#ref1 Introduction

Hainan's offshore area is located in the northern part of the South China Sea and has rich marine biological resources and a diverse marine ecosystem. However, in recent years, the marine ecology in this area has faced severe challenges, with fishery resources declining significantly and marine habitats suffering serious damage (Wang et al., 2023; Yu et al., 2024). Overfishing has caused a decline in the number of many economic fish populations, and "fishery resource exhaustion" has occurred in some coastal waters. At the same time, habitat degradation is a prominent problem, such as large-scale bleaching and death of coral reefs, reduction of mangroves and seagrass beds, etc., which threatens the survival of fish populations that rely on these habitats. The vulnerability assessment of the Hainan fishery system shows that Hainan's fishery is in an "extremely high" vulnerable state in most years, reflecting the severity of ecological and environmental pressures and over-exploitation of resources (Wang et al., 2023). Therefore, it is urgent to take effective measures to restore the fish habitat and fishery resources in Hainan's offshore area.

Overfishing is recognized as one of the main reasons for the decline of global fishery resources. In Hainan's offshore area, long-term and high-intensity nearshore fishing activities have changed the structure of fish communities, and large fish with high economic value have significantly decreased. In addition to fishing pressure, the degradation of coral reef ecosystems also has a negative impact on fish diversity and quantity. Coral reefs in the South China Sea have experienced widespread degradation due to multiple disturbances such as seawater warming, crown-of-thorns starfish outbreaks, and coastal pollution (Wang et al., 2023). According to statistics, since the 1980s, about 80% of coral reefs around mainland China and Hainan Island have been destroyed by human activities (Shi et al., 2022). The sharp decline in coral cover has caused the fish that inhabit them to lose their breeding and shelter places, and the fish community structure has shifted towards a state dominated by species with strong anti-disturbance capabilities (Huang et al., 2023). Coastal development is also a factor that cannot be ignored. Large-scale land reclamation, port construction, and tourism development activities along the coast of Hainan Island have caused coastal habitat fragmentation and water quality deterioration. These factors have worked together to reduce the diversity of fish in Hainan's offshore waters, reduce the number of populations, and weaken the ecological function of the waters.

Faced with the decline of fishery resources and habitat degradation, countries have explored new ways to restore the marine ecology. Artificial reefs are increasingly valued as an active intervention ecological restoration tool (Guo et al., 2024). Artificial reefs usually refer to structures that are artificially placed or constructed in specific sea areas to simulate complex environments such as natural reefs and provide space for marine organisms to attach, survive and reproduce (Pan et al., 2023). Practice has shown that artificial reefs can gather and attract marine organisms such as fish, effectively increasing local biodiversity and resource volume (Paxton et al., 2020). Many coastal countries have included artificial reef construction in fishery resource conservation plans (Brochier et al., 2021). Preliminary monitoring results show that artificial reef areas have attracted a variety of coral reef fish and benthic organisms to settle, and fish density and biomass have increased significantly. These successful practices provide a scientific basis for achieving ecological restoration through artificial reefs in Hainan's coastal waters.

This study will summarize the ecological function and mechanism of artificial reefs, sort out the current status of artificial reef construction and habitat degradation in Hainan's coastal waters, verify the role of artificial reefs in different scenarios through typical case analysis, and explore the challenges and future research directions. This study will help enrich the theoretical understanding of the ecological effects of artificial reefs and provide a reference for the practice of ecological restoration in Hainan and even my country's offshore areas, and has important ecological and socio-economic significance.

2 Ecological Functions and Theoretical Basis of Artificial Reefs

2.1 Basic structural types and ecological design of artificial reefs

The structural type and design of artificial reefs directly affect their ecological functions and restoration effects. According to the different water layers where the reefs are set, they can be divided into two categories: bottom-mounted artificial reefs and mid- and upper-layer floating artificial reefs. Bottom-mounted reefs are placed on the seabed and fixed by their own weight, and are the most widely used. Traditionally, artificial reefs have a variety of materials and structural forms, including prefabricated concrete modules, steel frames, abandoned ships, tires, stones, etc. (Lemoine et al., 2019; Vivier et al., 2021). The most commonly used material is cement concrete modified with a special formula, because of its low cost, high strength and surface roughness suitable for biological attachment, it can form a stable and lasting artificial habitat. In recent years, the concept of "ecological concrete" has emerged, which is to add special materials to concrete to improve its biophilic properties.

