Ecology and evolution of sympatric sticklebacks (Gasterosteus): specialization for alternative trophic niches in the Enos Lake species pair

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Abstract
Two morphologically distinct species of threespine stickleback (Gasterosteus) coexist in Enos Lake on Vancouver Island. For convenience these species are referred to as "limnetics" and "benthics." We used three experiments to compare the feeding performance of the two species and to test the hypothesis that limnetics are adapted for planktivory whereas benthics are adapted to forage on substrates. The results support this conclusion. When tested for maximum prey size; benthics consumed larger prey (relative to their body size) than limnetics. Benthics also were more successful than limnetics in foraging on a substrate. When allowed to forage for a fixed time on a detritus substrate, benthics of both sexes captured more prey than male limnetics, even though male limnetics directed as many feeding strikes at the substrate as did the benthics. Female limnetics would not forage on a detritus substrate. Limnetics were more successful in feeding on plankton than benthics. When held in mesh enclosures suspended in Enos Lake, limnetics consumed more plankton than benthics. Small limnetics (26–36 mm standard length) consumed nearly four times more plankton than large limnetics (44–50 mm standard length). The small limnetics were all either mature females or immature males; the large limnetics were all mature males. The results of the benthic foraging and plankton feeding comparisons support the additional conclusion that Enos Lake limnetics are sexually dimorphic in their feeding behaviour.