Effects of Algae Cover on Egg Acquisition in Male Three-Spined Stickleback

Summary The intensity of sexual selection depends on environmental conditions; factors such as predation risk and energy limitation determine the strength of mate choice and the intensity of sexual displays. Eutrophication of shallow coastal areas is changing the breeding habitats of the three-spined stickleback Gasterosteus aculeateus by increasing the growth of filamentous algae. Here I show that the density of breeding males is higher in habitats with a denser growth of filamentous algae, but the variation in egg number among nests is lower. This more equal distribution of eggs in densely vegetated habitats could be due to a lower variation among males in mating success, or to habitat-dependent frequencies of egg consumption, egg stealing or sneak fertilisation homogenising the distribution of eggs among nests in densely vegetated habitats. Of these possibilities, reduced variation among males in mating success is the most likely explanation, because increased vegetation reduces the opportunity for mate choice by reducing visibility and mate encounter rate. Variation in egg stealing and sneak fertilisation are less likely to explain the result because of their generally low frequency. Further experimental work is required to determine the causes and consequences of the habitatdependent distribution of eggs among nests.