Coral reefs, the Earth's biologically richest ecosystems are dying worldwide. They suffer a multitude of threats, the newest of which is stony coral tissue loss disease (SCTLD). Although SCTLD has so far infected only a tiny proportion of the world's reefs, it's effects there have been devastating. We don't yet know what causes it, but have strong evidence it is finding new locations to destroy because ships are failing to follow rules on safe ballast water disposal.
Already threatened by rising temperatures, over-fishing and nutrient run-off, coral reefs really don't need another threat to their existence. Unfortunately they have one, at least in the Caribbean, where SCTLD has been spreading since 2014, when it was spotted off Florida. Most coral disease affect only a few but SCTLD's range is broad, typically sometimes killing more than 80 percent of a reef's corals.
Now SCTLD has reached the Bahamas, and while its cause has not yet been found, a study of its spread in Frontiers in Marine Science reveals a lot about its movements.
SCTLD has been found to be spreading through the Florida keys at a rate of 100 meters (300 feet) per day, presumably without human assistance. However, whatever the pathogen is, it appears to have trouble getting across substantial stretches of water without assistance. The Bahamas are among the closest reefs in the Caribbean to Florida, yet until late 2019 there were no reports of SCTLD there, despite extensive surveys. In fact, for all the damage Bahaman coral was suffering from hurricanes and high water temperatures, only 1.2 percent of corals surveyed showed any disease at all. Meanwhile reefs further afield in Jamaica and Mexico fell victim.
In November 2019, however, SCTLD was reported in the Bahamas and within four months it was widespread. The first sightings were made close to Freeport and Nassau, the Bahamas' biggest commercial shipping ports, and reefs. Corals in deeper water proved more resilient to those closer to the surface, and it appears some common species have at least partial resistance. Nevertheless, the damage to reef diversity is immense. With a high proportion of infected corals dying swiftly, often overgrown by algae before other corals can take their place.
“If ballast water was taken on board in a port area where SCTLD was present and not exchanged in open water away from reefs as required in The Bahamas, ballast water may have been the means by which SCTLD reached the Grand Bahama area,” the paper notes. While this remains somewhat speculative, many invasive species have been transmitted around the world in ballast water, which is why The Bahamas and many other places require an open-ocean exchange. The paper's first author Dr Craig Dahlgren of the Perry Institute for Marine Science reports seeing ships dumping ballast water off Freeport, near the earliest Bahaman sites for the disease. Meanwhile, reefs close to Florida remained unaffected.
The Guardian reports treating corals with the antibiotic amoxicillin has shown some success, suggesting the cause is bacterial. Such treatments could only be practical on small reefs close to shore prized by the tourist industry.