Diet and Growth of Northern Pike in the Absence of Prey Fishes: Initial Consequences for Persisting in Disturbance‐Prone Lakes

The northern pike Esox lucius is a renowned piscivore, but will prey opportunistically on invertebrates (e.g., in small lakes of boreal Alberta, where winterkill can unexpectedly reduce or eliminate prey fishes). We emulated such a disturbance by stocking a fishless lake with northern pike and then monitored their diet and growth over two summers. Stomach content analysis revealed that stocked adults responded to the sudden absence of prey fishes by specializing on energy‐rich leeches (families Glossiphoniidae and Erpobdellidae), whereas juvenile offspring consumed a broader mix of invertebrates. Stable isotope analysis supported these results and indicated a relatively rapid drop in the trophic position of stocked adults. Compared with growth of northern pike in regional lakes containing prey fishes, growth of adults in the experimental lake was apparently compromised by a diet of invertebrates but growth of juveniles was high. Although long‐term dynamics of northern pike in these disturbance‐prone lakes are undocumented, our results suggest that northern pike are capable of adapting rapidly to the absence of prey fishes; however, such a diet imposes a trophic bottleneck that can lead to stunting.