Irrelationship between live coral cover and reef fish: an interim study of marine habitat dynamic on Mandangin Islands

Most coral reef ecosystems with high coverage of living coral would escalate the reef fish abundance and species richness as one of the correlatable communities to the coral reef. However, reef fish could only utilize one of the coral reef ecosystems to provide to the reef fish community that still withstand and adapt to those conditions. This study aims to assess the coral reef ecosystem quality on Mandangin Islands and the correlation between the benthic substrate to the reef fish structure. Coral reef ecosystem was assessed using Line Intercept Transect and Underwater Visual Census. A linear model or regression analysis was built to identify the key predictors driving factor in the ecosystem. Rock, Sand, and Rubble are the highest benthic substrate around Mandangin Islands. Reef fish have a high abundance along the observation site with exhaustive trophic level, high major functional group, broad bodied size with high abundance small to moderate bodied-size. Rock has a positive correlation to reef fish abundance; Sand has possibilities to escalate species richness, whereas live coral negatively correlates to reef fish. Historically Mandangin Islands contained a healthy coral reef ecosystem that degraded after years, yet reef fish withstand and adapt to the Mandangin Islands’ degraded coral reef ecosystems. Rock is coral that loses several of its function, especially as direct nourishment. This study showed that reef fish should be considered as one of the primary data to strengthen the decision-making of Integrated Coastal Management and Oil Spill Contingency Plans by the Oil and Gas Company.


Introduction
Coral reefs are a component that has an essential function in its ecosystem [1,2].Coral reefs as an ecosystem cannot be separated from their ecological role as a spawning ground [3], a nursery ground [4], and a feeding ground [5] for economically important biota.The coral reef ecosystem was built by several substrates, with each function supporting the ecosystem itself.It even enhances the function and complexity of the coral reef as an ecosystem that supports underwater ecological living niches in the coastal area [6][7][8].Reef fish was one of the economically important biotas that relied on the coral reef 2 ecosystem as its life cycle.However, several studies indicated no positive relationship between live coral and reef fish communities [9][10][11].Reef fish did not utilize all the coral reef ecosystem functions, only one or two ecological functions used by certain reef fish species.
The diversity of species or constituent structures in a coral reef community will affect the complexity level of the coral reef ecosystem.The high diversity of structure/substrate constituents will have a complex basic ecosystem structure and provide niches in various forms for the biota community, especially reef fish [12,13].The complexity of a shallow water habitat, including coral reef habitat, describes the roughness and shape of the habitat surface [14,15].The complexity of coral reef ecosystems is essential to study because it has a positive relationship with the abundance and species richness of reef fish.Conditions of non-uniformity of relief and topography of a coral reef ecosystem provide niches for reef fish [16,17].However, the complexity of coral reefs is not always formed due to the presence or high diversity of coral reefs in the ecosystem.The crevice between coral reefs formed from dead coral and rock substrates can positively impact reef fish's abundance and species richness.This basic concept is used in fish aggregating devices (FAD)/fish shelters to attract the presence of reef fish by providing attractions in the form of wide gaps made using cement, stone, tires, PVC, and other basic materials for the rehabilitation of coral reef ecosystems using the transplant method.
The first function of the coral reef ecosystem that attracts reef fish is in addition to the availability of food in the ecosystem, and there are also crevices between the building structures that make up the coral reef ecosystem which is attractive for shelter and symbiosis for reef fish [18][19][20].This study aims to assess the coral reef ecosystem quality on Mandangin Islands, along with the reef fish community structure as an abundant inhabitant on Mandangin Islands coral reefs.The correlation between benthic substrate composer of the coral reef ecosystem became crucial to determining the critical factor of Mandangin Islands coral reef attractiveness to the reef fish community.

Study sites
Mandangin Islands is one of the islands that remain around Madura.Most of the Mandangin Islands' livelihood depends on small-scale fisheries.Small-scale fisheries rely on the coral reef ecosystem as a living niche for reef fish [21,22].The observation indicated a high abundance of reef fish on Mandangin Islands, yet the coral reef ecosystem was threatened by anthropological factors (ex: coral mining, sand mining, destructive gear, and domestic waste).There were no sufficient series data of coral reef ecosystem quality data around Mandangin Islands.There was an assumption that coral reefs around these islands had good quality conditions before massive construction utilizing coral reefs as the home's foundation.

