Changes in roach (Rutilus rutilus (L.)) diet and growth in relation to river water quality

Diet composition and growth of the roach (Rutilus rutilus (L.)) was studied in three Lithuanian rivers, differing in water quality. In the pristine river roach fed on various taxa of macroinvertebrates and plant material with similar frequency, none of food categories clearly dominated in the guts of fish. The overall diversity of food items in the diet as well as abundance and diversity of macroinvertebrates in the benthic assemblages were the greatest in the moderately polluted river, but plant material formed the major part of food and was consumed most frequently, presumably as a result of inter-specific competition for the food resources. Percentage of the plant material in the diet was the smallest in the heavily polluted river, where roach predominated over the rest of fish. The growth rates of roach did not differ among reference and heavily polluted river, but in the moderately polluted river fish grew much slower. It is likely that habitat degradation to an extent when majority of less tolerant fish and macroinvertebrate species do not survive seems to be favorable for roach, if the degradation reduces inter-specific competition for food resources.