Diet shifts in 0+ nase, Chondrostoma nasus: Size-specific differences and the effect of food availability
Reckendorfer, W.; Keckeis, H.; Tutu, V.; Winkler, G.; Zornig, H.; Schiemer, F.
To document diet shifts, predator-prey relations and niche characteristics during early ontogeny, we examined the diet of age-O nase based on the food availability. The study was undertaken in the free-flowing section of the Danube River, Austria. Larval and juvenile nase (n = 289, 11.3-57.0 mm total length (TL)), zooplankton and drifting invertebrates were collected from May to August 1994. Two size-specific diet shifts occurred during ontogeny. The first one took place at about 14 mm TL, when nase switched from rotifers of the genera Brachionus and Keratella to drifting invertebrates, especially chironomids, and terrestrial insects. The second one occurred between 40 mm and 60 mm TL, when the drift-oriented feeding mode was replaced by a benthic-oriented feeding mode in which nase fed exclusively on benthic algae. Ontogenetic diet shifts were indicated by significantly different diet and prey selectivity of nase of different size classes within dates and by generally higher diet overlap within a size class than between size classes. Seasonal variation in the diet and prey selectivity of nase of the same size class could be attributed to food availability. For certain size classes and the most abundant food categories, both diet and prey selectivity were significantly related to prey density. Diet overlap among individual fish also differed between dates. When resources decreased, diet overlap was lowest, suggesting diet segregation. Intraspecific diet segregation was also indicated by a specialized feeding of individual fish compared with a broad diet of the population during this period.