species, we used Levins’ indexes of niche breadth (B) and standardized measurement of niche breadth (BA). Both species are strictly carnivorous, feeding mainly on invertebrates: the most important diet components were dipterans and crustaceans, but the European perch also feeds on fish. In the perch, the Levins index was greater (B=4.332) than that calculated for the ruffe (B=2.262). During the ontogenesis of the European perch, there is a rather pronounced diet shift: dipterans form the largest portion of the diet at all ages, though in older perch fish-eating becomes increasingly evident. Benthic crustaceans tend to be consumed in greater quantities by the 3+ age-class, though they are also found in the stomachs of specimens of all ages. Ruffe, by contrast, do not display a pronounced ontogenetic diet shift. The index of diet overlap between the two species was rather high, the maximum α value being 0.853; values indicate a high degree of diet overlap in the younger ageclasses (1+, 2+ and 3+), with a greater differentiation between the diets of the two species emerging as the age of the perch increased. Our research also clarified some of the biological characteristics of the European perch in Lake Piediluco, namely, theoretical growth in length, weight at length and relative weight. These analyses enabled us to compare the growth characteristics of the European perch during periods when ruffe stocks differed in terms of abundance. All of the results indicate that the growth conditions of the European perch population in Lake Piediluco are steadily worsening. This provides indirect confirmation of a negative interaction with the ruffe