Issue 26, May 2025

   Summary

• Recent SSTs across WA have been among the highest on record relative to the time of year.
• Abnormally warm SSTs across WA are set to continue through winter. 
• Heat stress is no longer accumulating across the northwest and recent SSTs are cooler to close to average for this time of year in the northwest, while warmer along the shelf south of the Southern Pilbara. Warmer than normal SST anomalies through winter are not likely to result in further coral bleaching.
• Heat stress around central WA remains high, but outlooks indicate heat stress reduction in the coming weeks. There is little risk for new coral bleaching and ongoing observations are required to monitor impact and recovery.
• Heat stress is high across the southwest and monitoring products indicate heat stress is still accumulating off the southwest coast, and outlooks show a reduction in anomalies over the next month.
• Widespread coral bleaching has been observed across most regions in WA. The severity and extent of recent heat stress across WA coral reefs is unprecedented. The impacts from this event are still unfolding and it will be months before we can truly understand their full extent. 
• Current Heat Stress (NOAA Coral Reef Watch Daily 5km Degree Heating Weeks):

Indian Ocean Territories (0 - 2 DHW); Oceanic reefs (3 - 9 DHW); Kimberley (3 - 7 DHW); Pilbara (2 - 14 DHW); Ningaloo (6 - 17 DHW); Shark Bay outer (5 - 20+ DHW); Houtman Abrolhos Islands (13 - 20 DHW); Marmion to Rottnest Island (9 - 20 DHW) 

Firstly, a heartfelt thank you to all who have contributed reports, observations, images, and video footage of coral bleaching across our states vast network of coral reefs.

Many have shared their long histories with a particular reef in WA, others have shared the rapid familiarisation and fondness found for a newly discovered site. All have expressed the growing apprehension felt at witnessing, for the first time for many, the onset of coral bleaching, and the grief and fear of an uncertain future for our beloved reefs, not for themselves but for future generations. These stories have resonated deeply with us, and we commiserate with you all through this unprecedented bleaching.

Hope can be found with the valuable and significant contributions of data from across multiple reefs and regions of WA. The coverage has been extensive, a collection of observations and images spanning a spatial scale never before documented in WA. Every report contributes to providing important context around this event and to understanding the progression and impacts of bleaching across regions. Data will be collated into regional summaries and shared among state and federal agencies and ultimately will help to inform current and future reef research, monitoring and management. 

Work is still ongoing, and at this stage, we cannot yet provide a full picture of this event, and it will take time to understand its full impact on WA’s reefs. We will continue to share the recent reports of what has been observed across our reefs so far.

The WA Coral Bleaching Group monitors and shares information on bleaching observations and temperature outlooks, as well as facilitate collaboration in the coordination and collection of bleaching observations. The group uses heat stress outlooks to assess coral bleaching risks for WA's coral reefs during the spring - autumn months.

Recording bleaching observations is critical for understanding and documenting the impacts of heat stress events on coral reefs and provide valuable information that contributes to global summaries and assessments of bleaching events. To facilitate this process, observations can be recorded via the mobile app or by email (n.ryan@aims.gov.au). More information about coral bleaching is available at https://www.aims.gov.au/coral-bleaching.

Recent Temperatures

The Bureau of Meteorology (BoM) provides regular updated Sea Temperature Analysis based on either in-water or satellite observations, or a combination of both.

Global ocean sea surface temperatures (SSTs) continue to remain high above average, and for the Australian region, SST anomalies were again the highest on record (1961-1990 baseline climatology) for the month of February, and second highest for March and April. 

SST anomalies were among the top five warmest on record for February, March and April in both the Northwest and Southwest regions.

SST anomaly analysis for February shows anomalies around 0.4 - 1.2°C warmer than normal (1991-2020 monthly average) along the Kimberley coast and close to average temperatures further offshore. SST anomalies were higher in March, 0.8 - 2°C warmer than normal inshore and extending across the northwest shelf but were close to average in April. In February, the highest temperature anomalies continued across central WA, with anomalies 2 - 3°C warmer than normal extending offshore from Karratha in the Pilbara to Shark Bay in the Gascoyne region. In March, 2 - 3°C anomalies continued around Ningaloo and the southern Pilbara, and 1.2- 2°C warmer than normal temperatures extended from the Pilbara to Cape Leeuwin through March and April.