In terms of structural design, fish reef modules usually have a multi-porous structure to provide hiding and passage space for organisms of different sizes (Figure 1) (Pan et al., 2022). For example, the hexagonal concrete fish reef blocks deployed in the waters near Sanya are about 2 meters high and weigh 90 tons, with caves all around for fish to enter and exit. Appropriate apertures and spatial layout can accommodate a variety of individuals from juveniles to adult fish, and provide habitats for attached organisms such as shellfish and algae.

2.2 Ecological mechanism of artificial fish reefs

The mechanism of artificial fish reefs on marine ecosystems is mainly reflected in three aspects: attracting aggregation, habitat improvement and ecological reconstruction. Artificial fish reefs create a "microecological environment" that is conducive to the survival and reproduction of marine organisms by changing local environmental conditions, thereby significantly attracting fish and other organisms to come and live. A large number of studies have confirmed that after the deployment of artificial fish reefs, the abundance of phytoplankton and invertebrates in the reef area increases, thus forming a "food enrichment effect", which further attracts small fish to come and feed and live. In addition, many reef-dwelling fish have a "reef-tending instinct", that is, they are naturally inclined to live in complex matrix environments to increase the success rate of foraging and avoiding enemies (Pan et al., 2022).

Artificial reefs can improve local water quality and bottom conditions through biological attachment and sedimentation processes. The attached algae and filter feeders on the surface of the reef absorb and utilize dissolved nutrients and suspended organic matter in the water during their growth, thereby reducing the degree of eutrophication to a certain extent. In addition, artificial reefs change the water flow, allowing fine-grained sediments to settle on the upstream surface of the reef, which helps to reduce the pressure on adjacent coral reefs caused by sediment coverage. The ultimate goal of artificial reefs is to restore and rebuild damaged marine ecosystems so that they can regain their self-sustaining functions. Model studies have shown that if artificial reefs are subject to a fishing ban, as the biomass of fish in the reef area increases, it can also produce an outward proliferation effect, driving the increase in fish catch in the surrounding open sea areas (Brochier et al., 2021).

2.3 Theoretical models for evaluating reef functions

In order to scientifically evaluate the functions and effects of artificial reefs, scholars have established a variety of theoretical models and evaluation frameworks. One of the core discussions is the debate between the "production increase effect" and "attraction effect" of artificial reefs (Brochier et al., 2021). The production increase effect refers to the fact that artificial reefs promote the actual increase in fish production by improving the habitat, while the attraction effect believes that reefs only attract fish from the surrounding areas and do not increase the overall production. Recent studies tend to believe that the two effects may exist at the same time, and their relative role depends on management measures and environmental conditions. To quantify these effects, mathematical models are introduced for evaluation. For example, some fishery bioeconomic models regard reefs as a means to increase environmental carrying capacity (K), setting the functional relationship between reef volume and additional carrying capacity, and the ratio of fish attracted by reefs β (V).

Another commonly used is the BACI experimental design (Before-After, Control-Impact), which evaluates the ecological changes caused by reefs by monitoring before and after the deployment of reefs and in the control area simultaneously. In addition to production, ecological network models (such as Ecopath/Ecosim) are also used to simulate the impact of fish reef intervention on the food web and predict key population dynamics and ecological cascade effects. In addition, the indicator system evaluation method sets multi-dimensional evaluation indicators (such as species richness, Shannon diversity index, fish density, biomass, attached organism coverage, etc.) and comprehensively scores to judge the ecological health of fish reefs (Yu et al., 2024).

