International Journal of Marine Science, 2025, Vol.15, No.3, 118-129 http://www.aquapublisher.com/index.php/ijms 123 4.3 Deficiencies and demands of current monitoring research Although there has been a certain accumulation of research on the diversity of coral reefs in Hainan Island, traditional monitoring is still insufficient, mainly reflected in the following aspects: the monitoring coverage is limited, and previous investigations have mostly focused on protected areas or tourist dive sites with less man-made interference (such as West Island, Fenjiezhou Island, etc.), while there is less attention to some remote waters (such as the Qiongxi Coast, near Linggao) and severely damaged shore reefs. This leads to a lack of comprehensive understanding of the spatial distribution pattern of fish diversity in coral reefs in Hainan Island. The monitoring frequency is not high enough, and coral reef fish communities may change significantly due to seasons, years and even disaster events (such as typhoons, red tides), but most studies are one-time or short-term surveys, lacking long-term continuous monitoring data and cannot reveal community succession and species dynamics. There is insufficient monitoring of key species, lack of systematic population assessment and on-site monitoring, and only sporadic records, so the effectiveness of protection measures cannot be evaluated. The methods and methods are relatively single, mainly relying on divers' visual or catch statistics, and the data accuracy is limited and it is difficult to detect early juvenile fish (Chen et al., 2024). There is room for improvement in the current research on the diversity of coral reefs in Hainan in terms of breadth and depth. Management departments and scientific researchers urgently need to introduce new technical means to achieve more comprehensive, long-term and fine monitoring of coral reef fish. This is an opportunity for eDNA technology to show its strengths - by deploying conventional water eDNA sampling programs in typical coral reefs in Hainan, we are expected to obtain high-resolution data on fish diversity across regions and seasons to fill the gap in traditional monitoring. 5 The Impact and Response Strategies of Environmental Characteristics of Hainan Island Coral Reefs on eDNA Application 5.1 Effect of tropical high-temperature environment on eDNA degradation rate The coral reefs in Hainan Island are tropical marine environments, with high water temperatures per year (usually in the range of 20 ℃~30 °C), and high temperatures will significantly accelerate the degradation of eDNA. Studies have shown that the higher the water temperature, the shorter the eDNA retention time in the water body, and the degradation rate increases exponentially. For example, McCartin et al. (2022) experiments found that marine eDNA can maintain detectable concentrations for more than two weeks under low temperature (≤10 ℃) conditions, and can be completely degraded at high temperatures (water temperature ≥20 ℃). This strong control effect of temperature on eDNA stability is consistent with the temperature dependence of the biochemical reaction rate: high temperature will accelerate the activity of degraded enzymes such as DNA enzymes and the chemical degradation of DNA, making eDNA molecules fragmented and degraded faster (McCartin et al., 2022). In tropical coral reef areas, strong solar radiation and ultraviolet rays may also accelerate the photodegradation of eDNA in surface water bodies, further shortening the half-life of eDNA. Therefore, compared with temperate or cold water environments, eDNA signals in the high-temperature sea areas of Hainan Island are more transient and fleeting. If not collected in time, the target DNA fragments may have been degraded and difficult to detect. 5.2 Characteristics of spatial distribution of eDNA under complex hydrodynamic conditions The hydrodynamic processes of coral reef environments are complex and changeable, including tidal fluctuations, waves and coastal currents, which significantly affect the spatial distribution and migration of eDNA. The review of Jo et al. (2019) pointed out that the horizontal diffusion distances of eDNA in different water bodies vary greatly: in still water waters, eDNA can usually only diffuse horizontally to several several meters to dozens of meters, while in open ocean waters, it can reach hundreds of meters or even kilometers, depending on the local water flow condition. In coastal environments like coral reefs, eDNA may be carried by water flow away from the original release site. There was field research to collect water samples from different distances along the coast of tropical islands and reefs, and found that the composition of eDNA species near the reef has been significantly different from that of hundreds of meters away from the reef, showing that the eDNA signal has spatial limitations: the eDNA of benthic reef fish is mainly distributed in waters close to coral reefs, while more species widely distributed in the ocean are detected far away from the reefs (Figure 1) (Jaquier et al., 2024). In this study, there
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