Outbreak of Stony Coral Disease

Are corals doomed? They seem to be under attack from more and different maladies that are the result of worsening environmental conditions. Dr Anjani Ganase shares information about what to look for when you dive Tobago’s reefs

The Stony Coral Tissue Loss is best explained through its descriptive name. It first appeared in the Caribbean in the Florida Keys in 2014, on a coral reef that was being impacted by a port-dredging project (Jackson and Prentice 2019). Since then, the disease has spread to at least eight other countries, including Mexico, Jamaica and with the latest occurrence in St Kitts and Nevis in August of this year (Kramer et al 2019). As the disease progresses through the Lesser Antilles and towards Trinidad and Tobago, we need to the understand the devastating impacts of the disease, and what scientists are doing in the attempt to curb the spread.

The giant brain coral in Speyside is one of the species of coral that is highly susceptible to the Stony Coral Tissue Loss Disease. Photo by Jonathan Gomez

The causes of the Stony Coral Tissue Loss Disease are currently unknown. The disease rapidly infects and kills the living tissue of the coral. An infected colony will appear to have lesions, exposing coral skeleton below, in patches throughout the colony. These lesions widen, and with sufficient tissue loss, the coral colony will die. Unlike other coral diseases that are specific to particular species, this disease targets a broad range of species (over twenty species), including species of coral that are common and are important reef builders on Tobago’s reef. The most susceptible species are the brain corals, including the same species as the giant brain coral in Speyside that is hundreds of years old. Other species of brain corals are very common and assist in providing reef structure for many reefs along the Caribbean coast and the offshore islands in the north east (St. Giles) of Tobago. Following the infection of the brain corals, the next species susceptible are the Boulder Star Corals and the Mountainous Star Corals (Orbicella species), which are our major reef builders. 

On Tobago reefs, a single colony of Mountainous Star Coral can extend to five metres in width and over five metres in depth and provide homes for a number of fish and other marine life. Fortunately, the disease does not appear to infect the already rare and vulnerable branching Acropora corals (Elkhorn and stag horn corals). While other diseases are described like a flu, scientists refer to the Stony Coral Tissue Loss Disease as the equivalent of the Ebola virus for coral in regard to its virulence. The outbreak is highly contagious as it quickly spreads to surrounding corals in an affected area. This makes it very difficult to stop the spread once present in the area.

Tobago’s coral reefs are no strangers to coral diseases. In the 1970’s, the aggressive white band disease nearly wiped out the branching corals throughout the Caribbean, including Tobago, which lost most of theirs in the 1980s. Other diseases have targeted our important mountain star coral and Boulder Star corals. The Yellow-Band Disease in particular is prominent on most reefs in Tobago. These diseases leave in their wake white or yellow sections of dead coral skeleton. These diseases will infect and kill the coral polyps and slowly spread from one end of the colony leaving dead corals in the wake.

 The mountainous star coral is common on reefs all around Tobago and the most valuable reef building species we have. They are also vulnerable to the Stony Coral Tissue Loss Disease. Photo by Anjani Ganase

Scientists are stumped by the virulent nature of the Stony Coral Tissue Loss Disease, and because of this, many have gone to drastic measures to try to stop the spread of the disease. In the desperate attempt to buy time, scientists in the Florida Keys and in the US Virgin Island have begun removing a stock supply of genetically diverse set of coral species that is susceptible to the disease. They will be cared for and grown in aquaria in controlled environments and used for future reef rehabilitation. Other conservationists in St Thomas are also frantically attempting to remove sections of infected coral tissue to curb the spread, while back in the Florida Keys, scientists are trying out broad spectrum antibiotics to slow the spread over the colonies. Responses from the antibacterial treatments is highly species specific with the mountainous star and boulder corals (Orbicella colonies) responding the best, while brain corals responded poorly. However, this form of treatment targets the lesion level and requires frequent re-applications, which may be too slow, considering the rate at which new lesions pop up (NOAA). The recommendation by NOAA is to focus the antibacterial treatment on the Mountain (Orbicella) coral species as they are the most important reef builders and seem to respond to the treatment.

While it is uncertain how reefs get infected, it has been suggested that both current flow and coral reefs with compromised health, and more vulnerable, drive the disease outbreak. In Florida, the disease first infected a reef impacted by dredging. Reefs in the past have shown disease outbreaks following a warming event. The diseases tend to be microbial in nature, and conditions of warming seas and pollution seem to shift surrounding conditions towards propagation and spread. 

While the rise and spread of the Stony Coral Tissue Loss may not be a result of climate change, the environmental conditions, such as exceptionally warm waters - even if there is no bleaching - or pollution may be enough to lower the coral’s immune system. Unfortunately, with our future of a rapidly changing climate, Trinidad and Tobago along with many other Caribbean coral reefs cannot afford another outbreak. Along the Florida Keys reef tract there is about 40 % of coral loss with a prevalence of between 66 - 100 % on most reefs. 

Many of Tobago’s reefs have about 10 - 20 % coral cover which are predominantly of the species susceptible to Stony Coral Tissue Loss Disease. A disease outbreak in Trinidad and Tobago, will be devastating to our coral reefs, even render a couple reefs functionally insignificant, unless we can plan for interventions. NOAA is currently hosting workshops to teach reef managers how to identify the disease, monitor and also carry out preventative and remediation plans. Unfortunately, without funding to support the prevention and mitigation plans, such as maintaining coral in an aquaria or the regular application of antibacterial medication, many of the Caribbean Islands may not be able to carry out such interventions. In the meantime, monitoring is important to track the spread of the disease and to understand more about the disease. 

You can help. If you’re out on the water in Tobago, please take photos. Be a mindful diver: no touching. If you’re visiting coral reefs with infected corals please ensure that your equipment is properly disinfected before being used in Tobago waters.  If you notice anything resembling coral tissue loss disease, - or even if you have no idea what you are seeing - please take photos and share with Wild Tobago or with your local environmental NGOs or the Institute of Marine Affairs. 

References:

 Kramer, P.R., Roth, L. and Lang, J. 2019. Map of Stony Coral Tissue Loss Disease Outbreak in the Caribbean. www.agrra.org. ArcGIS Online

Jackson L., Prentice, C. (2019) CORAL COLLAPSE, The reef plague https://graphics.reuters.com/ENVIRONMENT-CORALS-DISEASE/0100B2CP1GL/index.html

NOAA, 2019  https://oceanservice.noaa.gov/news/july19/coral-disease.html