Widespread coral bleaching across subtropical and temperate Japan under record marine heat stress

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Between 2023 and 2024, extreme and prolonged sea surface temperature (SST) anomalies occurred globally, resulting in the hottest summer on record and the most severe coral bleaching event observed to date. Here, we evaluated SST and coral bleaching prevalence at two subtropic reef sites (Ishigaki and Okinawa Island), and six non-reef temperate sites extending to 35 °N along the Japanese coast influenced by the Kuroshio and Tsushima warm Currents. The Degree Heating Week (DHW) index reached record-high values at all sites, with a maximum of 19.10, and the maximum monthly mean (MMM) temperature was the highest in the past 43 years at all sites except Ishigaki. In addition to mass bleaching at subtropical sites, we report, for the first time, widespread mass coral bleaching at temperate sites, including the recent poleward shifting scleractinian corals. Extensive bleaching followed by coral mortality occurred in shallow subtropical reefs, whereas most corals at temperate sites recovered after bleaching. Although temperate regions have been proposed as potential coral refugia under climate change, the widespread bleaching observed in 2024 suggests that the pace of climate change may now exceeded the capacity of corals to shift their distributions, rendering their long-term stability increasing uncertain. Bleaching high-latitude temperate poleward-shifting corals DHW Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Since the onset of anthropogenic carbon dioxide (CO 2 ) emissions, sea surface temperatures (SSTs) have risen continuously, profoundly affecting marine ecosystems. The combination of long-term climate-drive warming and the strong El-Nino-Southern Oscillation (ENSO) event of 2023–2034 resulted in record-breaking global temperatures, marking the warmest months observed in the past century (Hoegh-Guldberg et al. 2023 ; Henley et al. 2024 ; Peng et al. 2025 ). This unprecedented ocean warming triggered global coral mass bleaching across both the Atlantic and Pacific Oceans in both hemispheres (Mies et al. 2025 ; Cantin et al. 2024 ; Goreau and Hayes 2024 ). In the southwestern Atlantic, bleaching severity reached 70–90% at some locations (Mies et al. 2025 ). Catastrophic bleaching and subsequent mortality were also reported at One tree Island (23 °S) on the Great Barrier Reef, where more than 80% of monitored corals bleached by April 2024, and Acropora mortality reached 95% (Byrne et al. 2023 ). Similarly, in southern Vietnam, Acroporidae corals, accounting for over 60% of total coral cover at Con Dao Island, experienced extreme bleaching (86–97%), followed by near-complete mortality by October 2024 (Tkachenko et al. 2025 ). These events suggest that continued climate warming may drive local extinctions, particularly in tropical regions (Jones and Cheung 2015 ). Concurrently, many species are shifting poleward to colder regions to escape rising ocean temperature (Pecl et al. 2023 ). Poleward expansion of tropical corals has been documented in several locations, including the Caribbean, Mediterranean Sea, Australia and Japan (Prechr and Aronson 2004; Yamano et al. 2011 ; Baird et al. 2013 ; Beger et al. 2014 ; Verges et al. 2014 ). Along the Japanese coast, strongly influenced by the warm Kuroshio and Tsushima Currents, several reef-building scleractinian corals are expanding northward at rates of approximately 14 km yr − 1 (Yamano et al. 2011 ). These higher-latitude regions have therefore been considered potential refugia for tropical and subtropical corals under climate change (Riegl and Piller 2003 ; Beger et al. 2014 ). Here, we provide evidence that the record-breaking ocean warming in 2024 caused severe bleaching from subtropic reefs (24 °N) to temperate regions up to 35 °N, including poleward shifting scleractinian corals. These observations indicating that the pace of climate change may now exceeded the capacity of corals to shift their distributions. Methods Eight study sites were selected, including two subtropical reef sites (Ishigaki and Okinawa Island) and six non-reef temperate sites (Table 1, Fig. 1). The temperate six sites, Tatsukushi, Kushimoto, Tagojima (Izu South), Numazu (Izu North), Tateyama, and Tsushima are influenced by the warm Kuroshio and Tsushima Currents (Fig. 1). To evaluate high-temperature events, daily SST data from 1982 to 2024 were obtained from open-access archives provided by the Japan Metrological Agency and National Oceanic and Atmospheric Administration (NOAA) 1/4° Daily Optimum Interpolation Sea Surface Temperature dataset (OISST; https://www.ncei.noaa.gov/products/optimum-interpolation-sst). Maximum monthly mean (MMM) temperature was calculated from daily SST records. Temporal changes in MMM temperature at each site were evaluated using regression analysis, and statistical significance was assessed using Pearson’s correlation in R (v.4.5.2; R Core R Team 2025). Degree Heating Weeks (DHW) were calculated as the cumulative positive anomaly between daily SST and the MMM climatology (1982-2024) over a 12-week period. Coral bleaching observations were conducted by snorkeling or scuba diving at all eight sites. Because a standardized bleaching monitoring protocol was not applied uniformly across sites, statistical comparison of bleaching prevalence was not performed. Nevertheless, all sites have been monitored long-term by the authors and are designated coral monitoring locations by the Ministry of Environment, Japan. At four of the sites (Tatsukushi, Kushimoto, Tagojima and Tateyama), four permanent 3m x 3 m quadrats have been surveyed annually during October-November. In Okinawa, in addition to long-term monitoring program conducted by the Okinawa Prefectural Government, the authors conducted surveys of coral coverage, bleaching and mortality using manta tow methods at Sesoko in September 2024 and February 2025. At Numazu, corals were tagged and regularly monitored for beaching status within a 30 m x 5 m area after bleaching was first detected. Detailed site-specific results will be reported elsewhere. Results and Discussion Degree Heating Weeks (DHW) DHW values began to increase in early July at the subtropical sites of Ishigaki and Okinawa (Figs. 2-4). By 1 August 2024, the DHW at Ishigaki (4.36, Table 1) had already exceed 4.0, a threshold commonly associated with the onset of coral bleaching (ref). By 11 August, DHW values exceed 4.0 at most sites, except Kushimoto and Tateyama (Fig. 2-4). Peak DHW values at Ishigaki (9.24) and Okinawa (8.67) were reached by mid-September, whereas maximum DHW values at all six temperate sites occurred by mid-October. The highest DHW