2023 Caribbean Marine Heat Wave-Impact on Tobago’s coral reefs

In May 2023, rising sea temperatures in the Caribbean posed a severe threat to coral reef health and survival. Many Caribbean islands grappling with this, monitored the effects of the coral bleaching in their areas. On August 5th, Trinidad and Tobago was placed under a coral reef Bleaching Alert Level 1 by the National Oceanographic and Atmospheric Association, Coral Reef Watch (NOAA, CRW). A subsequent Bleaching Alert Level 2 was issued on August 25th indicating that severe coral bleaching and mortality was expected. This Bleaching Alert Level 2 lasted until November 20th 2023. 

What is coral bleaching? 

Corals have a symbiotic relationship with tiny algae called zooxanthellae. Zooxanthellae live inside the corals’ tissues and give the corals their colour. They also make food through photosynthesis and provide to the coral host. However, this relationship thrives at temperatures between 25°C to 29°C. At elevated temperatures occurring for four weeks, the symbiotic relationship becomes stressed, and the corals expel the zooxanthellae from their tissues. This results in corals losing their colour, exposing their calcium carbonate skeleton that gives a white or “bleached” appearance. Bleached corals also have a diminished food source. If ideal sea temperatures return quickly, corals can regain zooxanthellae and survive. However, if the high temperatures persist for eight weeks or longer, corals may not be able to regain zooxanthellae and may eventually suffer mortality. Furthermore, due to a weakened immune response, corals are prone to disease outbreaks during or in the aftermath of bleaching events. Research is being done regionally to investigate the impacts of bleaching and understand the effect on corals’ reproductive strategies and resilience.   

Bleaching Impact

During IMA’s annual coral reef monitoring in August 2023, highest live coral cover was recorded at Booby Island (19.53 ± 1.17 %) and lowest at Flying Reef (4.00 ± 0.69 %). The other sites ranged between (19.38 – 8.15 %) coral cover. In November 2023, IMA staff conducted bleaching surveys at six of the long-term monitoring sites. Coral bleaching was observed at all six sites. Bleaching was mainly observed in the brain and mountainous star corals with 18 out of 26 hard coral species exhibiting signs of a bleaching response. The highest bleaching response was observed at Booby Island with (56.05 ± 11.92) being impacted, followed by Mt. Irvine (45.22 ± 14.67) and Buccoo Western (42.37 ± 8.95). Flying Reef recorded the lowest bleaching occurrence with (13.44 ± 0.33) live corals being affected by bleaching. However, this site had the lowest live coral cover. The giant barrel sponges (Xestospongia muta) were also observed to have bleached at two sites. At Flying Reef, all barrel sponges surveyed had bleached and suffered mortality. This year, IMA will investigate coral mortality rates due to the 2023 bleaching event. 

Community Reporting

Bulletins were issued to the public to encourage boaters, divers and snorkelers to report their coral bleaching sightings to the IMA. Many stakeholders heeded the call and expressed their concern sharing information and pictures of the different sites they had visited. In all, IMA received 17 reports of coral bleaching at different sites around Tobago, including Castara, Englishman’s Bay and Cathedral Reef.