Diversity of reef fish on Lembeh Island as an indicator of the coral reef health condition

Lembeh Strait in North Sulawesi is one of the areas included in the world’s coral triangle. Coral reef structures provide a habitat for reef fish. Herbivorous and invertivorous fish were essential to determine the direction of coral reef succession. This study aims to examine the health of coral reefs based on the presence of reef fish. Observations were conducted on January 16-19, 2019, with a total of 22 observation points. Observation of reef fish communities using the underwater visual census (UVCs) and belt transect method. At the observation site, it was found that there were 260 species of reef fish. The resilience of coral reefs based on the presence of reef fish has a high value on the parameters of the abundance of herbivorous fish, taxonomic diversity, and abundance of coral reef fish.


Introduction
Indonesia's coral reefs are ecosystems with the highest biodiversity in the world. There are more than 2000 fish species and 650 coral species; Indonesia is declared the center of the world's marine biodiversity [1] [2]. Coral reefs play an important role both in ecological and socio-economic aspects. Unfortunately, these reefs are being degraded very vast by climate change and human activities in the last decade, such as destructive fishing, unsustainable fisheries, land base pollutants, and impact of harbour construction, which are the most severe threat to coral reefs [3]. The relationship of fish function in coral reefs is fish manure can to add nutrients to the waters, move the fish will also clean the polip coral.
Lembeh Island shows habitat heterogeneity and is rich in many different species, including endemic, rare, and most vulnerable species.The biota and reefs in this area contribute to the province's economy through tourism and fishery activities. A total of 193 species of scleractinian corals belonging to 68 genera has been recorded, including seven species of non-scleractinian coral such as Tubipora, Heliopora, Millepora, Distichopora, and Stylaster. The high diversity of species in the Lembeh Strait, not matched by comprehensive management, makes coral communities vulnerable to declining conditions[4], [5]. As a result, coral reefs were damaged, and fish cannot live properly. Pressure on reef fish also occurs due to destructive fishing (for example, cyanide and blast fishing) for ornamental and consumption fish [6].
The distribution of reef fish is closely related to the spatial distribution and condition of coral reef ecosystems. Juvenile reef fish that will settle in coral reef ecosystems. Reef fish distribution is limited due to specific habitat conditions. Reef fish depend on coral reef ecosystems to survive, especially reef fish in the juvenile stage. Reef fish need a good and healthy coral reef as a nourishment, shelter, and breeding area. However, coral reefs also need reef fish to control their competitors' biota, so that coral reef fish are currently used as an indicator of the resilience and health of coral reefs.
Assessment of coral reef health has been done a lot by considering the local condition and a wide variety of environmental parameters. The parameter that is used to analyze starts from slight to comprehensive. We believe that using a comprehensive parameter does not necessarily guarantee a good result based on those studies. In assessment condition coral reefs, that formula can be delivered comprehensive information based on a national standard. So, the objective of this study is to examine the health of coral reefs based on the presence of reef fish on Lembeh Island. The information is essential for the local government when management action should be made.

Material and method 2.1. Study site
Observations were made for three days from 16-19 January 2019 around the waters of Lembeh Island from north to south. Observations depth were carried out at a depth of meter to a depth of of 12 meters on the reef flat of Lembeh Island. Observation locations are depicted on the map (figure 1).

2.2.
Data collection technique for coral reef habitat 2.2.1. Coral reef fish community structure. For reef fish communities, data collection was carried out using underwater visual census (UVCs) using belt transects 50 meters long and 5 meters wide [7]. For observation, the reef fish are target fish with an economic value, hoping that many fish will describe the excellent condition of the coral reef ecosystem. Data recorded includes individual abundance, species diversity, and the estimated total length of each individual [8].