Above: BoM Sea Surface Temperature Anomaly Monthly analysis product for Australia for February 2025 with a scale ranging from -4.0°C (dark cyan) to +4.0°C (strong red). Climatology baseline: 1991 to 2020.

Above: BoM Sea Surface Temperature Anomaly Monthly analysis product for Australia for March 2025 with a scale ranging from -4.0°C (dark cyan) to +4.0°C (strong red). Climatology baseline: 1991 to 2020.

Above: BoM Sea Surface Temperature Anomaly Monthly analysis product for Australia for April 2025 with a scale ranging from -4.0°C (dark cyan) to +4.0°C (strong red). Climatology baseline: 1991 to 2020.

Recent SST analysis (19th - 25th May 2025) show SSTs in the northwest remain close to average for this time of year. Anomalies of 0.4 - 2°C (warmer than the 1991-2020 weekly average) have continued across the southern Pilbara and Gascoyne regions and extending to the south, with areas up to 2 - 3°C warmer further offshore. 

Above: BoM Sea Surface Temperature Anomaly Weekly analysis product for Australia for 19 March to 25 March 2025 with a scale ranging from -4.0°C (dark cyan) to +4.0°C (strong red). Climatology baseline: 1991 to 2020.

Recent Heat Stress

NOAA’s Coral Reef Watch (CRW) daily global 5km satellite Bleaching Alert Area product provides regular bleaching alert updates for Regional Virtual Stations (RVS), using data from all 5km grid cells within each station's jurisdiction. Current alerts extend to Alert Level 2 only, and when stations reach Alert Level 2, it signals the need for closer examination of detailed heat stress product maps. For more information see: NOAA Coral Reef Watch Satellite Virtual Stations Time Series Graph Description

The most recent (27th May 2025) heat stress updates show low accumulated heat stress across the Indian Ocean Territories and severe heat stress extending from southern Pilbara south around the Capes to Albany. Current alerts are:

Bleaching Watch:

Christmas Island, Cocos (Keeling) Island, Exmouth Gulf, Northern Ningaloo.

Alert Level 2:

Central and Southern Ningaloo, Shark Bay, Houtman Abrolhos, Marmion and Rottnest Island.

The Daily Degree Heating Week (DHW) product provides a measure of accumulated heat stress and likelihood of bleaching, based on CRW's Hotspot over the past 12 weeks. When DHW values reach 4 DHW (Alert Level 1), there is a risk of coral bleaching and when the DHW values reach 8 DHW (Alert Level 2), there is a risk of coral bleaching and mortality among the most vulnerable coral groups and when the DHW values exceed 12 DHW (Alert Level 3), there is a risk of multi-species mortality.

DHW values for 27th May show around 3- 5 DHW for Ashmore and Scott reefs, around 5-9 DHW for Rowley Shoals, and 3- 7 DHW for the Kimberley region. Across the central parts of WA, heat stress remains high, and in some areas, SST's greater than the highest mean temperature for that location. Across the northern Pilbara, DHW values are 3-6, and 5-14 DHW for the southern Pilbara. Across the Ningaloo region, DHW ranges from 6-17 DHW, with 18+ DHW further offshore. Further south, Shark Bay values were 5-20+ DHW, 13-20 at the Houtman Abrolhos Islands and 9-20 DHW around Marmion and Rottnest Island. Heat stress remains low, 0 - 2 DHW across the Indian Ocean Territories.

Above: NOAA Coral Reef Watch Daily 5km satellite Coral Bleaching Heat Stress Degree Heating Week product for 27th May 2025 with a scale ranging from 0 (aqua) to 20 (dark purple) DHW. Source: NOAA. Accessed 29 May 2025

Climate Update

The El Niño–Southern Oscillation (ENSO) is neutral, and BoM’s Southern Hemisphere Outlooks indicates neutral ENSO conditions until at least October 2025. The Indian Ocean Dipole (IOD) is neutral, and BoM's outlooks indicates a neutral IOD until at least July. 

