Patterns in reef fish assemblages: Insights from the Chagos Archipelago

Abstract

Understanding the drivers of variability in the composition of fish assemblages across the Indo-Pacific region is crucial to support coral reef ecosystem resilience. Whilst numerous relationships and feedback mechanisms between the functional roles of coral reef fishes and reef benthic composition have been investigated, certain key groups, such as the herbivores, are widely suggested to maintain reefs in a coral-dominated state. Examining links between fishes and reef benthos is complicated by the interactions between natural processes, disturbance events and anthropogenic impacts, particularly fishing pressure. This study examined fish assemblages and associated benthic variables across five atolls within the Chagos Archipelago, where fishing pressure is largely absent, to better understand these relationships. We found high variability in fish assemblages among atolls and sites across the archipelago, especially for key groups such as a suite of grazer-detritivore surgeonfish, and the parrotfishes which varied in density over 40-fold between sites. Differences in fish assemblages were significantly associated with variable levels of both live and recently dead coral cover and rugosity. We suggest these results reflect differing coral recovery trajectories following coral bleaching events and a strong influence of ‘bottom-up’ control mechanisms on fish assemblages. Species level analyses revealed that Scarus niger, Acanthurus nigrofuscus and Chlorurus strongylocephalos were key species driving differences in fish assemblage structure. Clarifying the trophic roles of herbivorous and detritivorous reef fishes will require species-level studies, which also examine feeding behaviour, to fully understand their contribution in maintaining reef resilience to climate change and fishing impacts.

Disentangling drivers of the abundance of coral reef fishes in the Western Indian Ocean

“…We propose that a ratio of piscivores/omnivores to total biomass may provide a useful metric of fishery exploitation and that values ~0.3 represent naturally productive reefs that are fished, and that higher values of ~0.4–0.5 might be achieved through protection within MPAs (Table ). The latter compare with 0.44–0.52 recorded on atoll rim and lagoon sites, respectively, in the unfished Chagos Archipelago (Samoilys et al, ).…”

“…Nevertheless, very high biomass of >1,500 kg/ha was recorded at individual sites in Tanzania, Mozambique, and Madagascar (Figure ), including sites where there is fishing. These latter values are on a par with some sites in the Chagos Archipelago which is uninhabited and represents close to “pristine” biomass for the WIO (Graham, Pratchett, Mcclanahan, & Wilson, ; Samoilys et al, ). Our results suggest that the productivity of reefs in the WIO in terms of fish biomass depends on their geomorphology, exposure, and nutrient levels.…”

“…It is likely that few or no mangroves near most sites in Comoros may be the reason for the lack of L. fulviflamma since their juvenile phase is almost entirely in mangroves (Kimirei, Nagelkerken, Mgaya, & Huijbers, ). The absence of caesionids and C. argus at sites in Comoros and Madagascar is not easily explained since both taxa are abundant in the Chagos Archipelago (Samoilys et al, ; Winterbottom & Anderson, ), but possibly island biogeography, reef area, and connectivity may play a role (Bellwood et al, ; Mora, ; Parravicini et al, ; Sandin, Vermeij, & Hurlbert, ). A small group of species were observed in NE Madagascar that were either rare or absent elsewhere which included three grazing‐detritivore acanthurids (sensu Green & Bellwood, ): A. dussumieri, A. blochii , and A. xanthopterus and the grouper Plectropomus punctatus .…”

“…Small individuals of parrotfishes (~5–10 cm TL) could not always be identified to species, so were recorded as Scarus spp. The size of all species >5 cm TL was estimated, and their biomass was calculated based on published length–weight relationships following procedures presented in Samoilys et al ().…”

“…The first was that fish assemblages will vary naturally in relation to a number of larger scale abiotic factors such as biogeography, reef structure, and oceanic nutrient levels, all either previously recognized or likely to have some influence in structuring coral reef communities (Heenan, Hoey, Williams, & Williams, ; Pinca et al, ; Taylor, Lindfield, & Choat, ). But at the smaller scale of reef habitat, the cover of hermatypic corals, algae, rubble, and rugosity also plays a significant role in structuring fish assemblages (Chabanet, Ralambondrainy, Amanieu, Faure, & Galzin, ; Halford, Cheal, Ryan, & Williams, ; Samoilys, Roche, Koldewey, & Turner, ). Our second hypothesis was that the abundance and biomass of coral reef fishes will vary in relation to protective management and fishing, which has been widely demonstrated in studies across the Indo‐Pacific (D'agata et al, ; DeMartini, Friedlander, Sandin, & Sala, ; Edgar et al, ; McClanahan, Graham, Calnan, & MacNeil, ).…”