Food niche segregation between two herbivorous cyprinid species in a turbid lake.

The diet and growth of two cyprinids, roach Rutilus rutilus and rudd Scardinius erythrophthalmus, in the clay-turbid Kirkkojärvi basin in Lake Hiidenvesi (Finland) were studied. Young R. rutilus fed mainly on detritus, molluscs, chironomids and zooplankton, while older individuals consumed mostly detritus, molluscs and filamentous algae. In the food of S. erythrophthalmus, detritus was less prevalent and animal food and plants contributed to most of the diets. The animal food of S. erythrophthalmus consisted mostly of surface insects. Of the plant food, the macrophytes Potamogeton obtusifolius and Ranunculus circinatus had the highest representation, while the proportion of filamentous algae was low. The growth rate of S. erythrophthalmus was significantly higher than that of R. rutilus. The results suggested that the species composition of vascular macrophytes was not favourable for R. rutilus. Small submerged species suitable for R. rutilus were absent and they fed on filamentous algae and detritus, which resulted in a slow growth rate. The results suggested that in a clay-turbid lake the food spectrum is more suitable for S. erythrophthalmus than for R. rutilus. In such conditions, R. rutilus may have no benefit from its generalistic feeding abilities.