2.2.2.
Coral reef health index. The coral reef health index was measured using a method developed by the Indonesian Institute of Sciences (LIPI). LIPI's method was determined based on two main components: percentage cover of benthic biota and biomass of coral reef fishes. The coral reef health index value from reef fish was calculated based on the biomass values of herbivorous fish (Scaridae, Siganidae, and Acanthuridae) and carnivorous fish (Serranidae, Lutjanidae, Lethrinidae, and Haemulidae). Furthermore, it will be combined with assessing the resilience index of coral reefs based on the abundance of herbivorous fish [9].
2.3. Data analysis 2.3.1. Community structure of coral reef fish. The community structure is analyzed using the following equations 1, 2, 3 and 4. Diversity is known using the Shannon-Wiener index, which considers species richness and evenness [10].
Pi is the proportion of abundance of species i.
H' is the diversity index, H'max = H'/Ln S, and S is the species richness. The equation calculates dominance: Pi is the proportion of abundance of species i. Fishes biomass was calculated using the length-weight relationship approach for each type of fish [11].

Reef fish community structure
The index of diversity and abundance of reef fish is influenced by the number of individuals and types of fish found in coral reefs. The high fish population is influenced by good environmental conditions, sufficient food, shelter, and a low level of predation [13]. The fish diversity index is in the low to the high category, indicating that the fish community is under pressure to stable conditions. The dominance index shows the dominance of certain species from low to moderate levels. The higher dominance value will-affect the stability of fish in the community and their diversity [14]. Based on observations, the coral reef fish community was quite varied, with 260 reef fish species. It'sIt is also stated by campbale that around 342 species of reef fish from 48 families have been found in Lembeh Island. Based on the  analysis of the coral reef fish diversity index, a value of 3.77 was obtained. It means the diversity of coral reef fish was high, with the highest value at the Angel Window location (4.82) and the lowest at the Dorbolaang (2.14). The evenness index was used to see the distribution of individuals in the community and obtained a value of 0.91 which means that the reef fish community was in stable condition, with the highest index at Tugu Trikora (1,13) and the lowest at Dorbolaang (0.59). The dominance index value of 0.06 means no dominant species in the community, where the highest is in Dorbolaang (table 4).
The reef fish abundance is quite varied, with a range of 100-2700 individuals/ha, with a total of about 3,572,360 individuals/ha. Pomacentrus coelestis was the most abundant reef fish, then Pomacentrus alexanderae, Amblyphidodon curacao, and Amblygliphidodon aureus as the list of 10 species with the highest abundance are shown in figure 2b. Biomass is quite varied from 0.01 to 7,073.67 kg/ha, with a total of 83,701.13 kg/ha. The highest biomass is shown by Ctenochaetus striatus, then Odonus niger, Zebrasoma scopas, and Amblyglyphidodon curacao (figure 2a). Based on the family, the dominant one is Pomacentridae which is associated with corals, especially the life form of branched corals. Campbell et al. 2009 stated that in the Lembeh Strait, the family Pomacentridae (damselfishes) having the highest number of species. Pomacentridae is closely related to the life form of branching corals because the Pomacentridae group often inhabits crevices in the branches of hard corals. Even some Pomacentridae species have a preference for certain types of branching. This family is also vulnerable to habitat damage on coral reefs [16]. An ecosystem can have high species diversity, but if there is missing one important functional group, then the function of the ecosystem will be disrupted. Herbivorous fish in coral reefs consist of four families were Acanthuridae, Scaridae, Siganidae, and Kyphosidae. The first three families were the main herbivorous fish. Based on their trophic level, the results of the analysis of coral reef fish on Lembeh Island show that the composition and proportion are based on biomass (figure 3b). The dominant fish were omnivorous (45%), followed by herbivorous (24%), carnivorous (20%), planktivorous (9%) and corallivorous (2%). Based on the composition of the abundance proportion (figure 3a), it can be seen that omnivores (64%), followed by carnivores (16%), herbivores (11%), planktivores (7%), and Corallivores (2%). The omnivores, herbivorous, carnivorous, and planktivorous reef fish were dominant based on abundance and biomassit's. It is mean these groups are large-bodied reef fish that dominate the coral reef ecosystems on Lembeh Island. On the other hand, only planktivorous fish are in small size.
The abundance and biomass of target fish on Lembeh Island show a higher omnivorous composition than the herbivorous reef fish. It indicates that the condition of the target fish community is experiencing disturbances, where the regular composition should be around 60% of herbivorous fish in a healthy coral reef ecosystem. However, variations can be different in other locations [17]. Low herbivore intensity will cause macroalgae dominance over coral communities stated that low abundance herbivores would   [19]. Biomass of herbivorous fish will increase the ability of coral reefs to recover because it increases post-settlement survival, and the loss of these fish groups will increase macroalgae cover [20] [21]. Previous studies on Sangiang Island and Belitung in Indonesia showed the vital role of herbivorous fish in controlling the abundance of macroalgae [22] [23]. Then another study exhibits that the critical function of turtles and dugongs in Jamaica does not cause a phase shift because herbivorous fish could replace their functions. In general, sustainable fisheries will choose carnivorous fish as a fishing target. However, if there is a derivation of the carnivorous fish catchment, herbivorous fish will be the next catchment [24]. The low abundance and biomass of herbivorous fish in Lembeh Island usually indicate that fisheries in the area are no longer sustainable and impact the decrease of coral reef conditions.

