Adaptive Divergence of Trophic Phenotype among Freshwater Populations of the Threespine Stickleback (Gasterosteus aculeatus)

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Abstract
We investigated differences in the morphological and behavioural contributions to foraging success among three morphotypes (open-water, littoral, and intermediate) of the threespine stickleback (Gasterosteus aculeatus) from a single drainage system on Vancouver Island. Stomach contents from wild samples showed diet to be dependent on morphotype. Interpopulation differences in trophic morphology were associated with differences in foraging success on given prey types. The longer jaw of the bottom-browsing littoral morphotype allowed it to ingest significantly larger prey then either the intermediate or open-water populations. In addition, the littoral type spent less time manipulating benthic prey then either the open-water or intermediate morphotype. The latter observation is independent of interpopulation differences in jaw length. Both the planktivorous open-water morphotype and intermediate morphotypes were superior foragers in experiments on water column prey. The differences in water column foraging success are associated with interpopulation differences in gill raker morphology. Our results support the conclusion that inter population divergence in trophic phenotype is an adaptive response to differences in the primary trophic resources available in lakes of different morphometry.