The official monsoon season (November to April) has come to an end, with this season having the latest start (7th February) on record. Twelve tropical cyclones developed across the Australian region this season, all except one (TC Alfred) within the western Australian region. Seven of the 11 cyclones within the western region reached Severe Tropical Cyclone strength. More recently, several lows and tropical cyclones have developed in the western region, two made landfall. 

Severe TC Courtney developed as a tropical low offshore of the northwest coast on the 22nd March, gaining strength as it travelled west across the Indian Ocean far south of the Cocos (Keeling) Islands. TC Courtney strengthened to a category 5 before leaving the Australian region and had little to no impact to Australia's mainland. 

Severe TC Dianne developed west of the Kimberley coast on the 27th March, quickly gaining strength to a Category 1 the following day, and crossing the coast near Koolan Island as a Category 2 on the 29th March bringing storm-force winds and heavy rain.

Severe Tropical Cyclone Errol developed as a low north of Darwin and strengthened to a Category 1 cyclone west of the Kimberley coast on the 15th April. Errol rapidly increased to a Category 4 as it moved westward, before making a U-turn towards the Kimberley coast and weakening to a tropical low, crossing the coastline on the 18th April.

Tropical Low 25U developed in the Indian Ocean, southeast of Christmas Island, briefly reaching cyclone strength near Cocos (Keeling) Island on the 19th March. This tropical cyclone remains unnamed. 

BOM's long-range seasonal outlook issued in October 2024, indicated an average tropical cyclone season for 2024-25. While an average season was expected, warmer than normal SSTs predicted for Australia through the season meant that the likelihood of severe tropical cyclones was higher than average. This season saw the highest number of tropical cyclones and the highest number of severe tropical cyclones in the Australian region since 2005/06, all except one in the western region.

Above: Cyclone tracks generated by Bureau of Meteorology, © Commonwealth of Australia. Licensed from the Commonwealth of Australia under a Creative Commons Attribution 4.0 International licence. 

Bleaching Update

Ashmore Reef

Heat stress across the northwest has been prolonged and severe with abnormally high SSTs much earlier and persisting longer than normal. Field surveys at Ashmore Reef in March did not coincide with peak heat stress (~ 14 DHW in January), however heat stress remained high (> 8 DHW) and surveys recorded variable ongoing coral bleaching and moderate mortality. Low coral bleaching was observed across most monitoring sites, with the highest observations within the lagoon. The impact was among the worst for the highly susceptible Acropora and Pocillopora corals and bleaching and mortality affected most coral groups. The impacts of recent cyclones were evident at a number of sites with injury to coral colonies and high coral rubble noted.

It is likely that coral bleaching and mortality has occurred at Scott and Seringapatam reefs less than 200 km south of Ashmore Reef, however there are no known surveys or observations during this time of peak heat stress. AIMS scientists will return to long-term monitoring sites at these reefs in October and will assess the condition of coral communities and the impacts from recent heat stress.

Left: To the far left, a diver surveys coral communities on the reef slope at Ashmore Reef. Across the slope, low bleaching extent with variable severity is evident in the live coral cover. Less obvious, but present, are colonies that have recently died - identified by the fine layer of turfing algae and skeletal detail integrity. Right: Coral colonies with partial bleaching and partial mortality surrounded by healthy coral colonies highlight the variability in the severity of bleaching across Ashmore coral communities.  Images provided by AIMS and Parks Australia.

Rowley Shoals

Field surveys at Mermaid and Clerke Reefs were conducted in April during high heat stress (> 10 DHW) following peak heat stress (~11 DHW at Mermaid Reef and ~14 DHW at Clerke Reef) in March. These surveys recorded high coral bleaching and mortality across most sites surveyed. The lagoon and western reef slope sites suffering the worst, with very high bleaching and mortality recorded at both Clerke and Mermaid Reefs from shallow lagoon bommie tops to depths greater than 20m on the slope. These previously pristine coral reefs have a brief history of low to moderate, isolated bleaching, and have never experienced heat stress on this scale before.  

