The Effects of Interspecific Interactions Upon Male Reproductive Success in Two Sympatric Sticklebacks, Gasterosteus Aculeatus and G. Wheatlandi

Abstract

1. Two closely related sticklebacks nest together in tide pools along the southern shore of the St. Lawrence estuary. 2. We tested the hypothesis that interspecific interactions limit male reproductive success by manipulating both fish density and species abundance ratios. The index of male reproductive success used was the number of eggs per nest and the per cent of males building nests. 3. There was no significant relationship between reproductive success and density or abundance ratios. 4. The data of the present study (sympatric populations) were compared to a previous study (allopatric populations). A higher per cent of G. aculeatus bred in the sympatric than in the allopatric pools while the opposite was true for G. wheatlandi. There were no differences in the number of eggs per nest for either species between the allopatric and sympatric situations. 5. We conclude that interspecific interactions between G. aculeatus and G. wheatlandi may lower the reproductive success of G. wheatlandi when both species breed in sympatry.