was recorded in Tsushima (19.10) followed by Tagojima (18.73) and Numazu (17.24) (Fig. 4, Table 1). Long-term trends in MMM and DHW trend MMM temperatures, typically occurring in August but occasionally in July or September, increased significantly from 1982 to 2024 at all sites (Fig. 5, Supplement Table 1). The rate of MMM increase was higher at higher-latitude sites (approximately 0.3-0.4 °C per decade) than the subtropical sites of Ishigaki and Okinawa (0.18 and 0.19 °C per decade, respectively). In 2024, the highest MMM was recorded at seven of the eight sites; Ishigaki was the only exception, where the maximum MMM occurred in 2022. Notably, MMM values in 2024 at Tsushima (30.18 °C), Tagojima (29.23 °C) and Numazu (29.02 °C) were approximately 3 °C higher than their respective 1982 to 2024 averages. DHW values were highest in 2024 at all eight sites. While DHW values did not exceed 10 at any site prior to 2022, values reached 10.8 at Tagojima and 12.3 at Numazu in 2023 (Fig. 6). Coral bleaching Coral bleaching was observed at all surveyed subtropical and temperate sites except Tateyama. Notably, to our knowledge, Tshushima experienced mass coral bleaching attributed to thermal stress for the first time and bleaching at Tagojima and Numazu has occurred in consecutive years since 2023. At the subtropical reef sites of Ishigaki and Okinawa, coral bleaching began in shallow reef zones between mid-June and early July 2024. At Ishigaki, Shiraho reef crest and slope, bleached scleractinian corals including Acropora, Montipora, Pocillopora and Dipsastrea, as well as soft corals, were observed in September 2024 (Fig. 1, Table 1). Post-bleaching surveys around Ishigaki Island reported little reduction in coral cover (Ministry of the Environment, 2025), indicating substantial recovery. A severe bleaching event occurred previously in 2022, coinciding with the highest maximum monthly mean temperature (MMM = 30.7 °C) over the past 42 years (Fig. 5), during which approximately 63% of corals bleached and 15% died at Sekisei Lagoon, Ishigaki (Afzar et al. 2024). Lower mortality in 2024 may reflect the prior loss of heat-sensitive coral in 2022. In addition, two typhoons passed near Ishigaki in July and August 2024, whereas only one typhoon in September 2022, likely reducing thermal stress and light intensity and limiting mortality. In contrast, bleaching at Okinawa Island was limited in 2022 but became severe and widespread in 2024, particularly in shallow areas (<10 m depth). After initial bleaching in June, bleaching extent increased rapidly by August, in association with elevated DHW values (Table 1; Figs. 2, 3). At the Sesoko South reef, Okinawa, most scleractinian corals including Acropora, Porites, Montipora, Pocillopora and Dipsastrea, as well as soft corals, remained bleached for more than three months. By September, 49 % of the monitored area exhibited fully bleached, and the area that coral cover exceeds 20% subsequently declined from 47% to 3% by February 2025. Although coral cover had been gradually recovering after the extreme bleaching in 1998 (Loya et al. 2001) and moderate bleaching in 2016, the extreme thermal stress in 2024 caused a drastic decline in shallow reef coral cover. At non-reef temperate sites, bleaching was first observed from late August to September 2024. At Tatsukushi, where coral communities occur at depths of 3 to 10 m and reach 60 to 70% cover locally, bleaching began in late August and affected Acropora hyacinthus and Acropora solitaryensis (Table 1). These two species comprise approximately 48% of the benthic community (Denis et al. 2013) but they were not recorded in surveys conducted in the1930s, suggesting recent poleward range expansion (Yamano et al. 2011). Although previous bleaching reports from this site were primarily attributed to cold stress, sedimentation, or low salinity (Yamazaki et al. 2009), the present event likely to represents one of the most severe heat-stress induced mass bleaching at Tatsukushi. At Kushimoto, bleaching occurred from September to October 2024 and primarily affected A. solitaryensis , Cyphastrea serailia , Stylophora pistillata , tabular Acropora sp., massive Dipsastraea sp., and massive Porites sp. (Table 1, Fig. 1). In contrast, dominant species such as A. hyacinthus and Acropora muricata , exhibited little bleaching. In shallow inner-bay areas, bleaching prevalence exceeded 90% at some locations (Ministry of the Environment, 2025), although most corals subsequently recovered with minimal mortality. At Tsushima, a large number of the Dipsastraea corals (mainly Dipasatrea speciosa ) bleached, resulting in mortality and subsequent algal overgrown by November, representing the first recoded thermal stress-induced bleaching event in this site, to authors knowledge. At Tagojima, thermal stress induced coral bleaching was first reported by local divers in 2023, followed by mass bleaching in 2024, particularly affecting A. solitaryensis (Fig. 1, Table 1). Similarly, at Numazu, the northernmost site, paling and bleaching was first observed in 2023 by local divers, coinciding with high DHW (11.0) and MMM (28.2 °C) (Fig. 5, 6). In 2024, more sever mass bleaching affected most corals, including Acropora including A. cf. glauca, A. hyacinthus and A. solitaryensis (Fig. 1, Table 1), under the highest DHW (17.24) and MMM (29.0 °C) recorded since 1982 (Figs. 5, 6). Recently established coral species in Numazu (Morita et al. 2025) exhibited severe bleaching and partial mortality in late summer, although most colonies began to recover from October onward. In contrast, the native coral Acropora pruinosa exhibited low bleaching prevalence, with no bleaching at depths of 5 to 8 m and only mild paling at 2 m. Tateyama was the only site where no coral bleaching was observed, despite DHW values exceeding 12 (Figs. 1 ,2, 4). Although the 2024 DHW (12.9) was the highest in the past 40 years, MMM at Tateyama (27.86 °C) was the lowest among all sites (Figs. 5, 6). Experiments studies using corals ( A. pruinosa and A. glauca ) from Tateyama showed no bleaching at 28 °C (personal communication), suggesting that summer temperature in 2024 remained below the local bleaching threshold. Overall, extreme SSTs in 2024 caused mass bleaching and mortality in subtropical corals, and widespread bleaching in temperate sites along the Japanese coast, including poleward expanding corals such as A. hyacinthus, A. muricata, and A. solitaryensis . Although higher thermal tolerance has been proposed to facilitate the poleward expansion of tropical corals, our observation indicate that even poleward-expanding Acropora species could not avoid bleaching under extreme heat stress. In contrast, endemic temperate species exhibited unexpectedly high