The increasing prevalence of artificial light at night (ALAN) is of growing ecological concern. By altering natural light regimes, ALAN affects the behaviour, physiology and life-history of organisms. While research has largely focused on terrestrial ecosystems, aquatic ecosystems are increasingly exposed. Here, we systematically review current knowledge of the impacts of ALAN on fishes and the underlying mechanisms and ecological consequences. We first discuss the factors influencing fish exposure to ALAN and how light can exert both direct and indirect effects. We then examine the potential consequences of these responses for ecosystem structure and functions, considering interactions with other environmental stressors, such as warming, noise and chemical pollution. To assess the current state of knowledge, we systematically review the literature. We find that most studies have focussed on the direct effects of ALAN on fish behaviour and physiology, whereas the fitness consequences of these responses and the impacts on population dynamics, species interactions and community composition remain largely unexplored. Notably, no study has assessed the broader implications for ecosystem functions. We identify key research priorities to advance understanding of both the short- and long-term effects of ALAN on fishes and to inform effective mitigation strategies. In particular, more research is needed on the ecological consequences of fish responses to ALAN. We conclude by discussing potential solutions to light pollution, emphasising the importance of integrating economic, social and environmental considerations in the development of environmentally friendly lighting practices.