Left: Bleached soft coral surrounded by bleached and recently dead hard corals on the reef slope at Mermaid Reef. Extreme bleaching affected soft corals at Mermaid and Clerke Reefs, at the Rowley Shoals. Right: A diver surveys the worst of the coral bleaching and mortality on the deep (20m) western reef slope of Mermaid Reef. Here, large Porites colonies suffered extreme bleaching. Images provided by AIMS and Parks Australia.

Kimberley

Aerial surveys conducted by the Department of Biodiversity, Conservation and Attractions (DBCA) across the Northern Kimberley Marine Park and the Lalang-gaddam Marine Park have confirmed extensive bleaching across shallow Kimberley reefs. Once analysed, these images will complement in-water surveys across Mayala and Bardi Jawi Garra Marine Parks where extensive coral bleaching has also been recorded. 

Bardi Jawi and Oorany rangers have completed surveys across Bardi Jawi sea Country, recording high bleaching across most sites surveyed. Most coral groups were affected by bleaching, the worst extent recorded for highly susceptible branching corals. 

Recent mortality across some shallow reefs in the southern Kimberley have been recorded by the Kimberley Marine Research Station. Recent surveys have returned to sites previously surveyed during peak heat stress where widespread and very high bleaching was recorded, now finding very high mortality.

As temperature anomalies return to normal for this time of year across the northwest, ongoing monitoring of coral communities will be required to fully capture the impact of bleaching in the Kimberley. 

Above: The southern Kimberley has experienced extensive coral bleaching, with the first observations made in December. Here, at a shallow reef site in Cygnet Bay, researchers from the Kimberley Marine Research Station have captured the varying severity of bleaching in early February 2025. Left: Bleached colonies with recent partial mortality adjacent to a large pale Turbinaria foliose coral. Right: Generally considered robust during stress events, bleached Porites massives were captured across the site. Images provided by Kimberley Marine Research Station, Cygnet Bay.

Pilbara

Scientists from DBCA have recently returned from field surveys across long-term monitoring sites at the Montebello and Barrow Islands Marine Park. Widespread coral bleaching was recorded across all sites, and the extent and severity of coral bleaching varied across coral groups and sites. Images from this recent survey will soon be analysed, building on the long-term dataset of coral and fish communities at the Montebello and Barrow Islands Marine Park.

Ningaloo

Aerial surveys in April conducted by DBCA across the Ningaloo Marine Park (along the North West Cape to Gnarloo in the south), Muiron Islands and Exmouth Gulf report variable coral bleaching across the entire length of Ningaloo Reef. Observations of very high bleaching was isolated to some reef locations, with other sites faring better. In water observations have confirmed some mortality at affected sites. Images from these recent aerial surveys will be analysed to confirm the extent of coral bleaching across the park.

Community members and tour operators have continued to document the widespread bleaching occurring across much of the northern sector of the marine park. In-water observations complement aerial observations and indicate variable bleaching and recent mortality extent across sites. Some of the highest observations of bleaching and recent mortality have been reported at northern shallow reef sites. 

Field teams from AIMS, DBCA and CSIRO are currently in the water at Ningaloo revisiting sites surveyed previously in this heat stress event. Initial observations confirm ongoing coral bleaching and recent mortality across all sites surveyed and indicate that this event is still unfolding for Ningaloo Reef. Data and imagery from these surveys will be analysed in the coming weeks.

This collaborative approach across Ningaloo will continue to be central in tracking ongoing conditions and building the picture of impact to coral communities across the marine park.

Left: Bleaching and recent partial mortality of Acropora colonies at Tantabiddi in the northern sector of Ningaloo where among the worst impacts of bleaching have been observed across the marine park. Image: Declan Stick, AIMS@UWA. Right: Ongoing coral bleaching has been observed across all study sites at Ningaloo and researchers are now recording variability in severity among sites and taxa. Image: Chris Fulton, AIMS

Temperature and Heat Stress Outlooks

Heat stress has decreased across the northwest, however, warmer than normal temperatures are expected to persist through to winter. Warmer than normal anomalies will continue through winter across most of Western Australia, however, as we move further from the warmest months of the year, there is little risk for ongoing coral bleaching. 

