Contrasting reef fish diversity and abundance in the Derawan Islands: A multi-temporal observations

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Introduction
Derawan Islands, located in the Berau District, East Kalimantan Province, has a lot of uniqueness.It lies on the western side of Makassar Strait and is strongly influenced by the Indonesian Throughflow (ITF), the major tropical oceanic exchange current between the Pacific and Indian Oceans, which has high water productivity due to the occurrence of the periodic deep-sea upwellings [1].Meanwhile, the seas of the Derawan Islands are also influenced by major river outflows from the Berau River and tributaries.The strong nearshore-offshore oceanographic gradient makes the Derawan Islands a diverse and dynamic environment with river deltas, mangroves, seagrass beds, shelf and oceanic coral reefs, AND pelagic waters [1].
Situated inside the coral triangle areas, Derawn Islands is rich in species diversity of hermatypic corals, including reef fishes of 444 and 872 species, respectively.There are about eight species of seagrasses with two species of sea turtles, the green turtles (Chelonia mydas) and hawksbill (Eretmochelys imbricata), that nest and forage around the Derawan Islands as well as nine species of cetaceans [1].In addition, a famous saltwater lake on Kakaban Island is inhabited by four species of unique endemic jellyfish [2].
However, although the Derawan Islands are rich in marine flora and faunas, this region faces many threats due to unfriendly exploitation of marine resources such as the use of explosives, potassium cyanide, and other destructive gears, overexploitation, severe sedimentation in the water caused by mining and logging activities, mangrove conversions for making aquaculture ponds [2].Furthermore, the available data and information about the marine resources in this region are scarce, for example, fish resources, mainly reef fish.Thus, it isn't easy to manage those resources sustainably without data.
This study aims to reveal the marine resources of the Derawan Islands by focusing on the diversity and abundance dynamics of coral fish with emphasis on three groups of coral fish: the herbivorous fish (Families Acanthuridae, Scaridae, and Siganidae), Carnivores (Haemulidae, Lethrinidae, Lutjanidae, and Serranidae (carnivores), and corallivores (Chaetodontidae).Those reef fish groups are usually used as coral reef health indicators through long-term monitoring.
In this study, we used firstly the observation data collected during the Derawan Islands expedition conducted in 1994 using the Research Vessel (R/V) of Soerja Atmadja belonging to the Agency of Research and Development Center for Marine Resources Ambon, Indonesian Institute of Sciences (LIPI).Secondly, the data from the cruise of R/V Baruna Jaya VII in 2003, and the third was during the implementation of the COREMAP CTI project in 2019.

Reef fish data collection
The reef fish data collection method used in this study is the Underwater Visual Census Method (UVC) developed by the Asean-Australia Project [3,4].The equipment used is diving equipment (SCUBA), underwater stationery, and a roll meter ruler.A 70 m long transect line was placed parallel to the beach or shoreline, then reef fish was censused along the transect line with observations at a distance of 2.5 m to the left and 2.5 m to the right from the transect line so that the total area observed in each UVC is 70 m x (2.5m + 2.5m) = 350 m 2 (Figure 2).Observations were made at depths between 5 -7 meters.
The collected reef fish data by UVC in 1994 and 2003 were divided into three groups [4]: (i).The Indicator group is the reef fish belonging to only one Family (Chaetodontidae, the butterfly fish) that is used to indicate the health of coral reefs since they are strongly associated with a good condition of live corals; (ii).The Target group is the consumable and economically valued fish, so they are the most exploited by the fishers; (iii).The Major group is the fish that do not belong to those two groups above.Generally, they have small sizes and beautiful colors, so they are well-known as ornamental or aquarium fish [4].This method has become a standard operating procedure (SOP) for monitoring reef fishes in the Great Barrier Reef, Australia [5], which is then well used in many other tropical countries [6].During the UVC transect, some identification books are used as references [7][8][9][10][11].
However, since the COREMAP CTI programs changed in 2015, the previous three reef fish classification systems were replaced again into three groups: (1) Corallivores, the group fish that are still the same as the indicator fish; (2) Carnivores fish; the fish that are the same with target fish, but limited only to four families of Haemulidae, Lethrinidae, Lutjanidae, and Serranidae, and Herbivores that restricted to three families of Acanthuridae, Scaridae, and Siganidae.Therefore, reef fish data in 1994, 2003, and 2019 were analyzed using the classification based on the protocol of COREMAP CTI.

