Abundance, size distribution and habitat preferences in the grouper assemblage of the Ustica marine reserve (SW Mediterranean)
The abundance and size distribution of groupers during summer were assessed along the shores of an islet (Scoglio del Medico) in the Ustica marine reserve. The relationships between grouper distribution and some micro and macrohabitat characteristics were also evaluated. The dusky grouper Epinephelus marginatus (Lowe, 1834) was the most common species and showed a well structured population. This species displayed a clear pattern of variation in density during summer, with peak abundance occurring in July and August. The goldblotch grouper Epinephelus costae (Steindachner, 1878) and the mottled grouper Mycteroperca rubra (Bloch, 1793) were the other resident species and their density varied markedly depending on sampling period. Both E. costae and M. rubra were mainly recorded in the western sectors of the islet, which were the most exposed to the prevailing wave direction. The dusky grouper sample was numerically dominated by medium sized individuals, whereas juveniles were not very common, likely due to the absence of sheltered sites along the islet coastline. A shift of groupers toward deeper waters with increasing size was observed both in E. marginatus and E. costae, whilst no significant differences were detected in the size frequency distribution with depth of M. rubra. The bulk of dusky grouper was observed resting on or close to bottom. Conversely, both E. costae and M. rubra dwelled preferentially far from bottom. The kind of reaction of groupers to diver seemed to have no relationship with fish size. At microhabitat level, E. marginatus preferred rocky reef areas with dense algal cover and high complexity. Conversely, neither E. costae nor M. rubra appreciated uneven substrata, likely in relation to their benthopelagic habit. In terms of macrohabitat, only M. rubra showed specific requirements, preferring gently sloping plateau.