3 Overview of Artificial Reef Construction in Hainan’s Offshore waters

3.1 Current status and trends of fishery resources in Hainan’s offshore waters

Hainan’s offshore waters have a tropical and subtropical fishery ecosystem, with rich fishery resources in history, and a variety of coral reef fish and migratory fish breed and thrive here. However, under the influence of multiple pressures, the amount of fishery resources in the region has declined significantly in recent decades, showing a trend from abundance to decline (Zhou et al., 2019). According to statistics, the unit fishing effort yield (CPUE) of important economic fish in Hainan’s offshore waters has dropped a lot compared to the end of the 20th century. In the coastal waters, the population density of traditionally high-yielding grouper, stone bass, golden pomfret, etc. has significantly decreased, while the proportion of low-value fish and juvenile fish in the catch has increased, showing signs of resource decline caused by overfishing. In addition, fishermen reported that there are fewer and fewer large fish that can be caught near the coast in recent years, and fishing grounds need to be continuously moved outward to maintain production, reflecting the current situation of overexploitation of fishery resources and the high vulnerability of the fishery system (Wang et al., 2023; Yu et al., 2024). On the other hand, management measures such as the summer fishing moratorium in the South China Sea implemented since 2017 have played a certain role in alleviating resource decline. During the fishing moratorium, fishing pressure decreased, and some fast-growing fish resources showed signs of temporary recovery (Yu et al., 2024).

3.2 Main causes and manifestations of habitat degradation

The degradation of Hainan's offshore habitat is an important cause and manifestation of the decline of fishery resources. Overfishing not only directly reduces the number of fish, but also indirectly causes degradation by destroying the seabed habitat. Marine pollution and climate change have superimposed effects. The discharge of nutrient-rich coastal sewage, aquaculture wastewater, etc. has caused the deterioration of coastal water quality and frequent red tides. Corals have been in a sub-healthy state for a long time and are more likely to bleach and die (Shi et al., 2022). In addition, coastal development activities are intense. In recent years, Hainan's reclamation and island construction, port and terminal construction, and tourism facility development have occupied a large amount of coastal shallow waters, destroying key habitats such as coral reefs, mangroves, and seagrass beds. The sedimentation caused by sand mining and channel dredging projects covers nearby reefs and seagrass beds, which also leads to a decline in habitat quality.

The degradation is reflected in different habitats: in terms of coral reefs, the coverage of live corals has dropped significantly, from about 60% in the mid-20th century to less than 20% in recent years (Xiao et al., 2022; Li and Zhao, 2024). The resulting changes in fish communities are: the number of specialized fish that rely on live corals has decreased, and the proportion of herbivorous fish (such as surgeonfish) has increased, reflecting that the reef area has entered a new steady state dominated by algae (Huang et al., 2023). The degradation of mangroves and seagrass beds has caused many juvenile fish to lose shelter and foraging places, and the nearshore juvenile fish resources have decreased significantly. In areas where benthic habitats have degraded, the bottom is poor and the biomass is low. The original resources of shellfish and sea cucumbers are exhausted, and the spawning area of benthic fish (such as tongue sole and flatfish) has been reduced.

3.3 Necessity and urgency of habitat restoration measures

In the face of the above-mentioned habitat degradation, it is urgent to take active ecological restoration measures. From the perspective of maintaining biodiversity, habitat is the spatial foundation for the survival of marine life. Once the "ecological cornerstones" such as coral reefs and mangroves collapse, it will trigger the decline of the entire biological community (Shi et al., 2022). From the perspective of sustainable development of fisheries, healthy habitats are a prerequisite for the replenishment of fish resources. From the perspective of sustainable development of fisheries, healthy habitats are a prerequisite for the replenishment of fish resources. Habitat restoration is also a need to improve ecosystem services. In addition to providing fishery resources, healthy coral reefs and mangroves also have functions such as coastal protection and carbon sinks. Studies have shown that the services provided by global coral reef ecosystems are declining significantly due to degradation (Figure 2) (Eddy et al., 2021). From the perspective of national and local policy orientations, Hainan, as a free trade port, has the dual mission of economic development and ecological civilization construction. In recent years, the government has emphasized "blue carbon sinks" and marine ecological restoration, and artificial reefs and coral transplantation have been included in relevant plans.

4 Design of Evaluation Indicators and Methods for the Ecological Effects of Fish Reefs

4.1 Construction logic of multi-dimensional evaluation indicator system

The evaluation of the ecological effects of artificial reefs requires the construction of a multi-dimensional indicator system, covering aspects such as biodiversity, community structure, ecological restoration and environmental factors. From the perspective of biodiversity, commonly used indicators include species richness, number of individuals and diversity index (such as Shannon index, etc.) to reflect the species composition and richness of the reef area (Taormina et al., 2022). For example, a study proposed the "Artificial Reef Multi-Indicator Index (ARMI)", which integrates 16 indicators in four categories, namely fish community structure (number of species, number of individuals, biomass, diversity index), trophic level structure (average trophic level), species vulnerability (average extinction risk of species) and economic importance (average fishery and tourism value), to evaluate the ecological quality of fish communities around artificial reefs.