Reef fish stock assessment
Two steps have to be taken in assessing the reef fish stock.The first step is to make the shallow bathymetric map in the range of 0-25 m, which assumes that this depth range is the habitat of the reef fishes, so they exist and are distributed at those depths.The shallow waters of Derawan Islands were mapped utilizing data from the general bathymetric chart of the oceans (GEBCO) provided by the British Oceanographic Data Center (BODC).The resolution of these data is 30 arc-second grids (926 m × 926 m) [12].The second step is to classify the shallow bathymetric and calculate each area (A) according to their depth ranges of 2-5 m, 5-10 m, 10-15 m, 15-20 m, 20-25 m, and > 25 m, respectively.Waters with a depth of <2 m is excluded from further calculations.Those are influenced by the tidal.The consumed reef fish density (D) with economic values for both carnivores and herbivores fish data was obtained from the UVC method (Figure 2).Thus. the standing stocks (S) of carnivores and herbivores reef fish were assessed using the equation: S = Standing stock of reef fishes in terms of the number of fishes or fish biomass (kg or tons); A = The areas obtained from the bathymetric map and D = The density (fish/ha) of carnivores and herbivores reef fish.
To get the biomass of carnivorous and herbivorous fish, it is necessary to convert fish length into biomass using the fish length and weight relationship equation as follows: (2) W = Fish biomass (kg or ton), L = Total length, a and b = the growth coefficient of fish.
The fish's total length (TL) can be estimated using the equation below: TL = (TLmin+ TLmax)/2 or TL = 0.65 × TLmax TLmin and TLmax are the minimum and maximum of the fish's Total length.

Coral cover data collection
On the same transect line with the reef fish data collection, observations were also made for the condition of the coral reefs in the form of live coral cover and other benthic habitats such as DA (Dead Coral), DCA (Dead Coral covered by Algae), SC (Soft Coral), SP (Sponge), FS (fleshy Seaweed), OT (Other biotas), R (Rubble), S (Sand), SI (Silt) and RK (Rock).The coral reef condition can be determined based on the covers of living coral.The Coral reefs are in bad, medium, good, and excellent condition if the percent covers of the live coral is <25%, 25-50%, 50-75%, and > 75%, respectively [3,4].

Reef fish species diversity
Regarding species diversity, we found 116, 93, and 104 species of the three groups (Corallivores, carnivores, and herbivores) reef fish in Derawans Island during the 1994, 2003, and 2019 observations, respectively.Figure 3 shows the graphs of species diversity of reef fishes, including the corallivores, carnivores, and herbivores in each observation site of Maratua, Kakaban, Panjang, Derawan, Samama, and Sangalaki Islands.
Looking at this graph at first glance, it seems nothing special.Nonetheless, there appears to be a tendency for species diversity to decrease from 1994 to 2003, mainly the herbivores fish in all observation sites, but increase again in 2013.The herbivorous reef fish diversity in 1994, 2003, and 2019 ranged from 20-36, 6-17, and 18-27 species, with an average value of 25, 13, and 23 species, respectively.The species that dominate this group are Ctenochaetus striatus and Zebrasoma scopas (Family Acanthuridae), Scarus sordidus, and S. dimidiatus (Family Scaridae), while the other herbivore fish species of the Siganidae family did not dominate at all.
The carnivore reef fish group diversity, from three periods of observation and in all observation sites, shows that the species diversity in 1994, 2003, and 2019 ranged from 11 to 22, 8 to 23, and 7 to 16 species with an average of 17, 15 and 12 species, respectively (Figure 3).The diversity of this group gradually decreased in those years, while the dominant species were Lutjanus decussatus, Cephalopholis urodeta, Lutjanus fulvus, C. cyanostigma, and Macolor macularis.
Meanwhile, the corallivore reef fish group from 1994, 2003, and 2019 in all observation sites (Figure 3) showed that the species diversity gradually decreased from the ranges of 15-21, 6-20, and 7-15 species with an average of 17, 14, and 11 species, respectively.The species diversity pattern of this group is similar to the carnivore group, which shows a tendency to decrease.Chaetodon kleinii, C. vagabundus, C. Trifasciatus, Heniochus various, and C. Baronessa are the dominant species in this group.

