Macroalgal susceptibility to grazing by herbivore fish Siganus spp. at Gulf of Mannar reef, Indian Ocean

Enhancing the herbivore fish stock is envisaged as a mitigation tool to reverse the macroalgal dominance in a coral reef environment. However, identifying the key species of herbivore fish that significantly controls the macroalgal abundance in a reef is critically important. The present study is aimed at determining the prospects of two common herbivore fish, Siganus javus and S. canaliculatus, in regulating the macroalgae prevalent in the Gulf of Mannar (GoM), a Marine National Park, using in situ cage experiments. Ten different macroalgal species that commonly grow over corals were collected from the GoM reefs, and a known biomass of macroalgae was fed individually to both S. javus and S. canaliculatus maintained in separate in situ cages. Grazing experiments revealed a significant difference in the macroalgae consumption rate between S. javus and S. canaliculatus. The S. javus consumed 0.9 ± 0.7 to 10.1 ± 1.8 g h−1 comparatively higher than S. canaliculatus (0.5 ± 0.14 and 4.7 ± 0.9 g h−1), indicating the differential feeding ability of the closely related herbivore fish. Green filamentous macroalgae including Enteromorpha compressa and Chaetomorpha crassa were intensely grazed within 5 h of feeding trials. Though the grazing rate of Siganus spp. recorded in this study was comparable with other geographical reefs, the feeding preferences differed as the macroalgae Turbinaria ornata, Sargassum wightii, and Halimeda tuna were least consumed by both the Siganus spp., probably contributing to their long-term persistence in the coral reef ecosystem. The results presented here emphasize that the feeding pattern of Siganus spp. varied with different species of macroalgae that might play a critical role in shaping the reef community. The herbivory rate and choice of macroalgae by other herbivores should be considered to decide on the management options such as marine replenishment zones.