In addition, environmental indicators are also an important component of the evaluation system, including the impact of artificial reefs on the surrounding water environment, such as water quality (dissolved oxygen, turbidity, etc.), bottom quality and algae coverage. In the assessment of the ecological effects of artificial reefs, it is necessary to comprehensively consider the regulatory effects of changes in environmental factors and the characteristics of the reef structure itself (such as structural complexity and habitat heterogeneity) on ecological benefits (Bao et al., 2025). By constructing a multi-dimensional indicator system covering biology, ecology and environment, researchers can more systematically evaluate the ecological effects of artificial reefs and provide support for improving the ecological theoretical basis for artificial reef construction (Beyeler and Dale, 2001; Davies et al., 2020; Milošević et al., 2021).

4.2 Ecological survey and monitoring methods and their applicability

The study of the ecological effects of artificial reefs has widely adopted a variety of ecological survey and monitoring methods at home and abroad, and each method has its own applicability and limitations. Among the traditional methods, underwater visual census (UVC) is one of the most commonly used methods. Divers observe the fish population and behavior around the reef by setting sample lines or fixed point counting, and can intuitively obtain species composition and quantity distribution information (Taormina et al., 2022). This method is widely used and is suitable for manual operations in clear shallow waters, but it is limited by diving depth and human subjective bias. In comparison, remote control/autonomous underwater camera technology has been increasingly used in recent years. These non-invasive imaging methods are suitable for long time series and deep-water environment monitoring, and can improve the detection rate of species that are difficult to approach or active at night, but video analysis requires more time and resources.

Fishing surveys are also a traditional method. Domestically, fish population dynamics in reef areas are often monitored by regularly laying fishing nets and cages, and the species and weight of the catches are used to reflect resource abundance. This method is relatively direct, but it is a destructive sampling method that may interfere with reef resources, and different net selectivity may cause deviations. In contrast, non-destructive acoustic monitoring technologies are emerging, such as scientific echo sounding (sonar) for quantitative assessment of the density and biomass of fish schools around reefs, and passive acoustic monitoring for recording fish sounds to infer activity and diversity (Van Hoeck et al., 2021). These acoustic methods can provide data in long-term automated monitoring, but require professional instruments and signal interpretation.

5 Evaluation of the Effect of Artificial Reef Habitat Restoration in Hainan’ s Coastal Waters

5.1 Impact of artificial reefs on fish community structure

The construction of artificial reefs has had a significant positive impact on the fish community structure in Hainan’s coastal waters. For example, the fish species composition in the reef area is richer than that in the control area and has typical reef community characteristics. The fish are mainly reef-dwelling species such as parrot fish, surgeonfish, and damselfish, accompanied by high-trophic level predators such as grouper and sparidae. Artificial reefs introduce complex habitats to areas that were originally fish-scarce, thereby attracting species with multifunctional ecological niches to settle in, changing the fish community structure and making it closer to a healthy natural reef ecosystem. This has also been verified globally: Paxton et al. (2020) found in a meta-analysis that artificial reefs are comparable to natural reefs in supporting fish communities, with fish density, biomass, and diversity all reaching comparable levels. The results in Hainan’s coastal waters are consistent with this, indicating that artificial reefs have effectively reshaped the local fish community structure.

5.2 The role of artificial reefs on fish diversity

Fish diversity is an important indicator for measuring ecological restoration. The increase in fish diversity is not only reflected in the increase in the number of species, but also in functional diversity. Fish in the reef area are more widely distributed in terms of feeding niches, behavioral types, body size, etc., and cover more levels in terms of ecological functions. This increase in functional diversity means that the ecosystem is more resilient and stable (Granneman and Steele, 2015; Yuan et al., 2022).

In the artificial reef area, the abundance of algae-eating fish (such as surgeonfish) provides clean substrate conditions for later transplanted coral seedlings, thereby improving the survival rate of corals. This reflects the linkage benefits of fish diversity restoration on overall ecological restoration. Of course, it should also be pointed out that the effect of artificial reefs on improving diversity may be limited by the surrounding species pool. If the regional species are extremely scarce, the species that the reef can attract are limited. Therefore, supplementing the construction of reefs with enhancement and release measures (such as the release of artificially cultivated fish fry) can further enrich the species composition (Cardoso et al., 2020; Medeiros et al., 2022).

