The influence of canopy structure and tidal level on fish assemblages in tropical Southeast Asian seagrass meadows

Seagrass meadows support abundant and diverse fish assemblages, but there are very few studies on the relation between seagrass beds with distinctly different plant canopies and their associated fish fauna. In the present study, fish assemblages were investigated by underwater visual census at intertidal and subtidal sites with varying seagrass species composition, shoot density, biomass, and leaf area index (LAI) on two small coral islands in the Spermonde Archipelago, Indonesia. We investigated (1) whether fish assemblages in distinctly different seagrass beds differ regarding community parameters, and (2) whether seagrass parameters affect fish abundances. Overall, more than 120 fish taxa were found. Bray–Curtis cluster analysis and non-Metric Multidimensional Scaling ordination (nMDS) showed site-specific similarities for fish assemblage structure with a distinct separation into subtidal and intertidal sites. Species accumulation curves for gamma diversity, single study sites, and the two most diverse fish families (Labridae, Pomacentridae), suggesting that species numbers are likely to increase with diel sampling. Total fish abundance and abundance for six out of the nine most common species varied distinctly among the study sites. The study indicates that seagrass beds with differing canopy parameters support distinct fish assemblages that differ with respect to species richness, dominant species, and the abundance of total and most common fish species. These differences are likely due to different canopy structures and water depth.

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► Fish assemblages in seagrass beds differing in terms of shoot density, leaf biomass and leaf area index are studied.
► Seagrass beds with significantly different seagrass parameters harbour distinctly different fish assemblages.
► Total fish abundance and abundance of the most common fish species is affected by seagrass parameters and water depth.
► Species accumulation curves did not show saturation for y-diversity, selected study sites, and two fish families.
► Seagrass beds harbour high species numbers at a small (regional) scale.