Retention of larval fish driven by flooding tide in a remote atoll lagoon in the South China Sea

Abstract

Retention of fish larvae in an optimal habitat is essential for successful recruiting fish populations. Atoll lagoons may serve as ideal retention areas for fish larvae. We used plankton nets to investigate larval fish assemblages during flooding and ebbing tides in a water-passage inside the Yongle Atoll Lagoon in the South China Sea in May 2022 to reveal the patterns of fish larvae exchanges between the lagoon and the ocean. We collected a total of 186 species/taxa, which were mainly composed of Pomacentridae (21 taxa, 12.3 % total individuals), followed by Apogonidae (18, 6.2 %), Gobiidae (17, 7.4 %), Lutjanidae (15, 5.8 %), Labridae (11, 16.9 %), and Myctophidae (9, 12.6 %). Species richness and density were higher during flooding tide at night (140 taxa, 129.5 ind. / 100 m³) than that during ebbing tide at night (99 taxa, 22.7 ind. / 100 m³), followed by flooding tide at day (42 taxa, 10.0 ind. / 100 m³) and ebbing tide at day (5 taxa, 1.6 ind. / 100 m³). Thus, more fish larvae were transported into the lagoon through flooding tides than those washed out through ebbing tides, as well as at night compared to during daytime. This demonstrated the retention function of the lagoon driven by a combination of flooding tide and the nocturnal activity of fish larvae. Our results suggested that the protection of atoll lagoons in open oceans should be further emphasized not only for their high biodiversity but also for their potential role in providing retention habitats and consequently enhancing recruitment of fish populations.