thermal tolerance, suggesting a broader thermal performance range than tropic corals. Despite extensive bleaching, most corals in temperate regions did not experience mortality. This likely reflect the shorter duration of SSTs exceeding the MMM (1.5-2 months in temperate regions versus >3 months in subtropical regions; Figs 3, 4). In addition, local environmental factors such as light intensity and zooplankton availability, which are known to influence bleaching severity and post-bleaching mortality (Grottoli et al. 2006; Anthony et al. 2009; Welle et al. 2017), may have contributed to the observed patterns. These results highlight the importance of local environmental conditions and suggest that DHW alone may not reliably predict bleaching severely and mortality in temperate regions. Differences in thermal resistance among local communities may also play an important role. Population genetic studies suggested the presence of cryptic coral species and indicate that recently expanding coral populations may represent distinct lineages from their tropical counterparts (Nakabayashi et al. 2019). Furthermore, both host haplotype diversity and symbionts (Symbiodiniacea) diversity in Pocillopora spp. decrease with latitude along the Japanese coast (Chong et al. 2025). These finding suggest that front-line coral lineages may have reduced capacity to tolerate heat stress. In conclusion, although temperate regions have been considered potential refugia for corals under climate change, the widespread bleaching observed in 2024 indicated that their long-term stability is uncertain. Declarations Conflict of interest On behalf of the authors, the corresponding authors states that there is no conflict of interest. Author Contribution All authors contributed to wrote, review and approve the manuscript. Acknowledgement We thank Jinza Mori and Asakura Kazuya for field support. This work was funded by Japan Science and Technology Agency (JST) Core Research for Evolutional Science and Technology (CREST) Grant Number JPMJCR23J2 to HK, HY and NY. Data Availability Data will be made available on request. References Afzar MS, Udo T, Ueno M, Nakamura T (2024) Mass coral bleaching and mortality associated with high sea surface temperatures in the summer of 2022 in Sekisei Lagoon, Okinawa, Japan. Galaxea, J Coral Reefs Stud 26:20-26 Anthony KRN, Hoogernboom MO, Maynard JA, Grottoli AG, Middlebrook R (2009) Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching. Function Ecol 23:539-550 Baird AH, Sommer B, Madin JS (2013) Pole-ward range expansion of Acropora spp. along the east coast of Australia. Coral Reefs 31:1063 https://doi.org/10.1007/s00338-012-0928-6 Beger M, Sommer B, Harrison PL, Smith SDA, Pandolfi JM (2014) Conserving potential coral reef refuges at high latitudes. Diversity Distrib 20:245-257 Byrne M, Waller A, Clements M, Kelly AS, Kingsford MI, Liu B, Reymond CE, Vila-Concejo A, Webb M, Whitton K, Foo SA (2023) Catastrophic bleaching in protected reefs of the Southern Great Barrier Reef. Limnol Oceanog Lett 10:340-348 https://doi.org/10.1002/lol2.10456 Cantin N, James N, Stella J (2024) Aerial survey of the 2024 mass coral bleaching event on the Great Barrier Reef. Australian Inst Mar Sci Chong F, Soong GY, Hakim AA, Burke C, De Palmas S, Gosser F, Hsiao WV, Kise H, Nishijima M, Iguchi A, Sommer B, Joyce D, Beger M, Reimer JD (2024) Subtropical specialists dominate a coral range expansion front. Coral Reefs https://doi.org/10.1007/s00338-024-02601-w Denis V, Mezaki T, Tanaka K, Kuo C-Y, De Palmas S, Keshavmurthy S, Allen Chen C (2013) Coverage, diversity, and functionality of a high-latitude coral community (Tatsukushi, Shikoku Island, Japan) PlosOne 14:e54330 https://doi.org/10.1371/journal.pone.0054330 Goreau TJF, Hayes RL (2024) 2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation change. Oxford Open Clim Change 4:kgae005 https://doi.org/10.1093/oxfclm/kgae005 Henley BJ, McGregor VH, King AD, Hoegh-Guldberg O, Arzey AK, Karoly DJ, Lough JM, DeCarlo TM, Linsley BK (2024) Highest ocean heat in four centuries places Great Barrier Reef in danger. Nature 632:320-330 https://doi.org/10.1038/s41586-024-07672-x Grottoli AG, Rodrigues LJ, Palardy JE (2006) Heterotrophic plasticity and resilience in bleached corals. Nature 440:1186-1189 https://doi.org/10.1038/nature04565 Hoegh-Guldberg O, Skirving W, Dove SG, Spady BL, Norrie A, Geiger EF, Liu G, De La Cour JL, Manzello DP (2023) Coral reefs in peril in a record-breaking year. Science 382:6676 Jones MC, Cheung WW (2015) Multi-model ensemble projections of climate change effects on global marine biodiversity. ICES J Mar Sci 72:741-752 https://doi.org/10.1093/icesjms/fsu172 Loya Y, Sakai K, Yamazato K, Nakano Y, Sambali H, van Woesik R (2001) Coral bleaching: the winner and the losers. Ecol Let 4:122-131 https://doi.org/10.1046/j.1461-0248.2001.00203.x Mies M, Destri G, Lacerda CHF, Carvalho JL, Ibanhez JY, Guth AZ, Liza AL et al. (2025) Coral bleaching and mortality across a 24° latitudinal range in the Southwestern Atlantic during the fourth global bleaching event. Coral Reefs https://doi.org/10.1007/s00338-025-02743-5 Ministry of the Environment, Japan (2025) The report of the 2024 Monitoring Sites 1000 Coral Reef Survey. 156p (in Japanese) https://www.biodic.go.jp/moni1000/findings/reports/ Morita M, Fujino Y, Nagamine T, Fukami H, Yokochi H, Nomura K, Asakura K, Kurihara H, Nakamura M, Yasuda N (2025) Rapid coral species turnover in one of the northernmost coral communities, Uchiura Bay, Shizuoka Prefecture, Japan under a changing climate. Galaxea, J Coral Reefs Stud 27:197-204 https://doi.org/10.3755/galaxea.G27-14 Nakabayashi A, Yamakita T, Nakamura T, Aizawa H, Kitano YF, Iguchi A, Yamano H, Nagai S, Agostini S, Teshima KM, Yasuda N (2019) The potential role of temperate Japanese regions as refugia for the coral Acropora hyacinthus in the face of climate change. Sci Rep 9:1892 https://doi.org/10.1038/s41598-018-38333-5 Peng Q, Xie S-P, Miyamoto A, Deser C, Zhang P, Luongo MT (2025) Strong 2023-2024 El Nino generatioed by ocean dynamics. Nat Geosci 18:471-478 https://doi.org/10.1038/s41561-025-01700-9 Pecl GT, Kelly R, Lucas C, van Putten I, Badhe R, Champion C, Chen I-C, Defeo O, Gaitan-Espitia JD, Evengard B, Fordham DA, Guo F, Henriques R, Henry S, Lenoir J, McGhie H, Mustonen T, Oliver S, Pettorelli N, Pinsky ML, Potts W, Santana-Garcon J, Sauer W, Stengaard A-S, Tingley MW, Verges A (2023) Climate-driven ‘species-on-the-move’ provide tangible anchors to engage the public on climate change. People Nature 5:1384-1402 https://doi.org/10.1002/pan3.10495 Precht WF, Aronson RB (2004) Climate flickers and range shift of reef corals. Front Ecol Environ 2:307–314 https://doi.org/10.2307/3868406 R Core Team (2025) R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna Riegl B, Piller WE (2003) Possible refugia for reefs in times of environmental stress. Int J Earth 92:520-531 https://doi.org/10.1007/s00531-003-0328-9 Tkachenko KS, Dung VV, Ha VT (2025) Ecological status and resilience of coral reefs in South-Central Vietnam (Khanh Hoa Province) in the third decade of the 21 st century. Region Stud Mar Sci 83:104074 https://doi.org/10.1016/j.rsma.2025.104074 Verges A, Steinberg PD, Hay ME, Poore AGB, Campbell AH, Ballesteros E, Heck Jr KL, Booth DJ, Coleman MA, Feary DA, Figueira W, Langlois T, Marsinelli EM, Mizerek T, Mumby PJ, Nakamura Y, Roughan M, van Sebille E, Gupta AS, Smale DA, Tomas F, Wernberg T, Wilson SK (2014) The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts. Proc R Soc B 281:20140846. http://dx.doi.org/10.1098/rspb.2014.0846 Welle PD, Small MJ, Doney SC, Azevedo IL (2017) Estimating the effect of multiple environmental stressors on coral bleaching and mortality. PLoS ONE 12(5): e0175018 https://doi. org/10.1371/journal.pone.0175018 Yamano H, Sugihara K, Nomura K (2011) Rapid poleward range expansion of tropical reef corals in response to rising sea surface temperatures. Geophys Res Lett 38:L04601 https://doi.org/10.1029/2010GL046474 Yamazaki A, Watanabe T, Sowa K, Nakachi S, Yamano H, Iwase F (2009) Reconstructing palaeoenvironments of temperate regions based on high latitude corals at Tatsukushi Bay in Japan. J J Coral Reef Soc 11:91-107 In Japanese. https://doi.org/10.3755/jcrs.11.91 Table Table 1 is available in the Supplementary Files section. Additional Declarations No competing interests reported. Supplementary Files SupplementalmaterialTableS1.docx floatimage7.jpeg Table 1 Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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20:17:41","extension":"html","order_by":19,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":89720,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/ddc4c7486b905b72d3c614b1.html"},{"id":100726129,"identity":"0a0dedf2-0101-447d-90d5-9800c2e8d601","added_by":"auto","created_at":"2026-01-20 20:21:17","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":694218,"visible":true,"origin":"","legend":"\u003cp\u003eThe eight sites included two subtropical coral reef sites (Ishigaki and Okinawa) and six temperate sites (Tatsukushi, Kushimoto, Tsushima, Tagojima, Numazu and Tateyama) influenced by the warm Kuroshio and Tsushima Currents. In 2024, mass coral bleaching caused by the thermal stress was observed at all sites except Tateyama. Corals, including the recently poleward shifting populations at temperate sites, exhibited bleaching; however, most subsequently recovered.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/dbff044110a0aafeccf07b2f.jpeg"},{"id":100725939,"identity":"f8219bbf-46c6-4180-96af-1827c49f2099","added_by":"auto","created_at":"2026-01-20 20:18:13","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":400610,"visible":true,"origin":"","legend":"\u003cp\u003eCumulative Degree heating weeks (DHW) from July 1 to September 24, 2024 along the Japanese coast.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/63768368f7ecfd11c0efd1f4.png"},{"id":100726178,"identity":"0964bc0a-da10-4375-868f-df2ecc1beba8","added_by":"auto","created_at":"2026-01-20 20:22:06","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":60586,"visible":true,"origin":"","legend":"\u003cp\u003eSea surface temperature (SST) at the eight sites in 2024. The line indicates the climatological (1982-2014) maximum monthly mean (MMM) temperature calculated from the daily SST data. Red shading denotes periods of high-temperature event.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/17f0952c7f97b717bae67481.png"},{"id":100725615,"identity":"a83e1224-3d74-4671-8423-156175a53f4f","added_by":"auto","created_at":"2026-01-20 20:14:44","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":126494,"visible":true,"origin":"","legend":"\u003cp\u003eDegree Heating Weeks (DHW) at the eight sites in 2024.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/333ca1dbdd2a203d54d514b9.jpeg"},{"id":100726197,"identity":"15140c4d-6028-4c43-bbb7-c9552477601b","added_by":"auto","created_at":"2026-01-20 20:22:21","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":59310,"visible":true,"origin":"","legend":"\u003cp\u003eMaximum monthly mean (MMM) temperatures at the eight study sites from 1982 to 2024. The maximum MMM value observed during this period is shown in red. All sites except Ishigaki, which peaked in 2022, showed their maximum MMM in 2024.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/abf1d2fedfed0564ccc1a018.png"},{"id":100725947,"identity":"a1939fee-f8f1-4c7e-ab82-fefa5f3d7005","added_by":"auto","created_at":"2026-01-20 20:18:42","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":54542,"visible":true,"origin":"","legend":"\u003cp\u003eDegree Heating Weeks (DHW) at the eight sites from 1982 to 2024. The maximum DHW value during this period is shown in red. All sites showed their maximum DHW in 2024.\u003c/p\u003e","description":"","filename":"floatimage6.png","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/490c16429d1d7f400406d702.png"},{"id":101943663,"identity":"0bcf6f96-5fdd-459f-b784-977a144357db","added_by":"auto","created_at":"2026-02-05 09:42:45","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1687179,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/7bf881ac-000d-4534-9435-7e3ab037b405.pdf"},{"id":100726327,"identity":"f61ea773-4a3e-4cb6-8cce-b93f69ace2ad","added_by":"auto","created_at":"2026-01-20 20:23:40","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":15730,"visible":true,"origin":"","legend":"","description":"","filename":"SupplementalmaterialTableS1.docx","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/82ca14eb1a0bdc5bf3a73870.docx"},{"id":100726443,"identity":"8371631f-8b5a-480a-9dc7-d25793ef8f46","added_by":"auto","created_at":"2026-01-20 20:24:51","extension":"jpeg","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":313810,"visible":true,"origin":"","legend":"\u003cp\u003eTable 1\u003c/p\u003e","description":"","filename":"floatimage7.