Artificial light at night (ALAN) is a global pollutant that affects terrestrial and aquatic ecosystems. Coastal marine environments experience increasing artificial lighting, but our understanding of how it influences community ecology, especially species interactions, remains limited. To investigate if ALAN affects coral reef fish predator-prey interactions in the wild, we exposed diurnal humbug damselfish and their surrounding wild mesopredator assemblages to ecologically relevant levels of artificial light (∼0.26-1.45 lx) over two lunar months in Mo'orea, French Polynesia. We used infrared cameras to evaluate damselfish behaviour, piscivorous predation risk, and damselfish responses to mesopredators shortly after sunset and at midnight. Exposure to ALAN increased the time damselfish spent out of their shelter from 3.0% to 22.7% shortly after sunset and at midnight. This extended period of activity was associated with continued foraging on planktonic prey, increasing from almost none to 11.7 and 4.9 foraging events per 10 min on average shortly after sunset and at midnight, respectively. A proxy of piscivorous predation risk - the number of mesopredator encounters - increased considerably at midnight with artificial lighting. Mesopredator detection rate was 12.6 times higher during the nighttime due to crepuscular and nocturnal mesopredators extending and increasing their activity locally. Finally, the probability of damselfish sheltering during mesopredator encounters decreased by 16% under ALAN at midnight, likely due to a shift in the foraging-sheltering trade-off. Our results show that even dim, ecologically realistic levels of ALAN along coastlines can simultaneously modify prey and predator behaviour, thereby intensifying nocturnal predation dynamics.
