Tidal variation in fish assemblages and trophic structures in tropical Indo-Pacific seagrass beds

Abstract

Background: Tidal cycle is an important factor which regularly changes coastal fish assemblages in shallow waters. However, the variations in fish assemblage and trophic structure across tidal cycles in tropical seagrass beds are rarely discussed. We used underwater visual censuses to quantify fish abundance and diversity from the shallow intertidal to the subtidal zone during both flood and ebb tides in seagrass beds surrounding the Dongsha Island, South China Sea. We also recorded fish feeding habits by analyzing stomach contents. Results: At least 70 fish species were observed in the seagrass beds during the study periods. Fish density and diversity increased with increasing depths, as well as during flood tides. The intertidal areas were characterized by more variation in both fish assemblage and trophic structure than the subtidal areas. A cluster analysis identified six principal feeding groups and invertebrate-feeding carnivores contributed about 70% of the total fish abundance. The density of herbivores, large-sized carnivores, and piscivores, but not small-sized carnivores, increased during flood tides. In contrast, detritivores decreased in density during flood tides. A heatmap of habitat usage pattern revealed that most fish showed a preference for deeper stations and flood tides. Only a minority of fish showed a preference for shallower stations and ebb tides. Conclusions: We documented differences in fish assemblage and trophic structure between tides in tropical seagrass beds. Our results suggest that water depth may account for a significant proportion of the variation. The deeper water during flood tides support more space for herbivores and high trophic level carnivores to forage in intertidal meadows. The remaining, non-ebbed seawater columns in the intertidal meadows formed important temporary refuges for fishes during ebb tides. This study offers new information for fish movements over tidal cycles in tropical seagrass beds.