Seasonal variation in larval fish assemblages in relation to oceanographic conditions in the Abrolhos Bank region off eastern Brazil


Nonaka, Ricardo H., Yasunobu Matsuura, and Kazuko Suzuki
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The Abrolhos Bank region off the eastern coast of Brazil, under the infl uence of the Brazil Current, constitutes a typical tropical coral reef ecosystem that is characterized by a high diversity of tropical fish. Results of three ichthyoplankton surveys conducted in this region during summer, autumn, and winter revealed that there were two dominant fish groups: mesopelagic fi sh and coral-reef-associated fi sh. From the 45,614 larvae collected, 83 taxa (77 families and 6 orders) were identified, in which the family Myctophidae was the most abundant accounting for more than 25% of the total catch in the three cruises. Among the Myctophidae, seasonal variation in abundance of Myctophum affine was significant; there was an outstanding peak in abundance of this species during the summer surveys. The summer peak in abundance of five other species (Diaphus spp., Lepidophanes guentheri, M. nitidulum, M. obtusirostre, and Hygophum reinhardtii) was not as pronounced. Larval distribution patterns of these six species showed no remarkable seasonal change, being evenly distributed over the oceanic area. The coral-reef-associated fish larvae were confined to the bank and adjacent waters. The spatiotemporal distribution patterns of these larvae was strongly influenced by hydrographic features and some seasonal variation was observed. Four larval fish assemblages were identified: Abrolhos Bank, neritic, transitional, and oceanic assemblages. The Abrolhos Bank assemblage was characterized by coral-reef-associated fish, mainly Gobiidae, occupying most areas of the bank on the three cruises. The oceanic assemblage was dominated by mesopelagic fish, especially Myctophidae in all seasons and their occurrence was limited to the open ocean. The transitional assemblage was characterized by a mixture of coral-reef-associated fish and mesopelagic fish, with a seasonal change of dominant groups. It occupied mainly the shelf break area. The small pelagic fish (sardine and anchovy) were the dominant group of the neritic assemblage and occupied the coastal region south of the Abrolhos Bank, where the influence of the South Atlantic Central Water was evident.