Checklist of the coral fish fauna of Xisha Islands, China

Abstract Background The Xisha Islands are composed of the Yongle Islands and the Xuande Islands in Hainan Province, China. It has one of the highest species diversity in the world and is also a typical oceanic distribution area of coral reefs globally. The ichthyofauna of the Xisha Islands were recorded by underwater visual census in May 2019 and July 2020. The survey data were combined with previous records of species into the checklist of the Xisha Islands presented herein. A total of 691 species, belonging to 24 orders and 97 families, was recorded. The major families were Labridae, Pomacentridae, Serranidae, Chaetodontidae, Hexanchidae, Lutjanidae, Scaridae, Gobiidae, Scorpaenidae and Carangidae. In this study, the Coral Fish iversity Index (CFDI) of six families (Chaetodontidae, Pomacanthidae, Pomacentridae, Labridae, Scaridae and Acanthuridae) was 229, indicating 756 coral fishes. In terms of the IUCN Red List, one species is Critically Endangered (Glyphis gangeticus), six species are Endangered (Stegostoma fasciatum, Aetomylaeus maculatus, Aetomylaeus vespertilio, Epinephelus akaara, Cheilinusundulatus sp. and Xiphias gladius), 16 species are Vulnerable, and 13 species are Near Threatened in the Xisha Archipelago, so conservation should be strengthened in this area in the future. New information One species is a new record for China (Dischistodus pseudochrysopoecilus) and 23 species are newly found in the Xisha Islands.


Introduction
The Xisha Archipelago is in the midwest of the South China Sea and it is at the northern margin of the Coral Triangle, which has the highest biodiversity in the world (Allen 2008). The Xisha Islands comprise the Yongle Islands and the Xuande Islands and begins in the north from the Beijiao Atoll and ends at the Songtao Bank in the south; Xidu Bank is in the east and Zhongjian Island in the west. Altogether, there are 29 islands, sandbars and four atolls.
The coral reefs of the Xisha Islands are the oldest and most primitive in China. It is also the birthplace of the coral reef ecosystem in the coastal areas of China (Huang et al. 2020). The Xisha Coral Reef Ecological Monitoring Zone was established in 2005 by the State Oceanic Administration of China to monitor and assess the health of coral reefs. The coral reef coverage declined from 53.8% in 2007 to 5.44% in 2016, due to anthropogenic activities, outbreak of crown-of-thorns starfish, coral diseases, typhoons and global warming (Li et al. 2017).
Several studies have focused on the fishes in the Xisha Islands. For example, 261 species belonging to 48 families of coral reef fishes were found in Beijiao Atoll, Yongxing Island, Huaguang Atoll and five other islands by gillnet and angling (Sun et al. 2004). A total of 146 species belonging to 31 families of fish was reported from the main reefs in Xisha Islands by bottom gillnet (Wang et al. 2011). There were 119 species belonging to 73 genera and 30 families of coral reef fish species reported in the Xisha Islands by underwater visual censuses and 643 species were recorded in combination with other previous studies (Gao et al. 2014). However, several new studies have been published. For example, in Zhaoshu Island, a total of 58 coral reef fish species was recorded, belonging to one class, three orders, 18 families and 37 genera by underwater visual censuses (Yang et al. 2018).The diversity of reef fishes declined from 3.10/m in 2005 to 1.23/m in 2013, due to reef degradation and overfishing (Li et al. 2017). The fish list of the Xisha Islands remains incomplete and taxonomic revisions are urgently needed to provide scientific support for follow-up research and protection of reef fishes in the Xisha Islands, especially against the background of intense human activity and rapid global change.

Materials and methods
In May 2019 and July 2020, a total of 27 sites in the Xisha Islands ( Fig. 1) was investigated by underwater visual censuses and more than 50 dives were performed, using a Canon 5D4, together with Seacam 150D and Sea&Sea YS-D2 flashlights. The photos were taken ranging from 5 to 30 m. The species were identified according to the following resources: Reef fishes of the East Indies (Allen and Erdmann 2012), Coral reef atlas of Xisha Islands (Huang 2018), Reef fish identification of Nansha Islands (Fang and Lv 2019), Coral reef fishes of the South China Sea (Fu 2014) and FishBase (Froese and Pauly 2020, Fig. 1) The Coral Fish Diversity Index (CFDI) is an evaluative index proposed by Allen (1988) to measure the diversity of coral reef fishes using the following formula: Total fish fauna = 3.39 (CFDI) -20.595 used by for areas under 2,000 km. The CFDI is based on the total number of species in each of the six indicator families (Acanthuridae, Chaetodontidae, Labridae, Pomacanthidae, Pomacentridae and Scaridae). All selected families are crucial parts of reef communities, widely distributed and firmly related to coral reef ecosystems.
This checklist has been arranged in the evolutionary order of class, order, family and species and families are arranged in the evolutionary order of genera and then species name.The newly-recorded species in the Xisha Archipelago are marked with r11 only in the Table. For habitats types, these abbreviations have been used: reef associated (RFA); brackish (BRA); demersal (DEM); amphidromous (AMP); pelagic (PEL); bathydemersal (BAD); benthopelagic (BEP); bathypelagic (BAP); pelagic-neritic (PE); oceanodromous (OD).
The IUCN status is indicated as: Critically Endangered (CE); Endangered (EN); Vulnerable (VU); Near Threatened (NT); Lower Risk (LR); Least Concern (LC). Threat to humans: Other; Traumatogenic; Poisonous to eat; Venomous; Reports of ciguatera poisoning. There were 197 species belonging to 28 families found in this survey, of which, one species represented a new record in China (Dischistodus pseudochrysopoecilus) and 23 species represent new records in the Xisha Islands (Table 1), combined with eight previous studies by Gao et al. (2014) and Sun et al. (2004)Li et al. 2017, Li et al. 2007, Wang 1981, Wang et al. 2011, Yang et al. 2018, Zeng 2004. In total, 690 species were recorded from the Xisha Islands, belonging to 24 orders and 97 families. The order Perciformes was the most dominant order, with 496 species belonging to it. The most dominant family was the family Labridae, which had 75 species; followed by Pomacentridae with 63 species; the third, fourth and and fifth largest families were Serranidae, Chaetodontidae and Hexanchidae, which had 42 species, 39 species and 29 species, respectively. Other major families were Lutjanidae and Scaridae, which both had 26 species, Gobiidae had 22 species; Scorpaenidae and Carangidae had 16 species (Table 1).

Discussion
According to previous research, the order Perciforms is the most dominant order in the Xisha Islands (Li et al. 2007) and the major families in the Xisha Islands are Labridae and Pomacentridae (Gao et al. 2014) which concur with our study. Combined with studies over the years, in the past 20 years, Hyperoglyphe perciformis has been the dominant species in both underwater visual censuses and gillnet surveys.
The CFDI value for the Xisha Archipelago was 229, such that we could estimate put it into the formula proposed by Allen 1988, we estimated that Xisha Islands had 756 coral reef fishes. However, only 690 species were found, which is relatively comprehensive, but there is still a need for more surveys in different seasons within different sites in the future.  Dischistodus pseudochrysopoecilus -a newly-recorded species in China