5.3 Restoration effect of artificial reefs on fish biomass

Artificial reefs not only affect the distribution of fish species and numbers, but also have a positive effect on the restoration of total fish biomass. For example, studies have reported significant increases in fish numbers and total biomass within 1~2 years of deployment, with some systems showing a 68-fold increase in the biomass of certain functional groups within a few years (Cresson et al., 2019; Schilling et al., 2020). Artificial reefs can support fish communities with similar density, biomass, and diversity to natural reefs, especially when well designed and appropriately located. In addition, some restoration projects have observed rapid changes in fish communities within a few weeks, while other restoration projects have recorded sustained increases in fish abundance and biomass (Jiang et al., 2021; Nguyen et al., 2021).

5.4 Effects of artificial reefs on ecosystem stability and recovery

Ecosystem stability is an important indicator of whether restoration is in-depth, including the steady-state maintenance of environmental conditions and the ability of biological communities to resist disturbances. Artificial reefs have also played a positive role in promoting the restoration and stability of Hainan's offshore ecosystem. In terms of environmental homeostasis, the deployment of reefs helps to restore the physical and chemical balance of local ecology. In terms of community stability, artificial reefs promote the reconstruction of more complex and resilient biological networks, making the system more resistant to disturbances. In addition, as a "stable habitat", artificial reefs also provide a shelter from strong environmental fluctuations. When the outside world undergoes drastic changes (such as storms and red tides), the holes and structures of the reefs provide refuge for many organisms (Condal, 2024).

From the perspective of ecosystem function, the introduction of artificial reefs has gradually restored key processes in the original ecosystem, such as material circulation and energy flow. Ecological network analysis shows that the total circulation and ecological redundancy of the reef area system have increased compared with the area without reefs, approaching the level of natural healthy systems. This means that artificial reefs not only increase biomass and diversity, but also rebuild energy transfer relationships, so that the function of the ecosystem is gradually improved (Pan et al., 2022).

6 Typical Case Analysis of Artificial Reef Construction and Habitat Restoration in Hainan's Offshore

6.1 Case and analysis of coral reef ecological restoration

The Sanya Yalong Bay Coral Reef Ecological Restoration Demonstration Area launched an artificial reef combined with coral transplantation project in 2021. The area was originally a tourist diving area, but due to previous human activities and tropical storms, the coral cover has declined and fish resources have declined significantly. The restoration project has laid out about 300 concrete artificial reef modules around the damaged coral reefs, and artificially cultivated and transplanted tens of thousands of coral seedlings. One year after the project was implemented, it was found that the coral survival rate was high, the diversity and number of fish also increased, and the ecological function began to recover. This case fully illustrates that in the coral reef ecosystem, the combination of artificial reefs and coral transplantation can achieve a synergistic restoration effect. On the one hand, artificial reefs provide immediate shelter and habitat for fish, and quickly restore fish communities; on the other hand, artificial reefs, as a coral attachment matrix, accelerate the reconstruction of living coral communities, and the two promote each other (Huang et al., 2023).

6.2 Specific case evaluation of benthic habitat restoration effects

Artificial reefs in the Bohai Sea, such as the one in Xiangyun Bay, significantly increased the abundance of small benthic animals and provided additional substrate for bivalves and kelp, resulting in more benthic organisms near the reefs than in the surrounding areas. However, diversity indices do not always show significant differences with distance from the reefs. Reefs also change local hydrodynamics, resulting in finer sediments and enriched organic matter, which are key factors affecting small benthic animal communities. This case shows that artificial reefs can also play an important role in soft-bottom habitats that have been neglected in the past. By artificially increasing hard substrates and spatial structures, the soft mud seabed has the conditions to support rich benthic communities, thereby restoring benthic food webs and fishery resources (Yang et al., 2019).

