Impacts of trout predation on fitness of sympatric sticklebacks and their hybrids

Predation may be a significant factor in the divergence of sympatric species although its role has been largely overlooked. This study examines the consequences of predation on the fitness of a pair of lacustrine stickleback species (Gasterosteus aculeatus complex) and their F, hybrids. Benthic sticklebacks are found in the littoral zone of lakes associated with vegetation and bare sediments, whereas limnetic sticklebacks spend most of their lives in the pelagic zone. The cutthroat trout (Oncorhynchus clarki) is a major predator of sticklebacks and the only other fish species native to lakes containing both benthic and limnetic species. In pond experiments we found that the addition of these predators primarily impacted the survival of linmetics. By contrast, benthic survival was unaffected by trout addition. The result was that relative survival of benthics and linmetics was reversed in the presence of trout. The presence of trout had no effect on the rank order of parent species growth rates, with benthics always growing faster than linmetics. F, hybrids survived poorly relative to benthics and linmetics and their growth rates were intermediate regardless of treatment. The results implicate predation by trout in the divergence of the species but not through increased vulnerability of F-1 hybrids.