Annual Food Habits of Northern Pike in an Eastern South Dakota Natural Lake

Food habits of northern pike Esox lucius were documented in Lake Thompson, a shallow, windswept, eutrophic lake in eastern South Dakota. The main food items, assessed with the Relative Importance Index, eaten by northern pike in Lake Thompson were common carp Cyprinus carpio, black crappies Pornoxis nigromaculatus, white crappies P. annularis, and fathead minnows Pimephales promelas. Common carp were eaten in mid-summer and again in winter. Black crappies were eaten mostly in the winter. Fathead minnows were an important secondary food source for northern pike of all sizes. Walleyes Stizostedion vitreum were not preyed upon by northern pike to any extent. A spatial and temporal segregation between these two species was suggested by the gill net data. A high use of invertebrates by adult northern pike was observed throughout the open water months. Northern pike in the smaller length groups ate mostly fathead minnows, young-of-the-year common carp, and invertebrates. Larger northern pike diets consisted mostly of larger common carp, young-of-the-year crappies, leopard frogs Rana pipiens, and occasionally walleyes. Northern pike fed heavily in the winter and during the post spawn period, while consumption was lower in the summer. Northern pike empty stomachs increased with northern pike size, which may have been related to an increase in temperature stress experienced by the fish as they grew. Prey size increased as northern pike size increased. However, the diet breadth also increased, suggesting that although large northern pike ate larger prey items, they continued to eat smaller ones also. Northern pike in Lake Thompson tended to have many smaller prey items in their stomachs, instead of one large food item, as has been reported in other studies. This was likely a reflection of the food base in Lake Thompson, which consisted of many smaller prey items, and few larger ones. Percent Resource Overlap Index values suggested that the potential for intraspecific competition was·greatest among northern pike up to 600 mm. This was attributed to a change in diet or behavior when northern pike attain a length of about 600mm