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  • Writer's pictureTravis Turgeon

Assessing the Spread of Coral Disease through Global Shipping

Updated: Dec 4, 2022

"When corals are infected with SCTLD, multiple lesions appear on the exoskeletons, quickly progressing and spreading across a reef or coral colony."

Stony Coral Tissue Loss Disease (SCTLD). It’s a relatively new disease first documented in Florida in 2014. While the cause or source of the disease is unknown, it currently affects more than 30 species of coral, with brain coral, pillar coral, star coral, and starlet corals showing the most frequency and vulnerability to the disease. Researchers know that the disease spreads quickly and has a considerably high mortality rate, with outbreaks confirmed across the Caribbean in countries including Mexico, Jamaica, St. Maarten, USVI, the Dominican Republic, Belize, Honduras, and the Bahamas.

A Closer Look at Stony Coral Tissue Loss Disease (SCTLD)

When corals are infected with SCTLD, multiple lesions appear on the exoskeletons, quickly progressing and spreading across a reef or coral colony. The disease is highly contagious, and in most cases, SCTLD results in the total mortality of a reef or colony. The disease is caused by a bacterial pathogen directly transmitted by contact or indirectly transmitted through the water column.

While the exact cause of this disease is unknown, researchers in the Caribbean have found rising water temperatures to be a driving factor for many types of coral stressors and diseases, and significant bleaching events have coincided closely with the spread of SCTLD. The microbial communities in SCTLD lesions respond to antibiotic treatments for Flavobacteriales, Rhodobacterales, and Rhizobiales bacterias.

In addition to warm waters and bleaching events, several other environmental factors have been shown to affect the transmission and spread of SCTLD, including water turbidity, local nutrient concentrations, and temperature variability (rapidly fluctuating water temperatures).

Tracking the Spread of SCTLD

Researchers have tracked the spread of Stony Coral Tissue Loss Disease in a linear pattern in Florida and the Caribbean, moving quicker toward northern latitudes than southern latitudes. One theory of the direction of the spread is ocean currents and sediments, which could be an efficient mechanism of transmission for this type of disease.

However, a recent study from the University of Miami suggests that another mechanism is at least partially responsible for the spread of the disease… shipping.

According to the study, water from one location can enter a ship’s hull and release to new destinations after the vessel makes a transit.

“Outbreaks in very distant locations suggest that the disease transport was aided by means other than just ocean currents, such as through a ship’s ballast water,” said the study’s lead author and marine science expert Michael Studivan.

To test the theory, researchers at the University of Miami conducted two lab-based transmission tests; one with healthy corals exposed to disease-exposed and non-disease-exposed waters and another with the same water (disease and non-disease-exposed) after holding it in a ship’s ballast tank to see if SCTLD could survive those conditions for efficient transport.

The experiments showed that water held in a ship's ballast did not make any noticeable impact on the onset of SCTLD to new and healthy coral communities, making it a likely transportation mechanism for the disease.

How Dark Ships Have the Potential to Play a Destructive Role in the Growing Issue

While all ships can transfer disease-infected water from one location to another, vessels that operate illegally or in restricted and regulated waters (dark ships) could further the threat considerably.

The most likely type of ship to spread this type of disease would be those operating in or around the Caribbean, where the disease remains prevalent. The difference between ships operating legally and illegally is that some regulations exist (or are in the works) to prevent this type of disease spread. Hawaii, for example, currently has a proposal to prohibit ships from emptying their hull waters without treating or cleaning them first, and vessel identification and AIS information could be used to calculate a ship’s risk factor for disease transmission.

If a ship makes transits while not transmitting AIS data, regardless of its use or purpose, it becomes nearly impossible to evaluate the associated disease risk. Further, if researchers want to keep a close eye on new infection areas or the global spread of the disease, dark ships muddy those waters and make it difficult to make accurate evaluations or hypotheses.

To eradicate Stony Coral Tissue Loss Disease, or at least slow the spread and reduce the environmental impacts, maritime authorities must begin to develop strategies to mitigate dark shipping in the most widely affected areas, namely the Caribbean, and prevent the spread to other vulnerable ecosystems.

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