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8487461/v1/781090d8f433255194da9a71.jpeg"}],"financialInterests":"No competing interests reported.","formattedTitle":"Widespread coral bleaching across subtropical and temperate Japan under record marine heat stress","fulltext":[{"header":"Introduction","content":"\u003cp\u003eSince the onset of anthropogenic carbon dioxide (CO\u003csub\u003e2\u003c/sub\u003e) emissions, sea surface temperatures (SSTs) have risen continuously, profoundly affecting marine ecosystems. The combination of long-term climate-drive warming and the strong El-Nino-Southern Oscillation (ENSO) event of 2023\u0026ndash;2034 resulted in record-breaking global temperatures, marking the warmest months observed in the past century (Hoegh-Guldberg et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Henley et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Peng et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). This unprecedented ocean warming triggered global coral mass bleaching across both the Atlantic and Pacific Oceans in both hemispheres (Mies et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2025\u003c/span\u003e; Cantin et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e2024\u003c/span\u003e; Goreau and Hayes \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2024\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the southwestern Atlantic, bleaching severity reached 70\u0026ndash;90% at some locations (Mies et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2025\u003c/span\u003e). Catastrophic bleaching and subsequent mortality were also reported at One tree Island (23 \u0026deg;S) on the Great Barrier Reef, where more than 80% of monitored corals bleached by April 2024, and \u003cem\u003eAcropora\u003c/em\u003e mortality reached 95% (Byrne et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Similarly, in southern Vietnam, Acroporidae corals, accounting for over 60% of total coral cover at Con Dao Island, experienced extreme bleaching (86\u0026ndash;97%), followed by near-complete mortality by October 2024 (Tkachenko et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2025\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThese events suggest that continued climate warming may drive local extinctions, particularly in tropical regions (Jones and Cheung \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Concurrently, many species are shifting poleward to colder regions to escape rising ocean temperature (Pecl et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). Poleward expansion of tropical corals has been documented in several locations, including the Caribbean, Mediterranean Sea, Australia and Japan (Prechr and Aronson 2004; Yamano et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Baird et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Beger et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Verges et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Along the Japanese coast, strongly influenced by the warm Kuroshio and Tsushima Currents, several reef-building scleractinian corals are expanding northward at rates of approximately 14 km yr\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Yamano et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). These higher-latitude regions have therefore been considered potential refugia for tropical and subtropical corals under climate change (Riegl and Piller \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Beger et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2014\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eHere, we provide evidence that the record-breaking ocean warming in 2024 caused severe bleaching from subtropic reefs (24 \u0026deg;N) to temperate regions up to 35 \u0026deg;N, including poleward shifting scleractinian corals. These observations indicating that the pace of climate change may now exceeded the capacity of corals to shift their distributions.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eEight study sites were selected, including two subtropical reef sites (Ishigaki and Okinawa Island) and six non-reef temperate sites (Table 1, Fig. 1). The temperate six sites, Tatsukushi, Kushimoto, Tagojima (Izu South), Numazu (Izu North), Tateyama, and Tsushima are influenced by the warm Kuroshio and Tsushima Currents (Fig. 1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo evaluate high-temperature events, daily SST data from 1982 to 2024 were obtained from open-access archives provided by the Japan Metrological Agency and National Oceanic and Atmospheric Administration (NOAA) 1/4\u0026deg; Daily Optimum Interpolation Sea Surface Temperature dataset (OISST; https://www.ncei.noaa.gov/products/optimum-interpolation-sst). Maximum monthly mean (MMM) temperature was calculated from daily SST records. Temporal changes in MMM temperature at each site were evaluated using regression analysis, and statistical significance was assessed using Pearson\u0026rsquo;s correlation in R (v.4.5.2; R Core R Team 2025). Degree Heating Weeks (DHW) were calculated as the cumulative positive anomaly between daily SST and the MMM climatology (1982-2024) over a 12-week period.\u003c/p\u003e\n\u003cp\u003eCoral bleaching observations were conducted by snorkeling or scuba diving at all eight sites. Because a standardized bleaching monitoring protocol was not applied uniformly across sites, statistical comparison of bleaching prevalence was not performed. Nevertheless, all sites have been monitored long-term by the authors and are designated coral monitoring locations by the Ministry of Environment, Japan. At four of the sites (Tatsukushi, Kushimoto, Tagojima and Tateyama), four permanent 3m x 3 m quadrats have been surveyed annually during October-November. In Okinawa, in addition to long-term monitoring program conducted by the Okinawa Prefectural Government, the authors conducted surveys of coral coverage, bleaching and mortality using manta tow methods at Sesoko in September 2024 and February 2025. At Numazu, corals were tagged and regularly monitored for beaching status within a 30 m x 5 m area after bleaching was first detected. Detailed site-specific results will be reported elsewhere.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e\u003cem\u003eDegree Heating Weeks (DHW)\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eDHW values began to increase in early July at the subtropical sites of Ishigaki and Okinawa (Figs. 2-4). By 1 August 2024, the DHW at Ishigaki (4.36, Table 1) had already exceed 4.0, a threshold commonly associated with the onset of coral bleaching (ref). By 11 August, DHW values exceed 4.0 at most sites, except Kushimoto and Tateyama (Fig. 2-4). Peak DHW values at Ishigaki (9.24) and Okinawa (8.67) were reached by mid-September, whereas maximum DHW values at all six temperate sites occurred by mid-October. The highest DHW was recorded in Tsushima (19.10) followed by Tagojima (18.73) and Numazu (17.24) (Fig. 4, Table 1).