Bleaching Alert Levels, Degree Heating Week (DHW) and Degree Heating Month (DHM) metrics reflect the predicted accumulation of heat stress, calculated by integrating SST anomalies above expected (i.e., climatological) mean temperature with the duration of heat stress. Risk of reef-wide bleaching is predicted with a Bleaching Alert Level 1, DHW ≥ 4 and DHM ≥ 1. Risk of reef-wide bleaching with mortality is predicted with an Alert Level 2, DHW ≥ 8 and DHM ≥ 2.

Indian Ocean Territories

Current temperature outlooks (27 May 2025) for Christmas and Cocos (Keeling) Islands show small positive anomalies (<1°C warmer than the long-term 1981 - 2018 average) are likely to continue for the rest of May and into the winter months. BoM’s Degree Heating Month (DHM) Outlook and NOAA's Coral Bleaching Outlook indicate no heat stress through to at least August. Maximum ocean temperatures typically occur from March to April and current outlooks indicate little to no risk for bleaching in the coming months. Model skill for BoM's outlooks in this region are reasonably good for June predictions, and skill reduces for predictions further out.

Above: SST anomalies for Christmas and Cocos (Keeling) Islands for June to November 2025 with a scale ranging from -1.5°C (dark blue) to +2.5°C (burgundy). Source: BoM

North Western Australia

Recent SST anomalies have reduced across the northwest, and across the oceanic shoals, current outlooks show low positive anomalies (< 0.6°C warmer than normal; 1981 - 2018) for Scott Reef, Ashmore Reef, and the Rowley Shoals for the rest of May and the first two weeks of June. Anomalies <1°C warmer than normal are predicted to persist through winter. 

The outlook for the rest of May and into the first week of June show that SSTs close to average, or cooler than normal, are likely to continue across the inshore Kimberley and Pilbara regions. Low warm anomalies (< 0.6°C warmer than normal) inshore in June are expected to increase slightly in July through August (< 1°C warmer than normal). 

Current heat stress outlooks for the northwest region suggest no heat stress through to August. Model skill for BoM's outlooks are reasonably good for June and reduce for predictions further into the future.  

Above: SST anomalies for North Western Australia for June to November 2025 with a scale ranging from -1.5°C (dark blue) to +2.5°C (burgundy). Source: BoM

Ningaloo 

Current temperature outlooks (27 May) show anomalies up to 2°C warmer than normal (1981 - 2018) for Ningaloo over the next two weeks. Outlooks for June suggest positive anomalies between 1 - 1.5°C. SST anomalies are expected to reduce further in July with warm anomalies (<1°C) continuing through winter.

NOAA's Coral Bleaching Outlook indicates low heat stress for the rest of May. BoM’s Degree Heating Month (DHM) Outlook indicates no heat stress from June through August. These outlooks follow the warmest months of the year at Ningaloo (April) and indicate little risk for further coral bleaching across Ningaloo.

Model skill is reasonably good for BoM's outlooks for June, but reduced July and beyond.

Above: SST anomalies for Ningaloo for June to November 2025 with a scale ranging from -1.5°C (dark blue) to +2.5°C (burgundy). Source: BoM

South Western Australia

SST anomalies across the southwest remain much warmer (1 - 2°C) than normal and current (27 May) temperature outlooks show anomalies up to 1.5°C warmer than normal (1981 - 2018) for the southwest (from Shark Bay to the Abrolhos Islands, and south to Cape Leeuwin and extending east) for the rest of May and June.  Outlooks for July show anomalies 1.5°C warmer than normal for areas around Shark Bay and the Abrolhos Islands, and less than 1°C warmer than normal for coastal areas south of Geraldton.

Current heat stress outlooks suggest heat stress to continue around the Abrolhos Islands and offshore of the southwest for June through August. BoM’s Degree Heating Month (DHM) Outlook indicates low heat stress (<1 DHM) is predicted June to August, and NOAA's Coral Bleaching Outlook indicates high heat stress (>8 DHW) through the rest of May. These outlooks follow the warmest months of the year (April) indicate little risk for ongoing coral bleaching across the southwest.

