THE INFLUENCE OF CHANGING WATER LEVELS AND TEMPERATURES ON THE FOOD-HABITS OF WALLEYE IN LAKE OAHE, SOUTH-DAKOTA

Fluctuating water levels and water temperatures in recent years have h ad an influenced the food habits of walleye (Stizostedion vitreum) in Lake Oahe, South Dakota. We collected walleye from the upper, middle, and lower regions of the reservoir in 1993 and, using the relative imp ortance (RI) index, compared walleye food habits in 1993 with those fr om a similar study of Lake Oahe conducted in 1991. Rainbow smelt (Osme rus mordax) had higher RI values than other prey in all regions of the reservoir throughout 1993. In 1991, rainbow smelt dominated the diet of walleye only in the lower portion of the reservoir. Ephemeridae and chironomidae were the only macroinvertebrates found in the walleye di et; however, neither constituted a large portion of the diet in either year. Seasonal importance did, however, vary slightly among sites as diets became more diverse in the upper reaches of the reservoir. The p redominance of rainbow smelt in the diet of walleye throughout the res ervoir in 1993 was probably due to cooler water temperatures and incre ased reservoir volume. Cooler water temperatures in 1993 allowed rainb ow smelt to occupy areas where water temperatures exceeded their upper thermal limit in 1991.