Biomass of key grazing fishes is an important determinant of coral growth and fouling control in coral nurseries

Abstract

Restoration is an emerging tool for coral reef conservation, yet despite small‐scale successes there are concerns about high costs and ecological setbacks. Integration between reef ecology and restoration could help address such concerns. A prime example is the use of grazing by herbivores to reduce coral nursery cleaning costs. However, the relation between herbivore communities and cleaning benefits remains unquantified. This study aimed to measure links between herbivorous fish communities, grazing intensity and coral nursery performance. Six reefs were selected in southern Kenya, equally divided across three levels of fisheries management (fished < reserve < no‐take). Fish surveys determined herbivorous fish biomass and composition, and remote underwater videos recorded grazing intensity on coral nurseries. Accumulated fouling and coral growth were measured at the end of the 4‐month study. Grazing intensity was sixfold lower and fouling density fourfold higher in the fished areas compared to protected zones. Higher fouling strongly correlated with lower coral growth: exponential growth constants in fished areas were respectively twice and three times as low compared to marine reserves and no‐take zones. Across study sites grazing was dominated by bristletooth tangs (Ctenochaetus spp.), except where these were outcompeted by territorial damselfish. Thus, better coral nursery performance in protected areas can be partially linked to higher grazing intensity, which in turn is determined by both fish biomass and local species composition. We recommend protecting herbivorous fish species and placing coral nurseries in areas with high biomass of key grazers to improve coral nursery performance and reduce maintenance costs.

Leveraging the Red List of Ecosystems for national action on coral reefs through the Kunming-Montreal Global Biodiversity Framework

“…Sites with poor benthic state but relatively abundant fish populations could be considered as candidates for restoration (e.g. coral gardening) as part of Target 2, particularly as they have herbivorous fish required to keep algae under control, increasing the likelihood of success (Ladd et al, 2018; Ladd and Shantz, 2020; Gouezo et al, 2021; Knoester et al, 2023). Sites with low fish populations and poor benthic condition may benefit from interventions to reduce threats, including tourism management (under Target 9), pollution reduction (Target 7), and/or fishery and herbivore management (Target 5), depending on the context (see Table S3 for full details).…”

Fish-based herbivory and the natural maintenance of algal fouling on coral nurseries used across the Great Barrier Reef

“…Their study similarly reported that surgeonfish species (Ctenochaetus striatus) consumed the majority of algae on their Coral Tree TM nurseries. However, unlike Knoester et al (2023) where fish biomass differed between fished, reserved and no-take zones, the fish assemblages documented here at Mojo in the Conservation Park Zone (limited hook-and-line fishing) and Rayban in the Marine National Park Zone (no fishing) did not differ in biomass (ANOVA, F = 0.32, P = 0.571), algal consumption (Kruskal-Wallis test, X 2 = 0.66, P = 4.15), abundance (Kruskal-Wallis test, X 2 = 1.71, P = 0.191), or species richness (ANOVA, F = 3.10, P = 0.097; Supplementary Data S4). Schools of Siganus rivulatus, (Surf parrotfish) a family not observed feeding on our CNP nurseries during this study, but known to assist in algal removal on GBR reefs (Fox and Bellwood 2008), have had a major impact on nursery algal reduction in the Seychelles (Shafir and Rinkevich 2010).…”

“…While the ecological functions performed by these fish on natural reefs are well-established, their contributions to corals grown on nurseries used for ongoing GBR coral propagation efforts have not been explored (Burkepile and Hay 2008;Green and Bellwood 2009;Bellwood et al 2014). In this study, we test the extent to which assemblages of larger fish (>2.5 cm in length), those known to have the highest contribute to herbivorous fouling removal (Fox and Bellwood 2007;Knoester et al 2019;Knoester et al 2023), facilitate the cleaning of biofouling algae on CNP coral nursery frames. We hypothesised that fish assemblages significantly reduce algal biomass on frames and would thus result in higher coral survivorship (Lirman 2001;McCook et al 2001;Brown et al 2019;Knoester et al 2019).…”

“…Similarly, for Coral Tree TM nurseries installed north of Wasini Island, Kenya, algal fouling was reduced in the presence of fish within the reserve or no-take zones. On these nurseries, coral growth rates increased in comparison to those grown on nurseries placed in heavily fished areas (Knoester et al 2023). In addition to observational studies, fish cage exclusion experiments have often been used to further understanding of herbivorous fish on coral-algal interactions on reefs (Hughes et al 2007;Doropoulos et al 2013).…”