Diel trophic structuring of seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia

The faunal communities of seagrass beds throughout SE Asia are highly threatened by continued overexploitation, yet their ecology is poorly understood. Developing a greater understanding of the faunal linkages between seagrass beds and associated coastal habitats can facilitate more informed ecosystem level management. The present study used beach seine netting to sample seagrass bed fish assemblages in the Wakatobi Marine National Park, Indonesia, to investigate diel migrations of fish into and out of seagrass beds. These fish assemblages were found to be diverse relative to other studies within the region, with many species being economically important to local subsistence fisheries. The abundance, species richness and trophic structure of these fish assemblages changed with time of day indicating that fish populations are in a dynamic state. Mean fish abundance increased by ≈45% from day to night (Day: 8.61 ± 0.13 fish 100 m −2; Night: 15.6 ± 1.4 fish 100 m −2) while mean species richness increased from 6.6 ± 1.9 per seine haul to 11.4 ± 0.2. Increasing abundance and diversity of fish at night suggests migration onto these habitats from nearby habitats such as reefs, mangroves or deep water; and/or increased activity of those fish resident within seagrass habitats. Division of species into trophic categories enabled the trophic structure of changing fish assemblages to be examined. Assemblages were dominated during both the day and night by invertebrate and fish feeders; however, a major diel change in trophic structure occurred in the abundance of omnivores. During the day omnivores were abundant, but they were replaced at night by exclusive invertebrate feeders. We therefore propose that diel changes in seagrass fish assemblages are predominantly structured by food availability, although other factors such as increased night-time shelter provision were also found to be important albeit to a much lesser extent.