INFLUENCE OF REEF FISH GRAZING ON JUVENILE CORALS

Abstract

Fish diversity and abundance were investigated at reefs around Royal Thai naval base, Sattahip area. A total of 46 species in 17 families were recorded in the area. The Pomacentridae was the dominant coral reef fish group followed by Chaetodontidae and Labridae similar to previous study in nearby areas. The results also showed that fish assemblages differed significantly among each location. The results from the Jaccard similarity index showed the highest similarity was found between at Ko Kham and Ko Tao Mo. The similarity in fish diversity among these two study sites may be caused by coral compositions and percentages of coral cover. Then, feeding behaviors and effect of grazing by fish on corals studies, field and laboratory experiments were conducted by 5 species those dominant in the area (Siganus javus (Linnaeus 1766), Halichoeres chloropterus (Bloch 1791), Scarus ghobban (Forsskål 1775), Abudefduf bengalensis (Bloch 1787), and Chaetodon octofasciatus (Bloch 1787)). The result showed that there was no variation of fish feeding behaviors throughout the day and the bite rates on corals of fish varied depending on species. Significantly differentiations of bite sizes on corals were founded between fish species. C. octofasciatus clearly bit on live corals (100%). In laboratory experiments, similar to field observations, C. octofasciatus preyed on corals more than other fish species. In this study also founded that C. octofascitus grazed more on a massive coral, Platygyra than branching coral, Acropora that different from previous studies. It may be most efficient for the butterflyfish to trade off prey preference and consume available prey during the trails. This way also reduces the mortality risk. Hence, the high consumptions of Platygyra by C. octofasciatus may reflect the food availability condition. Caging experiments were done for test the hypothesis that the exclusion of large invertebrates and fish would result in increasing outplanted-coral growth and survivorship. Three treatments were set: no cage, fish exclusion cage, sea urchin and fish exclusion cage. Growth rate and survival rate of corals were examined for 4 months. The results showed that percent changes of surface areas of Acropora millepora in cage with sea urchins were significantly greater than in either no cage or cage without sea urchin. Without sea urchins in the cages, the Acropora millepora growth could be affected by the amount of algal biomass. The result also showed that exclusion of herbivores could increase survivorship of juvenile corals and presence of grazers can reduce the pressure of coral-algal interaction.