Reef fish abundance (density)
In terms of abundance (fish/ha), Figure 4 shows the total abundance of each herbivore, carnivore, and corallivore group.A total abundance of 5,268, 2,664, and 5,332 fish/ha during the sampling periods of 1994, 2003, and 2019, respectively.From this figure, again, the herbivore group dominated the abundance of reef fish in Derawan Is.The abundance of this group in 1994 ranged from 1560-5680 fish/ha with an average of 3080 fish/ha.The range of abundance in 2003 decreased to 267-923 fish/ha with an average of only 647 fish/ha, while in 2019, the abundance ranged between 2543-3500 fish/ha with an average of 2983 fish/ha, which indicates an increasing of 4.5 times higher than 2003 or nearly back to the abundance of 1994.An abundance of 3000 fish/ha was found in Samama Is in 2019, Kakaban Is in 1994, and Maratua Is in 1994 and 2019.The high abundance of herbivore group dominated by the species of Ctenochaetus striatus, Zebrasoma scopas, Acanthurus pyroferus (Family Acanthuridae), Scarus sordidus, S. niger and S. dimidiatus (Family Scaridae).None of the species within the Family Siganidae dominated the herbivore group.The reef fish abundance of the carnivore group 1994 was 500-1240 fish/ha with an average of 950 fish/ha.In 2003, the abundance of this group decreased to the range of 336-1243 fish/ha with an average of 733 fish/ha, while in 2019, the abundance increased again to the range of 300-1600 fish/ha with an average of 881 fish/ha.The abundance of carnivore groups higher than 1000 fish/ha was found in Panjang Is in 1994 in Sangalaki and Maratua Is in 2019, which indicated that this group of fish recovered from previous years.The species of Lutjanus decussatus, Cephalopholis urodeta, L. fulvus, C. cyanostigma, and Macolor macularis dominate the high abundance of the carnivore group.None of the species from the Family of Haemulidae and Lethrinidae dominated this fish group.
The reef fish abundance of the corallivore group in 1994 was 1040-2060, with an average of 1426 fish/ha.In 2003, the abundance of this group decreased in the range of 592-2420 fish/ha with an average of 1290 fish/ha, while in 2019, the abundance increased again to the range of 986-2043 fish/ha with an average of 1430 fish/ha that a little bit higher even than in 1994.The abundance of corallivore groups higher than 1000 fish/ha was found in Derawan Is in 2003 and 2019, Panjang Is in 1994 and 2019, Sangalaki Is in 1994 and 2023, and Maratua Is In 1994 and 2019.The corallivore group has only one family (Chaetodontidae); thus, the species that dominated this group were Chaetodon kleinii, C. vagabundus, C. trifasciatus, Heniochus varius and C. baronessa.

Biomass and standing stock of herbivores and carnivores reef fishes
Figure 5 shows a shallow water bathymetric chart using the British Oceanographic Data Center (BODC).From the calculation, in the white shade box, the total depth areas from the ranges of 2-5, 5-10, 10-15, 15-20, and 20-25 m was 144,872 ha.It is assumed that the areas within the depth between 2 and 25 m are the habitat of reef fishes, where the reef fish exist and are distributed.Meanwhile, the areas in the depth of < 2 m are not accounted for because this depth was under the influences of the tide ranges, while in the depth of > 25 m are assumed to be dominated by the habitat of sand, in which the species diversity and abundance of coral reef are low so that it can be neglected.
Table 2 displays the standing stock of the herbivore and carnivore fish groups for the Derawan Islands.These fixed stocks were calculated based on the areas within the depth of 2-25 m that derived from the shallow water bathymetric chart of Figure 5, and the biomass of herbivore and carnivore fish group/ha, in which the biomass derived from the total number of fish in one ha x a*L^b.From this table, the total standing stocks of herbivores in which some herbivore fish are also consumable fish that have economic values was 54,295 tons, consisting of families Acanthuridae of 25,364 tons, Scaridae of 24,419 tons and Siganidae of 4,512 tons.Meanwhile, the total standing stock of carnivore was 17,689 tons, consisted of Haemulidae of 1,753 tons, Lethrinidae of 2,854 tons, Lutjanidae of 7,809 tons, and Serranidae of 5,273 tons.

Relationship between reef fish and living coral covers
In this study, we also want to know the relationship between the reef fish group and their habitat, which is indicated by the percent covers of the live corals.In this case, we used the data from 2019 because it is complete so that it can be analyzed compared to data in 1994 and 2003.The live coral cover percentages ranged between 26% (Maratua Is) and 74% (Sangalaki Is), with an average of 37%, indicating the coral reef in the Derawan Islands was in medium condition.
The analysis results show that the species diversity of the family Lutjanidae (carnivore), all herbivores and corallivores is strongly correlated with the percentage of living corals with coefficient determination (R 2 ) of 0.86, 0.92, and 0,93, respectively, but show a weak correlation with other family as well as the abundance of reef fishes with the R 2 <0.8 (Table 3).Figure 6 displays the relationship with the high R 2

Fig. 3 .
Fig. 3. Reef Fish species diversity of herbivores, carnivores, and corralifores in each sampling site during three observation periods.

Fig. 4 .
Fig. 4. The abundance (Number of individual fish/specimen) of herbivores, carnivores, and corralifores reef fishes in each sampling site during three observation periods.

Table 1 .
Coordinates of the UVC in each station.

Table 2 .
The standing stock of herbivore and carnivore groups of reef fish in the Derawan Islands.