Analysis on the outbreak period and cause of <italic>Acanthaster planci</italic> in Xisha Islands in recent 15 years

Abstract

Outbreaks of the crown-of-thorns starfish Acanthaster planci have received increasing attention due to their negative impacts on coral reefs. However, outbreaks of A. planci in Chinese seas are still poorly understood. Coral reefs in the Xisha Islands (Paracel Islands), South China Sea, have been monitored during 2005 and 2019, including hard coral coverage, larval recruitment rates, density of coral reef fish and the populations of A. planci . This study aims to analyse the regular pattern regarding outbreaks of A. planci in the main islands of Xisha Islands, the causes of outbreaks of the crown-of-thorns starfish Acanthaster planci , and the solutions for controlling the outbreaks of the crown-of-thorns starfish Acanthaster planci . Monitoring coral reef ecosystems on the main islands of Xisha Islands from 2005 to 2019 showed that the cycle of Acanthaster planci outbreaks in Xisha Islands is about 15 years. These 15 years can be divided into two phases, in which the period of A. planci outbreak is about 5 years, and the recovery period of coral reefs is about 10 years. When A. planci outbreak from 2006 to 2010, the coral reef cover rate decreased from more than 60% to less than 5%. During the recovery period from 2011 to 2019, the coverage of coral reefs gradually increased to around 15%, at the same time the coral recruitment and coral reef fish increased. Further analysis showed that the occurrence of A. planci in Xisha Islands is mainly related to typhoon weather, reduction of coral reef fish, global warming and human activities. The passage of the typhoon brought a large number of nutrients, which led to the increase of plankton, leading to the survival rate of larvae of A. planci . Global warming and a decline in coral reef fish could lead to a greater percentage of the eggs of A. planci hatching and surviving. Based on 15-years monitoring data, it is highly possible that Xisha Islands are entering the next outbreak cycle of A. planci , and therefore measures should be taken to prevent the coral reef ecosystem from being destroyed again. Outbreaks of the crown-of-thorns starfish A. planci should be controlled in several ways. Direct control methods including physical removal of individual starfish and cutting individuals into multiple pieces were most often used methods to kill starfish. The most effective methods to control A. planci require injecting individual starfish with lethal chemicals, including sodium bisulfate, powdered citric acid, vinegar and thiosulfate-citrate-bile-sucrose agar (TCBS). Releasing the predator of A. planci of their different life cycle was also an effective way to control A. planci . The Giant Triton Shells ( Charonia tritonis ), the Napoleon Maori Wrasse ( Cheilinus undulatus ), Starry Puffer ( Arathron stellatus ) and Titan Trigger Fish ( Ballistoides viridescens ) were the main predator of the crown-of-thorns starfish A. planci . Moreover, regular monitoring of the starfish A. planci should be conducted in the Xisha Islands for early warning of outbreaks.