Variation in female-biased sexual size dimorphism of northern pike (Esox lucius) associated with environment and life history

Sexual size dimorphism (SSD) is a widespread phenomenon in the animal world resulting from differential selection on the sexes. The northern pike (Esox lucius) is a freshwater apex predatory fish species that exhibits female-biased SSD, but the degree to which SSD varies among populations and what variables might dictate variation in SSD in this species remain poorly understood. We sought to quantify the degree of variation in SSD among northern pike populations across a large portion of their North American range, as well as evaluate associations between the magnitude of SSD in northern pike populations with environmental variables and life history traits of populations. We quantified SSD in 102 populations of northern pike across the province of Ontario, Canada, using a standardized gillnetting database, and investigated the degree to which both environmental variables (cisco [Coregonus artedi] abundance as catch-per-unit-effort, lake surface area, and latitude) and northern pike life-history traits (early growth and mortality rates) explained variation in female-biased SSD using linear models. Female-biased SSD in mean weight of northern pike increased with increasing cisco abundance, and the difference in female and male mean age increased with increasing latitude. Furthermore, SSD was greater in populations with lower female mortality and early growth rates. These results indicate that slow-growing, long-lived populations of northern pike should exhibit greater female-biased SSD, and that these conditions may be facilitated by the availability of large, energy-dense prey and cooler temperatures at northern latitudes.