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eLong-term trends in MMM and DHW trend\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eMMM temperatures, typically occurring in August but occasionally in July or September, increased significantly from 1982 to 2024 at all sites (Fig. 5, Supplement Table 1). The rate of MMM increase was higher at higher-latitude sites (approximately 0.3-0.4 \u0026deg;C per decade) than the subtropical sites of Ishigaki and Okinawa (0.18 and 0.19 \u0026deg;C per decade, respectively). In 2024, the highest MMM was recorded at seven of the eight sites; Ishigaki was the only exception, where the maximum MMM occurred in 2022. Notably, MMM values in 2024 at Tsushima (30.18 \u0026deg;C), Tagojima (29.23 \u0026deg;C) and Numazu (29.02 \u0026deg;C) were approximately 3 \u0026deg;C higher than their respective 1982 to 2024 averages. DHW values were highest in 2024 at all eight sites. While DHW values did not exceed 10 at any site prior to 2022, values reached 10.8 at Tagojima and 12.3 at Numazu in 2023 (Fig. 6). \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eCoral bleaching\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eCoral bleaching was observed at\u0026nbsp;all surveyed subtropical and temperate sites except Tateyama. Notably, to our knowledge, Tshushima experienced mass coral bleaching attributed to thermal stress for the first time and bleaching at Tagojima and Numazu has occurred in consecutive years since 2023.\u003c/p\u003e\n\u003cp\u003eAt the subtropical reef sites of Ishigaki and Okinawa, coral bleaching began in shallow reef zones between mid-June and early July 2024. At Ishigaki, Shiraho reef crest and slope, bleached scleractinian corals including \u003cem\u003eAcropora,\u003c/em\u003e \u003cem\u003eMontipora, Pocillopora\u0026nbsp;\u003c/em\u003eand\u003cem\u003e\u0026nbsp;Dipsastrea,\u003c/em\u003e as well as soft corals, were observed in September 2024 (Fig. 1, Table 1). Post-bleaching surveys around Ishigaki Island reported little reduction in coral cover (Ministry of the Environment, 2025), indicating substantial recovery. A severe bleaching event occurred previously in 2022, coinciding with the highest maximum monthly mean temperature (MMM = 30.7 \u0026deg;C) over the past 42 years (Fig. 5), during which approximately 63% of corals bleached and 15% died at Sekisei Lagoon, Ishigaki (Afzar et al. 2024). Lower mortality in 2024 may reflect the prior loss of heat-sensitive coral in 2022. In addition, two typhoons passed near Ishigaki in July and August 2024, whereas only one typhoon in September 2022, likely reducing thermal stress and light intensity and limiting mortality.\u003c/p\u003e\n\u003cp\u003eIn contrast, bleaching at Okinawa Island was limited in 2022 but became severe and widespread in 2024, particularly in shallow areas (\u0026lt;10 m depth). After initial bleaching in June, bleaching extent increased rapidly by August, in association with elevated DHW values (Table 1; Figs. 2, 3). At the Sesoko South reef, Okinawa, most scleractinian corals including \u003cem\u003eAcropora, Porites, Montipora, Pocillopora\u0026nbsp;\u003c/em\u003eand\u003cem\u003e\u0026nbsp;Dipsastrea,\u003c/em\u003e as well as soft corals, remained bleached for more than three months. By September, 49 % of the monitored area exhibited fully bleached, and the area that coral cover exceeds 20% subsequently declined from 47% to 3% by February 2025. Although coral cover had been gradually recovering after the extreme bleaching in 1998 (Loya et al. 2001) and moderate bleaching in 2016, the extreme thermal stress in 2024 caused a drastic decline in shallow reef coral cover.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAt non-reef temperate sites, bleaching was first observed from late August to September 2024. At Tatsukushi, where coral communities occur at depths of 3 to 10 m and reach 60 to 70% cover locally, bleaching began in late August and affected \u003cem\u003eAcropora hyacinthus\u003c/em\u003e and \u003cem\u003eAcropora solitaryensis\u0026nbsp;\u003c/em\u003e(Table 1). These two species comprise approximately 48% of the benthic community (Denis et al. 2013) but they were not recorded in surveys conducted in the1930s, suggesting recent poleward range expansion (Yamano et al. 2011). Although previous bleaching reports from this site were primarily attributed to cold stress, sedimentation, or low salinity (Yamazaki et al. 2009), the present event likely to represents one of the most severe heat-stress induced mass bleaching at Tatsukushi.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAt Kushimoto, bleaching occurred from September to October 2024 and primarily affected \u003cem\u003eA. solitaryensis\u003c/em\u003e, \u003cem\u003eCyphastrea serailia\u003c/em\u003e, \u003cem\u003eStylophora pistillata\u003c/em\u003e, tabular \u003cem\u003eAcropora\u003c/em\u003e sp., massive \u003cem\u003eDipsastraea\u003c/em\u003e sp., and massive \u003cem\u003ePorites\u003c/em\u003e sp. (Table 1, Fig. 1). In contrast, dominant species such as \u003cem\u003eA. hyacinthus\u003c/em\u003e and \u003cem\u003eAcropora muricata\u003c/em\u003e, exhibited little bleaching. In shallow inner-bay areas, bleaching prevalence exceeded 90% at some locations (Ministry of the Environment, 2025), although most corals subsequently recovered with minimal mortality.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAt Tsushima, a large number of the \u003cem\u003eDipsastraea\u003c/em\u003e corals (mainly \u003cem\u003eDipasatrea speciosa\u003c/em\u003e) bleached, resulting in mortality and subsequent algal overgrown by November, representing the first recoded thermal stress-induced bleaching event in this site, to authors knowledge.\u003c/p\u003e\n\u003cp\u003eAt Tagojima, thermal stress induced coral bleaching was first reported by local divers in 2023, followed by mass bleaching in 2024, particularly affecting \u003cem\u003eA. solitaryensis\u003c/em\u003e (Fig. 1, Table 1). Similarly, at Numazu, the northernmost site, paling and bleaching was first observed in 2023 by local divers, coinciding with high DHW (11.0) and MMM (28.2 \u0026deg;C) (Fig. 5, 6). In 2024, more sever mass bleaching affected most corals, including \u003cem\u003eAcropora\u0026nbsp;\u003c/em\u003eincluding \u003cem\u003eA. cf. glauca, A. hyacinthus\u0026nbsp;\u003c/em\u003eand\u003cem\u003e\u0026nbsp;A. solitaryensis\u003c/em\u003e (Fig. 1, Table 1), under the highest DHW (17.24) and MMM (29.0 \u0026deg;C) recorded since 1982 (Figs. 5, 6). Recently established coral species in Numazu (Morita et al. 2025) exhibited severe bleaching and partial mortality in late summer, although most colonies began to recover from October onward. In contrast, the native coral \u003cem\u003eAcropora pruinosa\u003c/em\u003e exhibited low bleaching prevalence, with no bleaching at depths of 5 to 8 m and only mild paling at\u003cem\u003e\u0026nbsp;\u003c/em\u003e2 m.