DIET OVERLAP AMONG SIZE CLASSES OF COLORADO RIVER CUTTHROAT TROUT (ONCORHYNCHUS-CLARKI-PLEURITICUS) IN A HIGH-ELEVATION MOUNTAIN STREAM

We examined the taxonomic composition, abundance, and size of food ite ms consumed by young-of-year, juvenile, and adult Colorado River cutth roat trout (Oncorhynchus clarki pleuriticus) in order to determine the degree of diet overlap occurring in a relatively unproductive, high-e levation, mountain stream. Overall, we identified 49 Families of insec ts representing nine Orders, and 4 other Classes of organisms in the d iets of the trout sampled and saw no evidence of piscivory. Each size class of fish consumed significantly different taxa and significantly different sizes of food items. However, despite these differences, the proportional similarity index (PSI) indicated that there was consider able overlap in taxa and sizes of organisms consumed by the three size classes. The greatest overlap occurred between young-of-year and juve niles, and between juveniles and adults. Both the relatively high prop ortion of small items in the adult diet and the slow growth rate of ad ults in these streams indicate that food may be limiting for adults an d that intraspecific competition between adults and smaller size class es may be high.