Remote underwater video for monitoring reef fish spawning aggregations

Abstract Fish spawning aggregations (FSAs) are critical events in the life cycle of many commercially and ecologically important species, yet FSAs are often exploited and highly vulnerable to fishing due to their predictability in time and space. Although FSAs are increasingly recognized as a conservation and management priority, monitoring these ephemeral dynamic events remains challenging. Here, we assessed the ability of remote underwater video (RUV) to monitor reef FSAs, using blacksaddled coral grouper (Plectropomus laevis) as a case study. We deployed RUV systems monthly on a year-round basis in a reef channel of the New Caledonian barrier reef where P. laevis was assumed to form spawning aggregations. Specifically, we investigated whether RUV could both track spatiotemporal changes in fish abundance and detect spawning signs to validate the purpose of potential gatherings. Our analysis revealed strong seasonal changes in P. laevis abundance, characterized by a 5-fold increase between October and February, and marked variations among sampled habitats. The probability of occurrence of males in courtship colouration showed spatiotemporal patterns fairly similar to those in abundance. Finally, we recorded clear courtship behaviours between November and April, providing evidence that P. laevis aggregate monthly to spawn during a protracted reproductive season. This study advances our understanding of the spawning aggregation dynamics of this understudied grouper, and highlights the potential of RUV to monitor and ultimately inform management of reef FSAs.