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;Tateyama was the only site where no coral bleaching was observed, despite DHW values exceeding 12 (Figs. 1 ,2, 4). Although the 2024 DHW (12.9) was the highest in the past 40 years, MMM at Tateyama (27.86 \u0026deg;C) was the lowest among all sites (Figs. 5, 6). Experiments studies using corals (\u003cem\u003eA. pruinosa and A. glauca\u003c/em\u003e) from Tateyama showed no bleaching at 28 \u0026deg;C (personal communication), suggesting that summer temperature in 2024 remained below the local bleaching threshold. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOverall, extreme SSTs in 2024 caused mass bleaching and mortality in subtropical corals, and widespread bleaching in temperate sites along the Japanese coast, including poleward expanding corals such as \u003cem\u003eA. hyacinthus, A. muricata,\u0026nbsp;\u003c/em\u003eand\u003cem\u003e\u0026nbsp;A. solitaryensis\u003c/em\u003e. Although higher thermal tolerance has been proposed to facilitate the poleward expansion of tropical corals, our observation indicate that even poleward-expanding \u003cem\u003eAcropora\u003c/em\u003e species could not avoid bleaching under extreme heat stress. In contrast, endemic temperate species exhibited unexpectedly high thermal tolerance, suggesting a broader thermal performance range than tropic corals.\u003c/p\u003e\n\u003cp\u003eDespite extensive bleaching, most corals in temperate regions did not experience mortality. This likely reflect the shorter duration of SSTs exceeding the MMM (1.5-2 months in temperate regions versus \u0026gt;3 months in subtropical regions; Figs 3, 4). In addition, local environmental factors such as light intensity and zooplankton availability, which are known to influence bleaching severity and post-bleaching mortality (Grottoli et al. 2006; Anthony et al. 2009; Welle et al. 2017), may have contributed to the observed patterns. These results highlight the importance of local environmental conditions and suggest that DHW alone may not reliably predict bleaching severely and mortality in temperate regions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDifferences in thermal resistance among local communities may also play an important role. Population genetic studies suggested the presence of cryptic coral species and indicate that recently expanding coral populations may represent distinct lineages from their tropical counterparts (Nakabayashi et al. 2019). Furthermore, both host haplotype diversity and symbionts (Symbiodiniacea) diversity in \u003cem\u003ePocillopora\u0026nbsp;\u003c/em\u003espp. decrease with latitude along the Japanese coast (Chong et al. 2025). These finding suggest that front-line coral lineages may have reduced capacity to tolerate heat stress.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn conclusion, although temperate regions have been considered potential refugia for corals under climate change, the widespread bleaching observed in 2024 indicated that their long-term stability is uncertain.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eConflict of interest\u003c/h2\u003e \u003cp\u003eOn behalf of the authors, the corresponding authors states that there is no conflict of interest.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eAll authors contributed to wrote, review and approve the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgement\u003c/h2\u003e\u003cp\u003eWe thank Jinza Mori and Asakura Kazuya for field support. This work was funded by Japan Science and Technology Agency (JST) Core Research for Evolutional Science and Technology (CREST) Grant Number JPMJCR23J2 to HK, HY and NY.\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData will be made available on request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAfzar MS, Udo T, Ueno M, Nakamura T (2024) Mass coral bleaching and mortality associated with high sea surface temperatures in the summer of 2022 in Sekisei Lagoon, Okinawa, Japan. Galaxea, J Coral Reefs Stud 26:20-26\u003c/li\u003e\n\u003cli\u003eAnthony KRN, Hoogernboom MO, Maynard JA, Grottoli AG, Middlebrook R (2009) Energetics approach to predicting mortality risk from environmental stress: a case study of coral bleaching. Function Ecol 23:539-550\u003c/li\u003e\n\u003cli\u003eBaird AH, Sommer B, Madin JS (2013) Pole-ward range expansion of \u003cem\u003eAcropora\u003c/em\u003e spp. along the east coast of Australia. Coral Reefs 31:1063 https://doi.org/10.1007/s00338-012-0928-6\u003c/li\u003e\n\u003cli\u003eBeger M, Sommer B, Harrison PL, Smith SDA, Pandolfi JM (2014) Conserving potential coral reef refuges at high latitudes. Diversity Distrib 20:245-257\u003c/li\u003e\n\u003cli\u003eByrne M, Waller A, Clements M, Kelly AS, Kingsford MI, Liu B, Reymond CE, Vila-Concejo A, Webb M, Whitton K, Foo SA (2023) Catastrophic bleaching in protected reefs of the Southern Great Barrier Reef. Limnol Oceanog Lett 10:340-348 https://doi.org/10.1002/lol2.10456\u003c/li\u003e\n\u003cli\u003eCantin N, James N, Stella J (2024) Aerial survey of the 2024 mass coral bleaching event on the Great Barrier Reef. Australian Inst Mar Sci \u003c/li\u003e\n\u003cli\u003eChong F, Soong GY, Hakim AA, Burke C, De Palmas S, Gosser F, Hsiao WV, Kise H, Nishijima M, Iguchi A, Sommer B, Joyce D, Beger M, Reimer JD (2024) Subtropical specialists dominate a coral range expansion front. Coral Reefs https://doi.org/10.1007/s00338-024-02601-w \u003c/li\u003e\n\u003cli\u003eDenis V, Mezaki T, Tanaka K, Kuo C-Y, De Palmas S, Keshavmurthy S, Allen Chen C (2013) Coverage, diversity, and functionality of a high-latitude coral community (Tatsukushi, Shikoku Island, Japan) PlosOne 14:e54330 https://doi.org/10.1371/journal.pone.0054330\u003c/li\u003e\n\u003cli\u003eGoreau TJF, Hayes RL (2024) 2023 Record marine heat waves: coral reef bleaching HotSpot maps reveal global sea surface temperature extremes, coral mortality, and ocean circulation change. Oxford Open Clim Change 4:kgae005 https://doi.org/10.1093/oxfclm/kgae005\u003c/li\u003e\n\u003cli\u003eHenley BJ, McGregor VH, King AD, Hoegh-Guldberg O, Arzey AK, Karoly DJ, Lough JM, DeCarlo TM, Linsley BK (2024) Highest ocean heat in four centuries places Great Barrier Reef in danger. Nature 632:320-330 https://doi.org/10.1038/s41586-024-07672-x \u003c/li\u003e\n\u003cli\u003eGrottoli AG, Rodrigues LJ, Palardy JE (2006) Heterotrophic plasticity and resilience in bleached corals. Nature 440:1186-1189 https://doi.org/10.1038/nature04565\u003c/li\u003e\n\u003cli\u003eHoegh-Guldberg O, Skirving W, Dove SG, Spady BL, Norrie A, Geiger EF, Liu G, De La Cour JL, Manzello DP (2023) Coral reefs in peril in a record-breaking year. Science 382:6676\u003c/li\u003e\n\u003cli\u003eJones MC, Cheung WW (2015) Multi-model ensemble projections of climate change effects on global marine biodiversity. ICES J Mar Sci 72:741-752 https://doi.org/10.1093/icesjms/fsu172\u003c/li\u003e\n\u003cli\u003eLoya Y, Sakai K, Yamazato K, Nakano Y, Sambali H, van Woesik R (2001) Coral bleaching: the winner and the losers. Ecol Let 4:122-131 https://doi.org/10.1046/j.1461-0248.2001.00203.x\u003c/li\u003e\n\u003cli\u003eMies M, Destri G, Lacerda CHF, Carvalho JL, Ibanhez JY, Guth AZ, Liza AL et al. (2025) Coral bleaching and mortality across a 24\u0026deg; latitudinal range in the Southwestern Atlantic during the fourth global bleaching event. Coral Reefs https://doi.org/10.1007/s00338-025-02743-5 \u003c/li\u003e\n\u003cli\u003eMinistry of the Environment, Japan (2025) The report of the 2024 Monitoring Sites 1000 Coral Reef Survey. 