Macroalgae as a primary driver of territorial damselfish assemblages on Fijian fringing reefs

Management of fishing activities and the establishment of marine protected areas (MPAs) are well-known conservation efforts that can aid in the recovery of coral reef ecosystems. Well-enforced MPAs have shown an increase in predators and herbivorous fish, which in turn control populations of smaller prey species and maintain the reef in a coral-dominated state, respectively. In the absence of these communities, coral reefs can shift to macroalgae-dominated states where structural complexity is reduced. Territorial damselfish (Stegastes spp.) establish and aggressively defend distinct territories of farmed turf algae, but it is not yet understood how these site-attached fish are influenced by MPAs and macroalgae-dominance. Field surveys and macroalgae transplant experiments were conducted inside three small locally managed MPAs and adjacent fished areas on the shallow fringing reefs of Fiji. MPAs in this study were coral-dominated while the fished areas were macroalgae-dominated. Density of Stegastes spp. was significantly higher in two MPAs compared to paired fished areas, and territories had a higher percent cover of turf algae compared to undefended substrate within MPAs. As expected, piscivore biomass was significantly higher inside MPAs, where fishing of these communities is prohibited. Macroalgae transplant experiments indicate that resident Stegastes attempt to remove macroalgae thalli from their territories but fail to effectively reduce algal biomass substantially. Stegastes spp. seemed to be able to establish territories when macroalgae were experimentally removed from fished reefs. Findings from this study suggest that territorial damselfish assemblages are indeed influenced by MPAs, and that macroalgae is likely a limiting factor for the establishment of their territories. Further research is required to fully understand the role that Stegastes play in coral reef processes and whether their abundance can be used as an indicator of reef health.