Study subjects and housing
We used a wild population of threespine stickleback, Gasterosteus aculeatus , caught with unbaited minnow traps from a vegetated freshwater lake in April 2021 (Syðridalsvatn, 66° 7’ N, 23° 14’ W). Thirty-three fish of the size 4.0-4.1cm were transported the short distance to the laboratory (<15min). This size of fish most likely represents individuals that hatched in spring/summer 2019. All fish were placed in individual holding tanks of 9.5l (Aquaneering.com). Each tank was equipped with a single 12cm knee-shaped PVC pipe for cover. Temperature was kept at 12.0°C, with an 8:16 h light/dark cycle regime. The tanks were continuous flow through, fitted with a biological filter, UV sterilization and aerated. After a single day of acclimating to the laboratory we began observations. Fish were fed defrosted frozen bloodworms (Chironomus spp .) on the day of capture and then to satiation daily after the experiment. Behavioral experiment without invasive or stressful procedures, such as the current experiment, do not require licensing by Icelandic law.