Specializations for alternate trophic niches by two forms of threespine stickleback, Gasterosteus, co-existing in Enos Lake, Vancouver Island

Two morphologically and ecologically distinct forms of threespine stickleback (Gasterosteus), a "limnetic" form and a "benthic" form, co-exist in Enos Lake on Vancouver Island. I used three experiments to compare the feeding performance of the two forms, to test the hypothesis that limnetics are adapted to planktivory and benthics are adapted to foraging on benthic substrates. The results support this conclusion. When tested for maximum prey size, benthics were able to consume larger prey (relative to their body size) than limnetics. Benthics were also more successful than limnetics in foraging on a benthic substrate. When allowed to forage on a detritus substrate for a fixed time interval, benthics of both sexes captured more prey than male limnetics, despite the fact that male limnetics directed as many feeding strikes at the substrate as did the benthics. Female limnetics did not forage on the substrate at all. In contrast, limnetics were more successful in feeding on plankton than benthics. When held in mesh enclosures suspended in the water column of Enos Lake, limnetics consumed more plankton than benthics. "Small" limnetics (26-35 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. These results, along with morphological, biochemical, ecological and behavioural data obtained in other studies, support the conclusion that the two forms of Gasterosteus in Enos Lake are distinct biological species. The results of this study also support another (unexpected) conclusion: mature male and female limnetics also differ in feeding behaviour. Female limnetics appear to be almost totally planktivorous; whereas, male limnetics are intermediate between female limnetics and benthics (both sexes) in feeding behaviour.Science, Faculty ofZoology, Department ofGraduat