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 had 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 importance (RI) index, compared walleye food habits in 1993 with those from a similar study of Lake Oahe conducted in 1991. Rainbow smelt (Osmerus 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 diet; 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 predominance of rainbow smelt in the diet of walleye throughout the reservoir in 1993 was probably due to cooler water temperatures and increased reservoir volume. Cooler water temperatures in 1993 allowed rainbow smelt to occupy areas where water temperatures exceeded their upper thermal limit in 1991.