Abstract
Changes in diet composition related to increase in fish size were. studied in a rainbow trout (Oncorhyncbus mykiss) population inhabiting a stream of the high Venezuelan Andes. The analysis of 319 gut contents of trout in five size classes showed that immatures of Ephemeroptera, Trichoptera, and Diptera are the main components of the diet. The contribution in number and biomass of these groups changed with fish size. Ephemeropterans are dominant in the diet of fishes in size classes under 15 cm. Ephemeropterans and Trichopterans share their importance in number and biomass in the diet of trout in size classes over 15 cm. Seasonal changes of total prey numbers and biomass in gut contents did not affect the proportional contribution of the different taxa within each size class. The infra-order taxa with most relative importance were the genera Baetodes, Baetis, and Thraulodes (Ephemeroptera); Leptonema, Arctopshyche, and Mortoniella (Trichoptera); the families Chironomidae, Blepharoceridae, and Tipulidae (Diptera) and terrestrial invertebrates. The trophic profiles of Relative Importance of preys differed significantly between size classes as a consequence of a differential use of the drift and benthos fauna, and changes in prey selection within each food source.
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