A test of ecological selection against young-of-the-year hybrids of sympatric sticklebacks

Ecological selection against hybrids, the reduction in hybrid fitness attri buted solely to environmental factors, was tested by introducing young-of-t he-year benthic, limnetic and F-1 hybrid sticklebacks Gasterosteus aculeatu s to divided experimental ponds and lake enclosures. The frequency of hybri ds in samples taken at the end was significantly lower than their frequency at introduction. Hybrid survival was significantly lower in pond-sides in which they were initially the most common cross type than in pond-sides in which they were initially rare, suggesting that hybrid survival may be freq uency-dependent. Growth rate of F-1 hybrids was marginally lower than benth ic growth rates, being significantly lower than in ponds and not different in lake enclosures. The diet of hybrids overlapped with both parent species in ponds and with benthic diets in lake enclosures. The results suggest th at ecological selection is acting against young-of-the-year hybrid stickleb acks. (C) 2000 The Fisheries Society of the British Isles.