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 of / Zoology, Department of / Graduate

Morphological, biochemical, and trophic evidence from other studies indicate that two populations of stickleback (Gasterosteus) co-exist in Enos Lake, Vancouver Island. One population is limnetic and the other is benthic (the names allude to their foraging behaviour and use of space). In this study, field observations, mate selection experiments, and courtship behaviour experiments were conducted to determine if the two stickleback populations are reproductively isolated. When breeding, limnetic males develop red throats whereas benthic males become uniformly black. In the field limnetic and benthic males nest in different habitats. Despite this difference, reproductive males and females of each population encountered each other, but in the few cases where courtship occurred, it never went beyond the initial stages of the lead-follow sequence. Apparently behavioural differences in courtship contributed to the break off of these natural courtships. In mate selection experiments, males and females of each population preferred mates from their own population. In the courtship experiments, behavioural differences between limnetics and benthics were found to be greatest in the early stages of courtship. With females from their own population, benthic males were more aggressive in their approach and leading sequences than limnetic males. In the beginning of the lead-follow phase, benthic females tend to position themselves above benthic males whereas limnetic females tend to position themselves alongside limnetic males. Male courtship behaviour was sometimes dependent on the phenotype of the female courted; benthic males bit and chased limnetic females whereas, limnetic males bit and led benthic females in a meandering path to the nest. Since the mate choice experiments indicated total positive assortative mating between limnetics and benthics, it is likely that the behavioural differences found in courtship behaviour contribute to reproductive isolation between the limnetic and benthic sticklebacks in Enos Lake. / Science, Faculty of / Zoology, Department of / Graduate