Coral reef ecosystem assessment
Coral reef and reef fish condition were assessed along 30 meters transect with three replication.The coral reef was observed using Line Intercept Transect (LIT) to identify the benthic substrate type in each observation location [23].LIT methods observed coral reef benthic substrate until level substrate type categorized into 11.Each category had its function to support the coral reef ecosystem and had historical value in determining the coral reef condition before the current condition.At the same time, the reef fish community was conducted by utilizing Underwater Visual Census (UVCs) by identifying reef fish to species level, estimation length, and estimation of individual number [24].Reef fish classification based on trophic level and the functional group was identified based on Fishbase data to assess reef fish community structure around Mandangin Islands.

Statistical analysis
A linear model or regression analysis was built to identify the key predictors driving the coral reef of Mandangin Islands by using reef fish community structure compared to benthic coverage type [25].For overall sites, fish abundance and species richness were tested proportionally to the benthic main substrate variable as a predictor variable, "live coral", "sand", and "rock".The live coral coverage presented the healthy coral reef condition in most coral reef ecosystems in Indonesia, then sand and rock, mostly known as a coral reef with uncomplete function, especially food availability for indicator reef fish.

Benthic coverage condition
The benthic substrate coverage is dominated by rock, sand, and rubble.The highest rock coverage was found in the South (39.55%), the highest sand coverage was found in the east (39.95%), and the highest rubble coverage was found in the Southwest (24,85%) (Table 1).The percent of live coral cover found was only 2,45 -7,92%.The highest live coral was found in the Southeast (7,92%), and the lowest was in the east (2,45%).The other benthic substrate coverage found are Other Biota/OT (2.04 -6.07%), sponge (0 -1.66%), and soft coral (0 -0.58%).

Reef fish community structure
A non-linear pattern on the mean reef fish abundance is visible for each location.The highest mean reef fish abundance on Mandangin island was located on the Southwest side (Figure 2) and decreased the number of individuals at South Observation Sites and Southeast Observation Sites but slightly increased at East Observation Sites.A similar pattern also occurs in species richness, where the highest species richness was located in the Southwest and the lowest in the Southeast.By grouping reef fish based on their diet, many on Mandangin Island belong to benthic invertivores, planktivores, and carnivores groups (Figure 3).The Carnivore group has the highest density of reef fish at the observation site on the Southwest side of Mandangin island, whereas the benthic invertivore group and planktivore group have the highest fish abundance on the South side.On the other hand, the detritivore group was recorded only on the south side of mandangin island.Coralivore and herbivorous groups were also recorded at the time of observations in the waters of Mandangin Island.Importantly, herbivorous fishes are recognized as an important determinant of coral reef resilience due to their ability to limit algal growth and promote coral recruitment.Most of the reef fish on Mandangin Island belong to the major fish category, with the highest density of individuals at the south-west side observation site (Figure 4).A major fish category is a group of fish that function as a filler in the trophic pyramid.In general, the composition of reef fish based on functional group categories was used to design ecosystem management.Moreover, functional redundancy and the resilience of coral reef ecosystems are critically dependent upon response diversity among species with similar ecological functions.The highest abundance of reef fish for the target fish category is on the south side of Mandangin island, while the lowest reef fish density is on the Southeast side.The target fish category is all types of fish that are the target of fishermen's catch.Variations in body size and corresponding changes in the scale of home ranges or feeding territories are key factors responsible for response diversity among species with similar ecological functions.The large-bodied fish targeted by reef fisheries are generally immune from the effect of coral loss and habitat degradation.This study results show that most reef fishes on Mandangin island had a size interval between 5-10 cm (Figure 5).Reef fish abundance on Southwest mandangin island was the highest by the size interval, followed by 10-15 cm bodied size reef fish.The south-west observation site was the largest body size of reef fish recorded on Mandangin Island.

