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 Ashless 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.
Bibliographical noteFunding Information:
We gratefully acknowledge S. Boss, I. Ludwig, I. Lusebrink, K. Ostermann, R. Popowich, and numerous volunteers for their hard work in the field and laboratory. Thanks also to K. Norris, University of Alberta, for sampling juveniles in 2003, and P. Aku, Alberta Conservation Association, for fastidiously aging our northern pike. P. Aku, D. Hayes, C. Paszkowski, G. Scrimgeour, A. Wolfe, and anonymous reviewers provided insightful comments on earlier versions of this manuscript. Figure 1 was created with help from D. Stoeher. In-kind support was provided unselfishly by Lac La Biche District Fisheries manager C. Davis and by staff at the Meanook Biological Research Station, University of Alberta. Funding was provided by grants and scholarships to P.A.V. from Alberta Sport, Recreation, Parks and Wildlife Foundation, TransCanada Pipelines, Alberta Conservation Association, Canadian Circumpolar Institute, Mountain Equipment Cooperative, Natural Sciences and Engineering Research Council of Canada (NSERC), Alberta–Pacific Forest Industries, and scholarships through the Department of Biological Sciences, University of Alberta, and the Government of Alberta. Additional support was provided by an NSERC research grant to W.M.T.