INFLUENCE OF REEF FISH GRAZING ON JUVENILE CORALS

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.

本研究针对泰国皇家海军基地周边、萨塔希普（Sattahip）海域的珊瑚礁开展鱼类多样性与丰度调查。调查区域共记录到17科46种鱼类，其中雀鲷科（Pomacentridae）为优势珊瑚礁鱼类类群，其次为蝴蝶鱼科（Chaetodontidae）与隆头鱼科（Labridae），这一结果与邻近海域既往研究结论一致。研究同时发现，不同采样点位的鱼类群落组成存在显著差异。雅卡尔相似性指数（Jaccard similarity index）分析结果显示，高岛（Ko Kham）与龟岛（Ko Tao Mo）的鱼类群落相似性最高。两地鱼类多样性的相似性可能与珊瑚群落组成及珊瑚覆盖率密切相关。针对区域内5种优势鱼类——爪哇篮子鱼（Siganus javus (Linnaeus 1766)）、绿尾海猪鱼（Halichoeres chloropterus (Bloch 1791)）、青点鹦嘴鱼（Scarus ghobban (Forsskål 1775)）、孟加拉豆娘鱼（Abudefduf bengalensis (Bloch 1787)）及八带蝴蝶鱼（Chaetodon octofasciatus (Bloch 1787)），本研究开展了野外与室内受控实验，探讨其摄食行为及鱼类牧食对珊瑚的影响。结果表明，鱼类的日间摄食行为并无显著节律变化，而不同鱼类对珊瑚的啃食速率存在显著物种特异性差异。不同鱼类间的珊瑚啃食规模存在显著分化：八带蝴蝶鱼（C. octofasciatus）仅以活珊瑚为食，摄食占比达100%。室内实验结果与野外观测一致，八带蝴蝶鱼的珊瑚捕食量显著高于其余4种受试鱼类。本研究同时发现，与既往研究不同的是，八带蝴蝶鱼对块状扁脑珊瑚（Platygyra）的牧食强度高于枝状鹿角珊瑚（Acropora）。这一现象可能源于该蝴蝶鱼在觅食过程中权衡了猎物偏好与可获取猎物的经济性，同时可降低自身被捕食风险。因此，八带蝴蝶鱼对块状扁脑珊瑚的高摄食率或可反映其所处生境的食物可获得性状况。本研究设置笼养实验以验证如下假说：排除大型无脊椎动物与鱼类后，移植珊瑚的生长与存活能力将得到提升。实验共设置3种处理组：无笼组、鱼类排他笼组、海胆与鱼类双重排他笼组。连续4个月监测珊瑚的生长速率与存活率。结果显示，带有海胆的笼养组中，多孔鹿角珊瑚（Acropora millepora）的表面积变化率显著高于无笼组与无海胆的笼养组。在无海胆的笼养环境中，多孔鹿角珊瑚的生长可能受藻类生物量水平影响。研究同时证实，排除植食性动物可提升幼珊瑚的存活率，而植食者的存在可缓解珊瑚与藻类间的竞争压力。