Model skill is reasonably good inshore for June, but there is reduced confidence for the region for July and beyond.

Above: SST anomalies for South Western Australia for June to November 2025 with a scale ranging from -1.5°C (dark blue) to +2.5°C (burgundy). Source: BoM

NOAA's Coral Bleaching Outlook predicts the likelihood of coral bleaching heat stress and is based on NOAA's daily SST forecast. The outlook indicates a high probability (>90%) of high thermal stress across south Western Australia from June to September 2025.

Above: NOAA Coral Reef Watch 90% Probability Bleaching Heat Stress 4-month heat stress outlook for June - September 2025. Alert Level 1 indicates that heat stress and reef-wide bleaching is anticipated, while Alert Level 2 predicts reef-wide bleaching and mortality of heat sensitive corals. 

OBSERVATIONS OF CORAL BLEACHING

Coral bleaching, Scott Reef 2016. Photo: James Gilmour, AIMS

We encourage the reporting of coral bleaching observations on WA and NT reefs by the wider community, including traditional owners, regional managers, tourist operators and anyone who visits WA’s coral reefs. The Australian Institute of Marine Science (AIMS) has a free mobile app for recording bleaching. Using this app, you can submit observations on the amount of bleaching observed, additional habitat information and photographs. Alternatively, if you would prefer to simply email observations and supporting images, or have questions, please contact n.ryan@aims.gov.au.  More information about coral bleaching is available at https://www.aims.gov.au/coral-bleaching

CORAL BLEACHING APP

The ArcGIS Field Maps App for Android or Apple devices is a simple tool for collecting geo-referenced coral bleaching data. To get started, download the app and register for a username by emailing Ben Radford at b.radford@aims.gov.au.

Once you have received your username and password in an email, you can log in to your app for the first time. To begin using the coral bleaching app, please refer to the user guide. 

Alternatively, a bleaching observation datasheet has been created which can be printed out for completion by hand.

We acknowledge the Traditional Owners of Western Australia and Northern Territory and pay our respects to elders past, present and future. We honour their continuing culture, knowledge, beliefs and spiritual relationship and connection to country. We also recognise Aboriginal and Torres Strait Islander peoples as the Traditional Owners of the land and sea country on which the Western Australian Bleaching Group works, and as Australia’s first scientists. Through collaboration and two-way knowledge sharing, we can learn from each other. We will continue to build our relationships with Rangers and Traditional Owners throughout regional WA, to build on our coral bleaching knowledge, confirm predictions and measure the impacts to their sea Country.

FURTHER INFORMATION

Sea surface temperature (SST) summaries and coral bleaching predictions are based on the products provided by: 

• Bureau of Meteorology Southern Hemisphere Monitoring
• Bureau of Meteorology Ocean Temperature Outlooks/ Coral Bleaching Risk
• NOAA Coral Reef Watch Regional Virtual Stations
• NOAA Coral Reef Watch Coral Bleaching Heat Stress Outlook
• IMOS Ocean Current

We rely heavily on the SST and heat stress predictions provided by NOAA and particularly Australia’s Bureau of Meteorology (BoM). These organisations' models provide different, but complementary products. Differences in heat stress predictions among the products often reflect uncertainty in future conditions, particularly several months into the future. However, when the different products provide similar predictions of bleaching within 1 or 2 months, then there is a higher degree of confidence in the predictions.

WA Coral Bleaching Group

Nicole Ryan, James Gilmour, Jessica Benthuysen, Camille Grimaldi, Tahlia Bassett, Claire Ross, William Robbins, Arani Chandrapavan, Scott Heron, Rachel Garcia, Jennifer Hoy, Darren Phillips, Damian Thomson, Claire Spillman

The WA Coral Bleaching Group meets regularly throughout the spring - autumn months to discuss coral bleaching risks to Western Australian reefs. The newsletter provides updates based on forecasts, satellite SST monitoring, and in-water and aerial surveys. If you would like to receive our updates click subscribe:

For additional enquiries, contact:

Nicole Ryan, AIMS
n.ryan@aims.gov.au