Effects of turbidity and habitat complexity on antipredator behavior of three-spined sticklebacks (Gasterosteus aculeatus)

Human-induced processes like eutrophication are increasing water turbidity and altering vegetated habitats in the Baltic Sea. Unfortunately, the influence of these environmental changes on predator-prey interactions remains poorly studied in mobile taxa of this region. We used three-spined stickleback (Gasterosteus aculeatus) as a model species to study the combined effects of turbidity and shoot density on habitat choice (shelter vs. open) in the presence and absence of piscivorous perch (Perca fluviatilis). Habitat choice of sticklebacks was video-monitored and compared between two paired observation periods: "control" (no predator) and "risk" (predator present). Though sticklebacks exhibited a general preference for sheltered habitat across treatments, repeated measures ANOVA found that sticklebacks responded to predator presence by significantly increasing use of the sheltered habitat. However, shoot density of the shelter interacted with observation period, where risk-induced shifts of sticklebacks into sheltered habitat were most apparent and sustained at higher shoot densities. Stickleback activity level was generally reduced with predator presence at higher turbidity and shoot density levels, suggesting a possible adaptation to reduce predator encounter rates in visually deteriorated conditions. Overall, our study demonstrates that relatively minute increases (1's - 10's of NTU) in algal turbidity intensify three-spined stickleback sheltered habitat use, but vegetation density may still play a larger role in avoiding predators at these turbidity levels. When compared with a similar study on 0+ perch, our work suggests that eutrophication (i.e., increased turbidity, loss of habitat complexity) may have variable, taxon- and/or habitat-specific effects on predator-prey interactions of the Baltic Sea.