Coral Bleaching Survey

Report on the 2005 Mass Coral Bleaching Event in Tobago – Results from Phase I Survey

In the second half of 2005, the Caribbean region experienced a widespread coral bleaching episode. As such episodes have previously caused widespread mortality amongst reef building corals, there was a great deal of concern amongst the scientific community, fisherfolk, tourism organisations and other interested stakeholders. In order to determine the extent and nature of the impacts of the episode on the reefs of their island, the Buccoo Reef Trust (BRT) and the Tobago House of Assembly (THA) decided to invite a team of 5 scientists from Coral Cay Conservation (CCC) to assist in undertaking targeted surveys of the coral bleaching in Tobago.

Over a three week period commencing in October 2005, a two-phase campaign was launched: in Phase 1 the reefs were assessed to establish the extent and severity of the coral bleaching on Tobago’s main reef systems, in Phase 2, an appropriate long-term monitoring programme was designed and installed to examine the recovery of bleached corals. The content of this report deals mostly with Phase 1, as the data from Phase 2 will not be complete until May 2006.

Overall mean bleaching of hard corals was found to be 66% (71% on deep transects and 63% on shallow sites)

Phase I

Phase 1 involved surveys of 22 discreet sites, which were assessed using a Point Intercept Transect method, utilising a 20m transect chain marked at 25cm intervals. At each site, two deep and two shallow surveys were completed, producing over 7000 data points from 88 transects. Overall mean bleaching of hard corals was found to be 66% (71% on deep transects and 63% on shallow sites). Bleaching by geographic region was found to be largely consistent, with sites exhibiting greater than 85% bleaching dispersed throughout the target area. However, of the 9 transects exhibiting less than 20% bleaching, 5 were located near Speyside in the northeast of Tobago, perhaps indicating either localised tolerance to bleaching or superior water quality (lower temperatures, less silt and fewer nutrients).

Bleaching by species was found to be highly variable, both between and within species. Agaricia agaricites (‘leaf’) and Siderastrea radians (‘rough starlet’) were the most impacted species, with 93% of the observations for both species being bleaching. Madracis mirabilis (‘yellow pencil’) and Acropora palmata (‘elkhorn’) were the least impacted species (3% and 0% respectively). Although the low number of observations recorded for A. palmata (n=7) reflects the low abundance of the species, many additional stands were observed by the surveyors during the campaign and all appeared to be unbleached. Very high variability was found amongst the species of the Montastrea annularis complex (‘boulder/mountainous star’), which demonstrated overall bleaching impacts of 73%. For example, at one site in Buccoo Reef, two adjacent stands of M. annularis (annularis) exhibited 97% and 6% bleaching impacts respectively, implying the existence of bleaching resistant combinations of the coral species, its algal symbionts, or both.

It is not possible to predict the nature and extent of any subsequent mortality/recovery to the reefs of Tobago. Whilst cyanobacterial overgrowth was observed on the colonial zoanthids of the Palythoa genus, there were no recorded observations of coral disease, tissue necrosis or turf-algal/cyanobacterial overgrowth on bleached scleractinian corals. However, the incidence of opportunistic coral diseases may be expected to peak at the end of the warm season, and thus remain to be quantified. The monitoring programme installed during Phase 2 has been designed to gather these data.

Phase II

Eight monitoring stations were established on representative coral reefs around Tobago, only one of which was located on the Atlantic coast (Cove) because of the generally rough and dangerous conditions on this side of the island. Each station consisted of a 20 metre long transect that followed the reef along the 10 metre depth contour, and was marked at each 5 metre interval with a sub-surface float tied 6ft above the seabed. Along each of the 20 metre transects, 20 coral colonies were randomly selected and individually tagged with an engraved plastic tag that was nailed to the adjacent reef substrate. The tagged corals were monitored by scuba surveys using an underwater digital camera (Sony Cybershot 5.1 megapixel) mounted on a PVC photoquadrat (50 cm x 50 cm). Each tagged colony was photographed on the 20th November 2005, 18th January, 3rd April and 2nd September 2006 (+ or – 2 days). The sea temperatures at four of the sites were logged using HOBBO© waterproof pendant dataloggers.

Digital photographs of the tagged colonies were examined for changes in colour level using the CORALWATCH Coral Health Chart, using seven categories ranging from; 6 = fully pigmented to 0 = totally unpigmented (white). The average pigmentation of the entire colony was used when blotches or spots were present. Tagged colonies were also analysed for changes in their live surface area (partial mortality). This was done visually using the following seven categories; 6 = 0% partial mortality (no change in live surface area); 5 = 1-20% partial mortality; 4 = 21-40%; 3 = 41-60%; 2 = 61-80%; 1 = 81-99%; 0 = 100% mortality. Attempts to use image analysis software to calculate partial mortality proved impractical because of the 3 dimensional shape of the colonies.


