MONTHLY AND SIZE-RELATED CHANGES IN THE DIET OF RAINBOW-TROUT IN LAKEOAHE, SOUTH-DAKOTA

We analyzed the food habits of rainbow trout (Oncorhynchus mykiss) fro m Lake Oahe, South Dakota, from May through September, 1993-1994. Rain bow trout were captured in surface, suspended and bottom monofilament gillnets. Diets were analyzed using the relative importance index (RI) . Composition of the diet differed among months and among size categor ies of rainbow trout. Greatest RI values for zooplankton prey occurred during May, July and August. Terrestrial invertebrates had the highes t RI value during June and September. Overall, RI values for aquatic m acroinvertebrates and rainbow smelt (Osmerus mordax) were lower than t he other two prey categories. Zooplankton were the most important prey for rainbow trout < 330 mm and decreased in importance as rainbow tro ut increased in length. Rainbow trout between 330-459 mm fed predomina ntly on terrestrial invertebrates. Rainbow smelt were incorporated int o the diet of rainbow trout between 201-330 mm and increased in import ance as rainbow trout length increased. Rainbow smelt dominated the di et of rainbow trout > 460 mm.