Diel changes in diet composition of an omnivorous cyprinid - a possible source of error in estimating food consumption

Diel changes in diet composition of omnivorous roach (Rutilus rutilus (L.)) and the effects on food consumption estimates given by a bioenergetics mod el were studied. In all length groups of roach, the percentage of zooplankt on in the diets was higher and the percentage of zoobenthos, plants and det ritus lower during daytime than during the night. As a result, compared wit h sampling throughout the day, consumption of zooplankton was overestimated (by 6-11%) and the consumption of other food categories underestimated (by up to 200%) when only daytime samples were taken into account. The error c aused by the diurnal diet variation was larger than that caused by a 10% de viation in the digestibility of prey and energy contents of predator and pr ey. The results suggest that diurnal changes in diet composition may consid erably bias food consumption estimates for fish species that feed both on a nimal prey and plant material. Additionally, estimates for fish-mediated nu trient recycling may be biased by inaccurate determinations of fish diets.