Coral reef and reef fish correlation
Regression analysis shows the positive and negative relationship between the highest driven and abundant benthic coverage to the reef fish abundance and species richness (Figure 6).The reef fish abundance is positively correlated with the percentage of rock.The percentage of rock will increase, followed by the reef fish abundance.Meanwhile, the reef fish species richness decreased when the percentage of rock increased.The reef fish species richness increased with sand as a benthic coverage, while the reef fish abundance is negatively correlated with sand.
The correlation between live coral and reef fish abundance and species richness is negative.The live coverage is high, while the reef fish abundance and species richness are low.This result is fascinating because reef fish do not need good live coral.Reef fish need a habitat that provides food, such as algae for herbivorous fish and plankton for planktivory fish.The rock is a place for algae, such as fleshy seaweed and coralline algae, to attach.The reef fish feed the plankton at the bottom of the water that is on the substrate.

Coral reef and reef fish current and historical prediction condition
Currently, Mandangin Islands is in a critical endangered condition of coral reefs condition.Ecological and anthropological pressure historically affected the coastal ecosystem quality.Several observation studies on the field indicated that mangroves had been degraded with an inclination of household construction built on the sea (Figure 7).Household construction escalation in small islands will harm the coral reef through coral and sand mining activity [26].Those anthropological pressures still occur in Mandangin Islands until the living coral is suppressed to under eight percent.The North side of Mandangin Islands has lost its coral coverage a hundred percent and alternates to silt and sand coverage, with abundant sea urchins.Sea urchins indicated high nutrient water quality, which is inclement for coral reef health [27].Degraded water quality in north Mandangin Islands causes by household disposal and redundant waste and litter without follow-up and awareness of coastal communities to overcome these historical conditions.Irresponsible fisheries occurred around Mandangin Islands with many vessels (Figure 8).Increasing vessels would escalate anchor utilization that fractured the coral reef building structure and increase rubble coverage [28].Most rubble coverage around Mandangin Islands came from living coral that fractured till overall wrecked by the high pressure of vessel anchoring and historical utilization of destructive fisheries.Historically, it predicted that the coral reef ecosystem was full coverage with live coral with various growth types around Mandangin Islands before destructive fisheries and high vessel utilization existed.Modernization and upgrading fishing gear focused on escalating the amount of fish exploitation rather than decreasing the fishing gear to the coral reef.It becomes unavoidable because most coastal communities' welfare comes from fisheries activities and exploitation of natural resources from the sea [29].and interaction areas.The nourishment area function provided by the coral reef ecosystem only prospers for planktivore, omnivore, and benthic invertivore trophic groups.Live coral has been degraded functionally and become rock and dead coral with algae (DCA), which provides a niche as a shelter for reef fish because it still has crevices between those rocks and DCA.There is a chance of reef fish species declination historically because of function loss by the coral reef ecosystem, yet several reef fish species still withstand Mandangin Island conditions by utilizing the coral reef ecosystem as a nursery area.Many species with high abundance are found in our observation in the location, such as the schooling of Diploprion bifasciatum, which is rare to be found in Indonesian waters to occur, and large-bodied size of Arothron stellatus interacted with various small-bodied size reef fish (Figure 9).

Main driving factors of mandangin islands coral reef ecosystem
Reef fish usually live in groups on certain corals, and their movements are limited.They utilize reefs as territories, shelters, nourishment, and reproduction areas [30].The abundance of reef fish is higher in areas with complex habitat structures and diversity.This structure and diversity affect the surface roughness (rugosity) of the sea floor to provide a niche for reef fish and other marine biotas to live.[31] was stated that fish abundance can be predicted from rugosity and diversity of growth forms.Areas with high rugosity usually have abundant reef fish [15,32].The substrate of Mandangin Island is dominated by rock.Rock substrate is a stable substrate with a wide surface where microalgae, macroalgae, gorgonians, sponges, and corals can live [31].Various substrate on coral reefs is biologically essential because many fish feed on the algae and benthic animals surrounding the rock.The rock substrate also provides rugosity for reef fish, providing shelter and a place to lay eggs.The low live coral cover on Mandangin Island does not make the reef fish in this area disappear because there is still rock substrate that provides space, nourishment, and shelter to live, making the abundance of reef fish relatively high.

