Feeding Flexibility in Northern Pike (Esox lucius): Fish versus Invertebrate Prey

Abstract

We describe the frequency of occurrence of fish and invertebrates in the stomach contents of northern pike (Esox lucius) from eight populations in Alberta and the Northwest Territories. Lakes were sampled one to eight times between 1979 and 1985, and a total of 899 pike were examined (mean standard length = 44 ± 8 cm, SD). Although northern pike are widely considered to be top aquatic piscivores, we found that the frequency of occurrence of invertebrates was high, at least on a seasonal basis, in some systems. In all of the lakes considered, the frequency of occurrence of fish (expressed as a percentage of non-empty stomachs) was 88% or greater in one or more of the sample periods. However, diets where the frequency of occurrence of invertebrates was greater than 90% occurred in three of the populations in one or more of the sample periods. The frequency of occurrence of invertebrates was negatively correlated with the percentage of empty stomachs, while the frequency of occurrence of fish was positively related to the percentage of empty stomachs. Invertebrate feeding was not limited exclusively to smaller pike; invertebrates were found in the stomachs of pike 24 to 60 cm long (standard length).