At all sites, pigmentation returned to the colonies rapidly, with most gains occuring between November and April, with little or no gain between April and September 2006. The site where corals showed the slowest colour recovery was Cove on the Atlantic coast, where water temperatures remained initially close to 1°C warmer than sites on the Caribbean coast.

Between the 20/11/05 and the 2/09/06 the level of survival of the all the tagged coral colonies was high, with only 11 colonies or 6.9% suffering 100% mortality. All of these 11 mortalities were from the category of 58 tagged corals that were severely bleached or “Totally Unpigmented”, equivalent to a mortality rate of 19% for this category. Of the 11 colonies that died there was a noticeable majority of Brain corals; 6 Diploria strigosa (16%, n=37) and 2 Diplora labyrinthiformis (10%, n=20). The other 3 mortalities were 1 Porites astreoides ( 20%, n=5), 1 Agaricia agaricites (1 or 50%, n=2), and 1 Montastrea annularis (2.4%, n=42).

Of the colonies that survived, 32.5% (52 individuals) suffered some level of partial mortality (or a reduction in the surface area of living tissue). Levels of partial mortality were highest among colonies that lost all or most of their pigmentation during the bleaching event of 2005 (Colour categories 1, 2 and 3 on the CORALWATCH Health Chart). Colonies that experienced little or no bleaching in 2005 (Colour categories 4, 5 and 6) suffered very little loss of living surface area. The level of partial mortality was highly variable between colonies, and often appeared to be associated with an outbreak of coral disease.

The level of partial mortality varied between sites, with the highest level occurring at Mt Irvine (mean 50% partial mortality) and the lowest at Kariwak (12%). Both these sites are located close inshore in close proximity to tourism and residential developments and are known to be affected seasonally by high levels of nutrients and silt. There was no correlation between level of partial mortality and the distance between the shore and the site.


The data (approx. 8,000 data points) gathered during this study showed that the bleaching event of 2005 observed in Tobago was both severe and widespread. An average of 66% of the hard coral cover in Tobago was visibly affected, with levels over 85% observed at many sites. All reefs on the Caribbean coast were badly affected, while those at Speyside showed greatly reduced levels of bleaching for reasons that are still unknown. The lack of bleaching in this extensive reef system emphasises the need to implement urgently the THA’s plans to establish a Marine Protected Area at Speyside and to improve the management of the surrounding watersheds in order to minimise siltation and pollution impacts on this important “upstream reservoir” of coral biodiversity.

The majority of tagged corals recovered their pigmentation between November 2005 and April 2006, giving rise to optimism among many observers that the worst was over. However, closer inspection of these corals until September 2006 revealed that partial mortality was evident in 32.5% of the tagged colonies, with a further 6.9 % suffering total mortality. The level of partial mortality was greatest in corals that had been severely bleached, and this was often associated with symptoms of tissue necrosis and coral diseases.

Brain corals (Colpophyllia natans, Diploria strigosa and Diploria labyrinthiformis), which account for 21% of the corals observed, were particularly badly affected and accounted for 73% of the observed total mortalities. This suggests that these slow-growing massive species may be more vulnerable to bleaching than other corals found in Tobago’s waters.

In order to understand the long-term effects of bleaching events, monitoring of coral diseases will be particularly important. The risk of disease spreading to other colonies highlights the need to urgently improve coastal water quality and reduce other stressors on Tobago’s reefs. Particular attention must be given to reducing land-based sources of pollution, such as inadequately treated sewage and domestic grey water, as well as contaminated storm run-off and silt. Local management initiatives that improve the health of coastal ecosystems will give corals a better chance of successfully overcoming future coral bleaching events and adapting to the environmental changes that lie ahead.

This study was supported with grants from the Tobago House of Assembly and The Travel Foundation. All personnel costs and survey equipment were provided by Coral Cay Conservation and the Buccoo Reef Trust. The survey team consisted of Shay O’Farrell, Rebecca Korda, Hayley Rose, Christian Williams, James Comley, Barry Lovelace, Hyacinth Armstrong, Rolland “Saga” Guilland, Joseph Benjamin and Owen Day. The authors would like to thank Andrew Lovell and Ricky Knowles for providing a consistent supply of air tanks during the course of the survey.

The Phase I report was produced jointly by Shay O’Farrell of Coral Cay Conservation and Owen Day of the Buccoo Reef Trust. The Phase II poster was produced jointly by Owen Day and Barry Lovelace of the Buccoo Reef Trust and Shay O’Farrel and James Comley of Coral Cay Conservation.