Protecting Tobago from Stony Coral Tissue Loss Disease

Prepared by: Hannah Lochan, Marine Technician

Cruise ship season is here again after a long break due to the COVID-19 pandemic. As we welcome visitors to experience our beautiful twin islands, there is one passenger we hope is not on board any cruise ship – the lethal pathogen that causes Stony Coral Tissue Loss Disease (SCTLD)! This pathogen can be transported via ballast water in ships and is extremely virulent causing its own underwater coral epidemic having already decimated coral reefs in the region.

Potential spread of SCTLD

When a ship is preparing for a voyage, it often takes in saltwater into large holding tanks in its base to help with stabilisation. When the ship reaches its destination, it discharges this ballast water into the sea at the new site. This can result in the rapid spread of invasive species, bacteria, and other pests which can harm our valuable coastal ecosystems. Many countries, including Trinidad and Tobago, have agreed to join the International Convention for the Control and Management of Ships’ Ballast Water and Sediments, which stipulates that all ships must have a ballast water treatment plan. In Trinidad and Tobago, the Shipping (Marine Pollution) Bill (2004) is presently being updated and represents National Legislation towards management of Ballast Water.

Common ballast water treatment strategies include mechanical filtration, oxygen reduction, ultraviolet (UV) radiation, chemical treatment, ozonation or a combination of methods. According to regional ballast water requirements, the exchange or discharge of ballast water must be done at least 50 nautical miles from shore, at a depth no less than 200 metres.  

Fig 1 Ship discharging ballast water (Photo credit: iStock)

What is Stony Coral Tissue Loss Disease (SCTLD)?

In 2014, scientists observed hard corals losing tissue and dying at alarming rates in Florida. As the situation worsened, scientists suspected that the disease is caused by bacteria, although the exact cause is still unknown. Infected hard corals usually develop multiple lesions where tissue begins to slough off the coral, hence giving the name Stony Coral Tissue Loss Disease. Tissue loss occurs rapidly, and corals can die within a few days to weeks. This disease affects approximately 20 hard coral species present in the Caribbean region. The coral species primarily affected include the pillar coral, the symmetrical brain coral, the boulder brain coral, boulder star coral and the lettuce coral. All these species are found in Tobago with the latter four being very common on our reefs. So far, SCTLD does not affect finger corals or the endangered elkhorn and staghorn corals. The pathogen can be transmitted via direct contact with an infected colony or through the water column. Once present, it spreads rapidly across coral reef sites.

Although SCTLD has not yet been recorded in Trinidad and Tobago, it may be more a matter of “when” it arrives rather than “if” it arrives. Currently, the Institute of Marine Affairs (IMA) monitors the health of ten coral reef sites around Tobago on an annual basis and assesses disease presence and prevalence.

As we continue to host cruise ships, it is crucial that we ensure ballast water rules are followed, especially as these tours tend to island-hop through the Caribbean where many islands are now affected by the disease. Oftentimes, the reefs where the disease is first observed are located close to ports. As of December 2022, 26 countries in the region are affected by SCTLD, with Grenada being the nearest affected island. Notably, recent data published by marine scientists at the University of Miami indicates that UV radiation as a treatment for ballast water to kill SCTLD pathogens is not as effective as presumed.

SCTLD emergency response teams throughout the region have been activated, spearheaded by the National Oceanographic and Atmospheric Administration’s (NOAA) Coral Reef Conservation Program and the Atlantic and Gulf Rapid Reef Assessment (AGRRA) organisation. These agencies aid in capacity building, education, and training in SCTLD identification, monitoring and treatment. Currently of the 26 countries affected by SCTLD, 12 countries are engaged in administering antibiotic treatments to affected coral colonies. This involves actively applying broad-spectrum antibiotic paste onto the corals in hopes that it will reduce the spread of disease and lower coral mortality rates. Though successful in some territories, it is an expensive and very labour intensive solution. Additionally, antibiotic treatment does not prevent SCTLD from infecting the corals again.

Fig 2 Infographic of corals affected by SCTLD (Photo credit: Andrew Bruckner, Florida Keys National Marine Sanctuary)

Fig 3 Map showing locations of SCTLD outbreaks within the Caribbean region. Green= SCTLD absent at this site, Red= SCTLD present at this site, Yellow= SCTLD may be present at this site and Purple= New submission of SCTLD report- under review (Photo credit: Kramer et al., 2019)

What else can we do?

Keep a look out! If snorkelling or diving in Tobago and you observe something you think may be SCTLD, snap a picture if possible and submit a report to the IMA directly or via the SeaiTT app (available for free on the PlayStore). Reports will be investigated. Early detection will allow for monitoring, and treatment strategies can be implemented as soon as possible.

Important to note, SCTLD may be confused with coral bleaching as the coral appears white in both instances as the coral skeleton is exposed. However, bleached coral usually has visible tissue remnants whereas in SCTLD infected corals, tissue would be completely dead or sloughed off the coral, comparable to skin falling off bone. The rate of tissue loss and mortality is much faster in SCTLD affected corals. Bleached corals are usually only observed during periods of higher water temperature, whereas SCTLD is active throughout the year and is often more virulent in winter months. Additionally, do not touch infected corals and clean gear! If you have snorkelled or dived on reefs in other islands affected by SCTLD, ensure your gear is properly rinsed and bleached to prevent cross contamination of the disease at other sites. Gear should be cleaned between sites and before diving in another country.

While these fragile coral reef ecosystems face so many threats, we must still acknowledge what we gain from them and cherish the roles they play ecologically, socially, and economically. By raising awareness, we must do what we can to protect our own resources as nobody else will do it for us. After all, “We will only conserve what we love, we will only love what we understand, and we will only understand what we are taught”- Baba Dioum, 1968.