Coral reef ecosystem management based on mandangin islands study case
Fisheries are the primary livelihood sector for the people of Mandangin Island from generation to generations.The total population of this island until 2021 is 21,534 people, with an estimated number of fishers are 4,019 people.The number of fleets on Mandangin Island is approximately 40 purse seine vessels, 780 small inboard-engine vessels, and 50 canoes without motors.If it is assumed that each family consists of four members, then the total fisheries household is more than half of the total population of Mandangin Island [Primary data 2021].On average, capture fisheries production on Mandangin Island is 1,600 -2,000 tons/year [33].The production comes from fishers using gill nets, handlines, squid-nets, crab-nets, crabs-dredge, crab-pots, fish-pots, mini trawls, and purse seines.Of all these types of fishing gear, only purse seines fish over 12 nautical miles.Other fishing gear operates in traditional fishing lanes ranging from the shore to 4 or 12 miles.This means fishing occurs in coral reef areas, including vessel anchoring activities.In addition to fisheries, the coral reefs on Mandangin Island also interact with other activities: coral mining.This practice occurs for two reasons.The first reason, there was market demand.This occurred in the 1990s and ended in 2000 when prohibitions and sanctions were imposed [Primary data 2021].The second reason is the need for a foundation in which no other source is cheaper than taking coral reefs in the sea around the island.The first practice is non-existed, while the second practice still exists to present.A series of destructive actions from fisheries and coral mining caused damage to coral reefs by up to 90% by 2021.They are leaving not more than 10% of coral reefs still alive.Various studies state that the status of coral reefs correlates with the status of fish resources [25,[34][35][36], but in the waters of Mandangin Island, a relatively large level of reef fish diversity is found.The damage to coral reefs does not necessarily linearly decrease fish abundance.In the food chain, reef fish in Mandangin still can fill the food chain to predatory fish and economical fish caught by fishers.In other words, they still have a relatively high ecological value.
Anthropogenic factors are one of the main factors in the destruction of coral reefs in the world [37] and damage the overall food chain cycle in an aquatic area [38].In order to improve, maintain or protect and rehabilitate the fish resources on Mandangin Island, an Integrated Coastal Management (ICM) framework can be applied to ensure that these safeguards can be supported within an integrative framework from the technical to the policy level.Integrated Coastal Management (ICM) or Integrated Coastal Zone Management (ICZM) has so far been defined as a continuous and dynamic process in which every decision-making is aimed at the use, development, and protection of the coastal and marine resources and areas [39].Integrated coastal area management is a process to solve all the problems on the coast [40,41].From Integrated Coastal Management (ICM) perspective, fisheries and coral reef resources are essential for sustainable resource management.[42] stipulates five aspects of sustainable development in coastal areas: natural and human-made disaster prevention and response management, natural coastal habitat protection, restoration and management, water use and supply management, pollution and waste reduction management, food security, and livelihood management.The study's results found that the damaged coral reefs and the abundance of fish in Mandangin waters indicated the need to study further the value and correlation between reefs and reef fish so that management and restoration can be carried out.The presence of reef fish indicates that the resource value on Mandangin Island is still relatively high even though the coral reefs are damaged.However, its management cannot be done partially but integrally and continuously.
Marine resources are considered the common-property resource, and the main challenge of common property resources is on governance and the level of efficiency and institutional functionality to overcome the rationality of resource use [43,44].Failure to manage common-property resources further will result in resource depletion and environmental degradation [45][46][47] due to overexploitation without any attempt to recover.The absence of management, over-exploitation and the free riders' emergence will lead to the Tragedy of the Commons as formulated by [48].As subtractable and non-excludable resources, fish resources in Mandangin Island are also threatened because they do not have communitybased (communal) or state-based management mechanisms.
As a foundation for sustainable management, natural coastal habitat protection, restoration, and management can be started by calculating the total value of the resource and paying attention to the level of sensitivity related to surrounding activities.In the context of Mandangin, in particular, an offshore oil platform is managed by Husky-CNOOC Madura Limited (HCML).Based on the International Convention on Oil Pollution Preparedness, Response and Co-operation (OPRC), which Indonesia has ratified through Presidential Regulation No. 76 of 2022 on Ratification of the International Convention on Oil Pollution Preparedness, Response, and Co-operation, 1990, Indonesia must ensure that oil exploration and exploitation operations and their transportation processes are obliged to implement the convention.This convention is a joint effort between parties to protect water resources (marine and coastal) from potential oil production and transportation operations at sea [49].At the operational level, [50,51] has developed a method to measure the sensitivity of a resource to oil spills, including reef fish.
Despite the damage to coral reefs, the high abundance of reef fish around Mandangin water requires attention and management before it loses its ability to support the socio-economic system of the Mandangin community.Policymakers need an Integrated Coastal Management approach to find challenges and take opportunities to make changes in the use of aquatic resources [52].In the ICM concept, the functionality of governance is the extent to which formal and informal arrangements and institutions serve as the basis for planning and decision-making [46] or the achievement of common goals based on specific institutional structures [53].At this point, a significant gap can be seen on Mandangin Island, which can threaten aquatic resources to be sustainable.
Overall, the coral reefs of the Mandangin Islands are in a critical endangered condition.The quality of the coastal ecosystem has historically been impacted by anthropogenic and ecological forces.The majority of the rubble encircling the Mandangin Islands was created by living coral, which was broken up and consequently wrecked by historical harmful fishing practices and the intense pressure of vessel anchoring.Even without live coral next to them, reef fish are still in great quantity at the Mandangin Islands.Most reef fish are territorial and utilize coral reefs as shelter and interaction areas.Living coral has been functionally degraded into rock and dead coral with algae (DCA), which nevertheless retains crevices between those rocks and DCA, providing a niche as a home for reef fish.Because Mandangin Island still has rock substrate that offers living space, food, and shelter, the reef fish population is still comparatively abundant despite the island's low live coral cover.
In this study showed that reef fish component have a valuable meaning to the ecosystem and other ecological component around the ecosystem itself.Many off-shore study such as Integrated Coastal Zone Management (ICZM), oil spill around coral reef ecosystem, and Environmental Sensitivity Index (ESI), and Environmental Impact Assessment (EIA).Even, without existence of living coral, there is still many reef fish that could live around those degraded ecosystem and still have value and meaning itself to be studied.

