Diel changes in fish assemblages of Southwest Atlantic rocky reefs

The alternation of light and darkness drives species’ activities, and identifying diel rhythms is critical for understanding the ecology and representing the biodiversity of a range of communities. This study aimed to investigate diel changes in rocky reef fish assemblage composition and structure and to identify the most underestimated nocturnal species. We performed visual counts of rocky reef fishes along six times of the day (dawn, morning, afternoon, dusk, early night, and night) within three consecutive days. Distinct fish assemblages were observed, with species from multiple trophic levels during the day, while planktivorous and invertebrate feeders dominated the nighttime. Dawn and dusk shared diurnal and nocturnal species with long active periods, with no records of exclusively crepuscular species. Fish richness and abundance increased from dawn to a peak in the morning or afternoon and decreased at dusk with the lowest values during the night. Nocturnal planktivorous and invertivorous species would be severely underestimated using only diurnal samples, suggesting a potential bias to the understanding of the trophic dynamics. We discussed the underlying processes driving short-term dynamics of reef fish assemblages (e.g., predation risk and prey availability) and highlighted the need for a better representation of twilight and night periods in reef fish ecological studies.