6.3 Typical case study of economic fish population restoration

In 2018, the marine ranch demonstration area near Hainan in Qinzhou Bay, Beibu Gulf, Guangdong coast, began to build large-scale artificial reefs, focusing on the restoration of several target species with high economic value and severe resource decline, including golden drum (Pseudosciaena spp.), grouper and cuttlefish (Pan et al., 2022). After 5 years of construction, the resource volume of several target economic species has rebounded significantly, which is regarded as a typical successful case of marine ranch construction in my country. This case fully demonstrates the great potential of artificial reefs in restoring fishery economic species. When the scale of reef construction is large enough and management measures are in place, the once declining economic fish population can be restored and bring considerable fishery returns (Brochier et al., 2021).

7 Ecological Benefits and Management Recommendations

7.1 Improvement of ecosystem service functions of artificial reefs

The construction of artificial reefs is regarded as an important means to improve the service functions of marine ecosystems, and its ecological benefits are reflected in many aspects. Artificial reefs increase the heterogeneity of marine habitats by providing complex hard substrates and hidden spaces, which helps to protect and restore marine biodiversity (Bao et al., 2025). Artificial reefs promote the recovery and enhancement of ecological processes and functions. The attached biological communities on reefs (such as algae and filter-feeding invertebrates) can act as "biological filters", improve water quality and provide food sources for higher trophic levels (Taormina et al., 2024). Artificial reefs can also provide indirect benefits such as cultural and regulatory services. For example, they are often designed as attractions for recreational fishing and diving tourism, bringing entertainment and sightseeing value, while playing a certain coastal protection function by mitigating wave energy and stabilizing the seabed (Bracho-Villavicencio et al., 2023). It should be noted that the benefits of artificial reefs in providing ecosystem services often depend on factors such as the purpose of construction and the management model.

7.2 Sustainable utilization strategies for fishery resources

Artificial reefs have a dual role in promoting fishery resources: on the one hand, they provide new breeding habitats for fish, which is expected to increase the total amount of fishery resources; on the other hand, they also gather fish to improve fishing efficiency, which may lead to the risk of overfishing. Therefore, the sustainable utilization of artificial reef fishery benefits requires the support of scientific management strategies. First, the fishery management objectives of artificial reef construction should be clarified, such as whether it is used for resource recovery or catch improvement, so as to formulate corresponding supporting measures. Second, implement adaptive fishery management strategies: as the dynamics of fish communities in artificial reef areas change, management measures should be adjusted regularly, such as modifying the open fishing season and quota in a timely manner according to monitoring data to prevent resource decline. Third, encourage local fishermen to participate in the management of artificial reefs to achieve co-management and co-governance. Studies have shown that when stakeholders recognize the value of artificial reefs to fisheries and participate in management, they are more willing to support regulatory measures in reef areas, thereby improving management effectiveness (Paxton et al., 2020). In addition, sustainable utilization strategies also include promoting selective friendly fishing gear and fishing methods near artificial reefs to minimize the accidental capture of juvenile fish and non-target species.

7.3 Optimization suggestions for artificial reef construction

In order to maximize the ecological benefits of artificial reefs and reduce potential negative impacts, it is necessary to optimize the project site selection, structural design and post-management. In terms of site selection, it is recommended to select areas with suitable ecological and environmental conditions and no important natural habitats for artificial reefs. For example, sensitive ecosystems such as living coral reefs and mangroves should be avoided, and "desert" sea areas or degraded areas that originally lack hard substrates and have low biomass should be given priority, so as to avoid destroying the original habitat and play the ecological restoration role of reefs (Bao et al., 2025). When selecting a site, the water depth, bottom quality and hydrodynamic conditions should also be comprehensively evaluated.

In terms of design and construction, the principles of bionics and ecological engineering should be followed to enhance the ecological function of reefs. Specifically, it includes: using environmentally friendly and durable materials (such as pH-neutral cement, ceramics or degradable materials) to avoid secondary pollution, and providing rich spatial heterogeneity and shelters through structural optimization. In terms of management and maintenance, a long-term monitoring and maintenance mechanism should be established after artificial reefs are put into use. Regular monitoring can timely detect problems such as reef sinking, displacement or abnormal attached organisms, and maintenance or restocking can be carried out when necessary; monitoring changes in ecological indicators can also evaluate reef performance and provide a basis for future design improvements.

Acknowledgments

We would like to express my heartfelt thanks to all colleagues who provided guidance and assistance in this study.

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.

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