The resilience of coral reefs is essential for their recovery from mass coral mortality events 
珊瑚礁恢復力是對抗大規模珊瑚死亡事件的關鍵

紅海東北海域由杜拜延伸至沙烏地亞拉伯的阿卡巴灣入口，地理位置偏遠，導致該海域的基線數據長期以來沒有更新。為了完整該海域的底棲珊瑚礁及魚類群落的基線數據，研究團隊使用了水下視覺調查和誘餌式遠端水底攝影系統進行調查。並將此次調查結果與20年前的基線數據進行比較。調查發現，現在的珊瑚覆蓋率與20年前相比沒有顯著差異，但同時亦發現大量死珊瑚骨骼。由此可見，紅海東北海域也曾發生大規模的白化現象，導致大量珊瑚死亡，但由於東北海域珊瑚的高恢復力使珊瑚覆蓋率沒有明顯下降，能夠維持在尚可水平。

The northeastern Red Sea, spanning Duba to the entrance of the Gulf of Aqaba at the Saudi Arabian coast, is remote from major cities. The overlooked location has resulted in outdated of 2 decades and incompleted information of the area. To enhance the baseline data on benthic coral communities and associated fish communities, Yu-Jia Lin, an assistant professor of the institute of Marine Ecology and Conservation at National Sun Yat-sen University, along with his pervious teammates from the King Fahd University of Petroleum and Minerals, conducted underwater survey with Underwater Visual census methods and Baited Remote Underwater Video (BRUV) .They compared the current data to information collected 2 decades ago and found out that, there is a similar level of coral coverage but having a higher proportion dead coral at present. This indicates that a mass coral mortality event has occurred in the northeastern Red Sea. However, the coral in this area has shown high resilience, remaining as healthy as it was two decades ago.

紅海擁有超過50屬的造礁珊瑚，超過1200種的珊瑚礁魚類，有不少海洋生物更是紅海特有。珊瑚普遍對水溫非常敏感，若水溫高於30度，依附在珊瑚上的共生藻就會離開，造成白化現象。但由於紅海的海水溫度夏季時達35度，造就了當地珊瑚的高耐熱性。可是，耐熱不等於隔熱，紅海的珊瑚礁也曾多次因熱浪侵襲，經歷大規模珊瑚白化。紅海的珊瑚礁是不平等地受熱浪影響，近岸的珊瑚礁比離岸的更易白化。紅海其他地方及鄰近的阿拉伯灣就曾發生過因大規模的珊瑚死亡，導致棲地退化成由大型藻類主導的珊瑚礁群落或海膽荒漠。

The Red Sea is home to more than 50 genera of hermatypic corals, over 1200 fish species and high level of endemism for marine organisms. Coral in the Red Sea demonstrates an exceptionally high thermal tolerance, allowing them to endure the surface water temperature that can reach up to 35℃ in the summer . However, the coral in the Red sea are not entirely “heat proof” and they also experienced several intense heating event which caused widespread coral blenching. Furthermore, pervious study found out that nearshore reefs is more susceptible to blenching than offshore reefs. The mass coral mortality may lead to a phase-shift, changing the coral reefs to macroalgae-dominant reefs or sea urchin barren grounds. This phase-shift have occurred in other parts of the Red Sea and nearby Arabian Gulf.  

研究團隊在2021年7月利用潛水調查(UVC)和誘餌式遠端水底攝影系統(BRUV)在紅海東北水域7個近岸珊瑚礁區3個離岸珊瑚礁區，共10個地點進行水下調查，收集珊瑚礁及魚類群落的數據。調查共發現29屬珊瑚，其中濱珊瑚屬、軸孔珊瑚屬、角星珊瑚屬和指形軟珊瑚屬最為常見。近岸樣點的珊瑚覆蓋率中位數是36.2%，與20年前的28.1%相比有明顯增加。同時，死珊瑚覆蓋率從20年前的6.7%，大幅增加至現在的38.1%。研究證實，大規模的珊瑚死亡事件確實曾在紅海的東北海域發生，同時也展示出當地珊瑚礁生態的驚人回恢復力。

To collect data on coral and fish communities, the team conducted Underwater Visual Census methods (UVC) and Baited Remote Underwater Video (BRUV) at a total of ten stations along the northeastern Red Sea in July 2021. The stations included seven nearshore and three offshore sites. A total of 29 coral genera were observed, with stony corals such as Porites sp., Acropora sp., Goniastrea sp., and Sinularia sp. being frequently recorded.The mean coral coverage for all nearshore stations was 36.2%, which was higher than the 28.1% recorded two decades ago. Meanwhile, the coverage of dead coral significantly increased from 6.7% two decades ago to 38.1% at present.

This result demonstrated a mass coral mortality has occurred and has recovered with their marvelous resilience.

刺尾鯛跟鸚哥魚等草食性魚類，能藉由攝食來抑制大型藻類跟絲狀海藻的生長；單棘魨、隆頭魚和龍占魚會捕食海膽，因此可控制海膽的數量。這些魚種在UVC或BRUV調查中都十分常見。研究團隊指出，很大可能是因為這些魚類扮演了天敵的角色，防止了藻類及海膽的生長。棘冠海星沒有在當地爆發、人為活動的干擾稀少、有充足的珊瑚幼蟲從紅海其他地方加入、以及有利珊瑚重新形成礁體的環境條件等，促使當地的珊瑚礁擁有驚人回恢復力。

Surgeonfish and parrotfish likely play a crucial role in controlling the colonization of macroalgae and turf algae, while triggerfish, wrasses, and emperors serve as important predators of sea urchins. These fish species are commonly observed in Underwater Visual Census (UVC) or Baited Remote Underwater Video (BRUV) surveys. The research team suggests that the remarkable resilience of the coral in the Red Sea could be attributed to fish communities facilitating the removal of macroalgae and sea urchins, infrequent occurrences of crown-of-thorns starfish outbreaks, the absence of other atherogenic disturbances, a sufficient supply of coral recruitment from other areas of the Red Sea, and favorable environmental conditions for coral growth. Lastly, it's important to note that both UVC and BRUV methods have their pros and cons. Integrating both methods can form a holistic framework for monitoring coral reef ecosystems. Regular and comprehensive monitoring of coral reefs is crucial to safeguard against unforeseen circumstances.

上圖: 死珊瑚骨骼的海床；下圖:在死珊瑚骨骼上生長的珊瑚聚落。

Upper panel: a flat of dead coral rubbles; lower panel: colonies of Acropora and Pocillopora on a dead coral colony