Vertical distribution and structure of the reef fish assemblage in the oceanic island of Fernando de Noronha, Brazil.

The Fernando de Noronha Archipelago harbors one of the richest reef ichthyofaunas among the oceanic islands of the South Atlantic; however, most ecological studies on the local fish assemblages have been conducted in shallow reefs. This study assessed the structure of fish communities along the reef depth gradient, from shallow (≤30 m) to mesophotic depths (30-111 m), through underwater visual surveys. In total, 61 species belonging to 27 families were recorded, with significant variations in the reef fish assemblage among depth strata. The upper mesophotic zone (30-60 m) stood out with the highest biomass values, exhibiting abundance and richness similar to those found on shallow reefs. Species bathymetric distributions revealed a strong gradient, with both exclusive and wide ranging species present in the upper mesophotic zone. In terms of trophic structure, planktivores were abundant across all depths, but showed greater biomass in deeper reefs (>30 m). Herbivorous and omnivorous fish, on the other hand, predominated in shallow reefs; mobile invertebrate feeders and macrocarnivores were more common in the upper mesophotic zone, while sessile invertebrate feeders were more abundant in the lower mesophotic zone (>60 m). Mesophotic reefs displayed a distinct composition, characterized by the occurrence of endemic and exclusive species, in addition to a higher frequency of large-bodied individuals. Nevertheless, most mesophotic reefs are located outside the limits of the no-take marine protected area. Therefore, these findings reveal that a significant portion of the local fish fauna, unique to mesophotic ecosystems, remains unprotected.