Spatial variability of the ichthyoneuston around oceanic islands at the tropical Atlantic

Abstract

Oceanic tropical waters are characteristically oligotrophic. However, current-topography interaction around oceanic islands promotes bio-physical processes, which may enrich surface waters. The ichthyoneuston assemblage at three areas around oceanic islands in the tropical Atlantic was investigated. The aim was to evaluate if the ichthyoneuston was influenced by physical oceanographic processes, such as possible mechanisms of plankton retention promoted by the presence of the islands. Samplings were performed by 20 min long diurnal and nocturnal 500-μm plankton net hauls using a David-Hempel Catamaran at two transections in each area (Saint Peter and Saint Paul's archipelago, Fernando de Noronha archipelago, and Rocas Atoll). The transections were composed by three stations and were located at opposite sides of the archipelagos (windward and leeward sides of the archipelago). A total of 131 larvae and 3027 eggs, belonging to 16 families and 28 species were collected. Higher densities of eggs were found at the leeward side, which indicates the occurrence of a retention mechanism or spawning at this side. Higher densities of larvae were found at Fernando de Noronha, especially during the nocturnal period. Only one species, Ceratoscopelus warmingii, co-occurred at the three islands. Fernando de Noronha was highlighted as the area with the higher density, abundance and species richness. The waters around this island were positively correlated with chlorophyll-a, which indicates a higher availability of food items and stimulates the selection of this area as a favorable spot for reproduction and early nursery zone. This work highlighted the importance of the relation between abiotic factors and the distribution of fish eggs and larvae inhabiting the neustonic layer around South Atlantic oceanic islands, showing that favorable conditions, e.g. higher phytoplanktonic biomass, may be of fundamental importance to the spawning by adult individuals.