Coral reef health index
Coral Reef Health Index is expected to be a tool in measuring management with specific standards in measuring whether the ecosystem is in good condition or not. Reef fish can be grouped into three based on utilization status in capture fisheries, namely major, target, and indicator fish. The target fish obtained varies between 0,1 to 3,777.6 kg/ha with a total of 20,798. Acanthuridae family dominated the abundance (86%) and biomass (88%) of target fish in Lembeh Island, while the lowest was Lethrinidae (0.01%). The Acanthuridae family was dominant on Lembeh Island because these fish were found in almost all observation locations except Honey Bay and Trikora Monument. The fish species of the Acanthuridae family found on Lembeh Island were abundantly dominated by Zebrasoma scopas, but in terms of biomass, it was dominated by Ctenochaetus striatus. Predatory fish groups such as snapper (Lutjanidae), grouper (Serranidae), emperor fish (Lethrinidae), sweetlips (Haemulidae) each have a biomass of 77.82 kg/ha (0.37%); 381.74 kg/ha (1.84%); 1.65 kg/ha (0.01%); 278.54 kg/ha (1.34%); and 563.92 kg/ha (2.71%). Furthermore, their abundance was found to   The presence of reef fish in quality and quantity can be used as bioindicators of the condition of coral reef ecosystems. Mainly, massive coral colonies provide habitat for large-bodied Serannidae and Lutjanidae [25]. The reef fish community structures could monitor overall coral reef health due to their strong affinity for corals, such as the association of Chaetodontidae tribe as obligate corals and indicators of coral reef health [26] [16]. The target fish is one of the economically important fish communities that become the target of fishing catches. The predatory fish that are the target catches, such as Lutjanidae, Serranidae, Lethrinidae, Siganidae, and Haemulidae, are generally classified as high economic fish [27]. Controlling these groups will affect the herbivores group's composition and, in general, will change the structure fish community. Thus the lcorallivorous, herbivorous, and carnivorous can be determined as an indicator to monitor coral reefs conditions [28] [29].
A general abundance of herbivorous fish was poor, the biomass of reef fish was dominant in the low category, and the diversity of indicator fish was dominant in the low category. Based on table 5, there are five locations (Angel Window, Nusu Beach, Pasir Panjang, Pulau Dua, and Tanjung Kuning) with high target fish biomass but very few herbivorous fish abundances. In contrast, the diversity of indicator Fish was moderate to high. There were four locations with moderate target fish biomass, but the abundance of herbivorous fish was low, while the diversity of indicator fish and abundance of corallivores fishes on live coral were moderate to very high. Furthermore, there were 13 locations with low target fish biomass, but the abundance of herbivorous fish and the diversity of indicator fish was very low.