Selective predation upon pelvic phenotypes of brook stickleback, Culaea inconstans, by northern pike, Esox lucius

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Abstract
Laboratory and field work assessed the importance of Esox lucius predation in maintaining pelvic skeletal phenotypes in a population of Culaea inconstans. Individuals possessing a pelvis and pelvic spines (with) have a selective advantage over those lacking these parts (without). The latter are preferentially preyed upon in the lab by small northern pike due to this morphological difference. The advantage conferred by the spines depends upon relative size of predator to prey and likely has no effect upon predation by larger pike. Preferential selection of with individuals whose pelvic spines were removed, rather than without individuals (i.e. morphologically similar prey), suggests withouts compensate behaviourally for their morphological disadvantage. Analysis of food of large wild pike indicated that these postulated behavioural differences contributed to selective predation of individuals of the with phenotype in Wakomao Lake, Alberta, Canada. The frequency of these phenotypes in any situation represents a balance between counterselective forces, some of which are due to predation. Additional predatory species present in the lake and additional uses of pelvic spines probably also influence the phenotypic frequencies.