The influence of Sargassum biomass and thallus density on the recruitment of coral reef fishes

A habitat’s structural complexity is a key determinant of the recruitment and composition of associated communities. While the influence of the physical structure of corals on coral reef fish recruitment is well studied, the significance of other benthic components, like macroalgae, remains unclear. We used experimental patches of the canopy-forming macroalga Sargassum to assess the influence of macroalgal complexity, which was manipulated by altering thallus density and biomass, on coral reef fish recruitment. We established twenty-five 75 × 75 cm patches on the reef flat of Orpheus Island, (inshore, central Great Barrier Reef) during austral summer. Patches were randomly divided into five treatments of varying Sargassum thallus density (3–9 thalli) and/or biomass (177–779 g per patch) and surveyed daily for recruiting fishes for 18 d. We recorded 35 fish species recruiting to our patches, with Sargassum biomass having the greatest influence on fish recruits’ abundance and species richness. Comparisons between treatments with equal thallus density but varying biomass revealed a positive association between Sargassum biomass and fish species richness and abundance (up to ~ 2.5-fold differences). Additionally, treatments with similar total Sargassum biomass but different density revealed a negative relationship between density and fish species richness and abundance (20–30% reduction). These positive associations with Sargassum thallus biomass suggest that recruiting fishes favour the fine-scale complexity of intra-thallus spaces, rather than the larger, inter-thallus gaps. This study highlights that fine-scales of complexity within tropical macroalgal beds may influence the reef fish recruitment value of these often-underappreciated areas.