Cannibalistic growth polyphenism in Atlantic cod

Background: Phenotypic variability in Atlantic cod in the Canadian Arctic is characterized by a distinctive resource polymorphism manifest by the co-existence of cannibalistic and noncannibalistic individuals in the same populations. Questions: What are the fitness consequences of this variation in foraging tactic? What maintains the two phenotypes? Method: Measure and compare condition and growth trajectories between the two morphs from three meromictic lakes on Baffin Island, Canada. Use life-history invariants to estimate natural mortality and other components of fitness. Results: Compared with non-cannibals, cannibals are predicted to achieve greater maximum size, experience higher mortality in early life, and mature at a larger size, albeit at the same age. Growth trajectories of cannibals and non-cannibals diverge as early as one year of age. Condition indices do not differ between forage morphs. The evidence does not support a genetic difference between cannibals and non-cannibals. Conclusion: The co-existence of alternative foraging tactics by cod within Arctic lake populations can be explained as phenotypic variability maintained by environments that pose similar fitness consequences to cannibalistic and non-cannibalistic individuals.