Figure 1 .
Figure 1.Coral reef ecosystem sampling locations spread south-west to the east of Mandangin Islands.

Figure 2 .
Figure 2. Mean reef fish abundance and species richness along Mandangin Islands coral reefs ecosystems.

Figure 3 .
Figure 3.The trophic level of reef fish is based on the mean abundance around Mandangin Islands coral reef ecosystems.

Figure 4 .
Figure 4. Reef fish mean abundance categorized based on function group on Mandangin Islands coral reef ecosystems.

Figure 5 .
Figure 5. Reef fish abundance along Mandangin Islands based on size interval category.

Figure 6 .
Figure 6.Relationship between the highest driven and abundant benthic coverage to the reef fish abundance and species richness along three repetitions in each observation site on Mandangin Islands coral reef ecosystems.

Figure 7 .
Figure 7. Existing conditions for aerial and underwater sightseeing show (a) no coverage of mangroves on the coast, with (b) massive construction on the north side of Mandangin Islands, and (c) low coral coverage with sea urchins blooming on the south Mandangin Islands.

Figure 8 .
Figure 8.An aerial view of the south side of Mandangin Islands showed (a) sand coverage mostly, and (b) many vessels operated with various fishing gear.The predicted historical condition of high coral coverage of Mandangin Islands showed by the abundance and richness of reef fish species around Mandangin Islands, and even the current condition has been shifting to sand and rock coverage.Reef fish around Mandangin Islands still have a high abundance without live coral beside them.Most reef fish are territorial and utilize coral reefs as shelter

Figure 9 .
Figure 9.The reef fish community's underwater visualization, with (a) high abundance and richness even on non-live coral coverage on coral reef ecosystem along Mandangin Islands coastal area, even (b) Arothron stellatus was found healthy live around this degraded coral reef ecosystem.

Table 1 .
The mean number of benthic substrate percent coverage per site ± standard deviation.The percentage data coverage order is based on the highest to lowest mean data.