156p (in Japanese) https://www.biodic.go.jp/moni1000/findings/reports/\u003c/li\u003e\n\u003cli\u003eMorita M, Fujino Y, Nagamine T, Fukami H, Yokochi H, Nomura K, Asakura K, Kurihara H, Nakamura M, Yasuda N (2025) Rapid coral species turnover in one of the northernmost coral communities, Uchiura Bay, Shizuoka Prefecture, Japan under a changing climate. Galaxea, J Coral Reefs Stud 27:197-204 https://doi.org/10.3755/galaxea.G27-14\u003c/li\u003e\n\u003cli\u003eNakabayashi A, Yamakita T, Nakamura T, Aizawa H, Kitano YF, Iguchi A, Yamano H, Nagai S, Agostini S, Teshima KM, Yasuda N (2019) The potential role of temperate Japanese regions as refugia for the coral Acropora hyacinthus in the face of climate change. Sci Rep 9:1892 https://doi.org/10.1038/s41598-018-38333-5\u003c/li\u003e\n\u003cli\u003ePeng Q, Xie S-P, Miyamoto A, Deser C, Zhang P, Luongo MT (2025) Strong 2023-2024 El Nino generatioed by ocean dynamics. Nat Geosci 18:471-478 https://doi.org/10.1038/s41561-025-01700-9\u003c/li\u003e\n\u003cli\u003ePecl GT, Kelly R, Lucas C, van Putten I, Badhe R, Champion C, Chen I-C, Defeo O, Gaitan-Espitia JD, Evengard B, Fordham DA, Guo F, Henriques R, Henry S, Lenoir J, McGhie H, Mustonen T, Oliver S, Pettorelli N, Pinsky ML, Potts W, Santana-Garcon J, Sauer W, Stengaard A-S, Tingley MW, Verges A (2023) Climate-driven \u0026lsquo;species-on-the-move\u0026rsquo; provide tangible anchors to engage the public on climate change. People Nature 5:1384-1402 https://doi.org/10.1002/pan3.10495 \u003c/li\u003e\n\u003cli\u003ePrecht WF, Aronson RB (2004) Climate flickers and range shift of reef corals. Front Ecol Environ 2:307\u0026ndash;314 https://doi.org/10.2307/3868406\u003c/li\u003e\n\u003cli\u003eR Core Team (2025) R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna\u003c/li\u003e\n\u003cli\u003eRiegl B, Piller WE (2003) Possible refugia for reefs in times of environmental stress. Int J Earth 92:520-531 https://doi.org/10.1007/s00531-003-0328-9 \u003c/li\u003e\n\u003cli\u003eTkachenko KS, Dung VV, Ha VT (2025) Ecological status and resilience of coral reefs in South-Central Vietnam (Khanh Hoa Province) in the third decade of the 21\u003csup\u003est\u003c/sup\u003e century. Region Stud Mar Sci 83:104074 https://doi.org/10.1016/j.rsma.2025.104074\u003c/li\u003e\n\u003cli\u003eVerges A, Steinberg PD, Hay ME, Poore AGB, Campbell AH, Ballesteros E, Heck Jr KL, Booth DJ, Coleman MA, Feary DA, Figueira W, Langlois T, Marsinelli EM, Mizerek T, Mumby PJ, Nakamura Y, Roughan M, van Sebille E, Gupta AS, Smale DA, Tomas F, Wernberg T, Wilson SK (2014) The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts. Proc R Soc B 281:20140846. http://dx.doi.org/10.1098/rspb.2014.0846\u003c/li\u003e\n\u003cli\u003eWelle PD, Small MJ, Doney SC, Azevedo IL (2017) Estimating the effect of multiple environmental stressors on coral bleaching and mortality. PLoS ONE 12(5): e0175018 https://doi. org/10.1371/journal.pone.0175018 \u003c/li\u003e\n\u003cli\u003eYamano H, Sugihara K, Nomura K (2011) Rapid poleward range expansion of tropical reef corals in response to rising sea surface temperatures. Geophys Res Lett 38:L04601 \u003cu\u003ehttps://doi.org/10.1029/2010GL046474\u003c/u\u003e\u003c/li\u003e\n\u003cli\u003eYamazaki A, Watanabe T, Sowa K, Nakachi S, Yamano H, Iwase F (2009) Reconstructing palaeoenvironments of temperate regions based on high latitude corals at Tatsukushi Bay in Japan. J J Coral Reef Soc 11:91-107 In Japanese. https://doi.org/10.3755/jcrs.11.91\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Table","content":"\u003cp\u003eTable 1 is available in the Supplementary Files section.\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Bleaching, high-latitude, temperate, poleward-shifting corals, DHW","lastPublishedDoi":"10.21203/rs.3.rs-8487461/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8487461/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe frequency and intensity of mass coral bleaching events are increasing due to rising seawater temperatures associated with climate change and the intensification of marine heatwaves. Between 2023 and 2024, extreme and prolonged sea surface temperature (SST) anomalies occurred globally, resulting in the hottest summer on record and the most severe coral bleaching event observed to date. Here, we evaluated SST and coral bleaching prevalence at two subtropic reef sites (Ishigaki and Okinawa Island), and six non-reef temperate sites extending to 35 \u0026deg;N along the Japanese coast influenced by the Kuroshio and Tsushima warm Currents. The Degree Heating Week (DHW) index reached record-high values at all sites, with a maximum of 19.10, and the maximum monthly mean (MMM) temperature was the highest in the past 43 years at all sites except Ishigaki. In addition to mass bleaching at subtropical sites, we report, for the first time, widespread mass coral bleaching at temperate sites, including the recent poleward shifting scleractinian corals. Extensive bleaching followed by coral mortality occurred in shallow subtropical reefs, whereas most corals at temperate sites recovered after bleaching. Although temperate regions have been proposed as potential coral refugia under climate change, the widespread bleaching observed in 2024 suggests that the pace of climate change may now exceeded the capacity of corals to shift their distributions, rendering their long-term stability increasing uncertain.\u003c/p\u003e","manuscriptTitle":"Widespread coral bleaching across subtropical and temperate Japan under record marine heat stress","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-01-20 17:30:07","doi":"10.21203/rs.3.rs-8487461/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fffab66e-9d2b-4d93-a15a-3d98aba1c538","owner":[],"postedDate":"January 20th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2026-02-05T02:25:23+00:00","versionOfRecord":[],"versionCreatedAt":"2026-01-20 17:30:07","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-8487461","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8487461","identity":"rs-8487461","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}