Human-induced eutrophication has increased offspring production in a population of threespine stickleback Gasterosteus aculeatus in the Baltic Sea. Here, we experimentally investigated the effects of an increased density of juveniles on behaviours that influence survival and dispersal, and, hence, population growth—habitat choice, risk taking, and foraging rate. Juveniles were allowed to choose between two habitats that differed in structural complexity, in the absence and presence of predators and conspecific juveniles. In the absence of predators or conspecifics, juveniles preferred the more complex habitat. The preference was further enhanced in the presence of a natural predator, a perch Perca fluviatilis (behind a transparent Plexiglas wall). However, an increased density of conspecifics relaxed the predator-enhanced preference for the complex habitat and increased the use of the open, more predator-exposed habitat. Foraging rate was reduced under increased perceived predation risk. These results suggest that density-dependent behaviours can cause individuals to choose suboptimal habitats where predation risk is high and foraging rate low. This could contribute to the regulation of population growth in eutrophicated areas where offspring production is high. 1. Introduction Individuals are usually forced to balance costs against benefits when choosing a habitat [1, 2]. Structurally complex habitats are often more favourable than open habitats, as they provide more resources and better refuges against predators [3–5]. However, complex habitats can also be costly if they harbour more predators and competitors than open habitats [6, 7]. In addition, the costs and benefits of choosing a habitat depend on what other individuals in the population are doing [8–10]. The profitability of a habitat decreases the more individuals that occupy it, because of a reduction in the amount of resources available per individual. As high quality habitats become saturated, more and more individuals are forced to occupy poor-quality habitats [11, 12]. Interactions among individuals will then influence habitat selection, and, thus, density-dependent processes will affect individual fitness and the temporal and spatial distribution of the population. Human activities are currently altering habitats at an unprecedented rate and scale around the world. The consequences that this will have for the populations that were well adapted to the past conditions are poorly known. An environment that is changing rapidly because of human activities is the Baltic Sea. Increased input of
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