Background. There is need for more information on behavioural rhythms of fishes in relation to their habitat use in few or not previously monitored areas, such as coralligenous habitats. Some studies have compiled inventories of observable fish species in this habitat, but no studies have been performed on the temporal patterning of fish behaviour, especially at the scale of the day. The presently reported study aims at providing knowledge on fine-scale fluctuations of abundances and behaviour of two fish species in coralligenous habitats throughout the diurnal period. Materials and methods. Fish fauna was filmed hourly, for several days, from dusk to dawn, with autonomous, programmable, and rotating video systems. Diurnal variations in abundances per count, considering size classes and behaviour, were studied for two diurnal fish species— the Mediterranean rainbow wrasse, Coris julis (Linnaeus, 1758) (Labridae), and the common two-banded seabream, Diplodus vulgaris (Geoffroy Saint-Hilaire, 1817) (Sparidae) in two study areas of coralligenous habitats on the French Riviera. Results. Significant temporal variability at the scale of the hour was revealed for abundances of C. julis in one of the two study areas, with few individuals in crepuscular periods and highest abundances in the morning and in the afternoon. These differences were mainly due to significant diurnal variations in abundances of small and medium C. julis, which were low in crepuscular periods, whereas large-sized individuals were equally abundant during the day. On the other hand, no discernible diurnal rhythmicity of mean abundances of D. vulgaris was observed, regardless of the study area. Crepuscular periods were associated with sudden changes of abundances and behaviour for both species, with a greater percentage of mobile and solitary individuals, and few feeding individuals compared to the rest of the day. Feeding occurred mainly in the morning for C. julis. In contrast, no dial-time preference for feeding was revealed for D. vulgaris. Conclusion. The study provided preliminary insights into fine-scale diurnal variations of abundances of different life stages and of fish behaviour. Variability in abundances of life stages and of fish behaviours at different times of day should be taken into account for population assessments and spatial comparisons. Moreover, our study confirmed the use of video-imaging as an efficient non-destructive tool for the study of fish fauna in structurally complex, highly valuable habitats.

activity rhythms, ethology, fish, bioherm, underwater video