Effects of artificial light at night on the diurnal risk-taking behaviour of a gregarious reef fish

Artificial light at night (ALAN) is increasingly prevalent in both terrestrial and aquatic systems and influences physiological and behavioural traits of wild animals across a range of habitats. In coral reef ecosystems, increasing human pressure contributes to an increase in ALAN exposure for reef organisms. In the present study, we investigated the effects of chronic exposure to ALAN in the wild on diurnal risk-taking and social behaviour in a gregarious coral reef fish, Dascyllus trimaculatus . Fish were collected from sites exposed to ALAN or control light levels and underwent an emergence test to assess diurnal risk-taking behaviour and a 3-chamber shoal choice test to assess sociability. Activity levels were measured during both tests. ALAN-exposed fish took less time to emerge from an anemone compared with control fish. We found evidence for reduced activity in ALAN-exposed fish in sociability trials. Increased risk-taking may be due to either direct effects of ALAN exposure or selective mortality of more risk-averse individuals in the wild due to an increased risk of starvation or reduced growth. While we did not find evidence for increased diurnal activity, ALAN exposure may lead to increasing metabolic needs, driving increased risk-taking and foraging, or selective mortality. However, the increased risk-taking behaviour of ALAN-exposed fish did not increase distance to a conspecific shoal, i.e. no effect on sociability. Our study provides evidence that chronic ALAN exposure in the wild increases risk-taking behaviour in a marine fish.