Old nautical maps reveal dramatic coral loss
Between 1773 and 1775, George Gauld, a surveyor with the British Admiralty, immortalized the coast of the Florida Keys in ink. Though his most pressing goal was to record the depth of the sea — to prevent future shipwrecks — Gauld embraced his naturalist side, too. He sprinkled his maps with miscellany that later charts would omit: where sea turtles made their nests, or the colours and consistency of sand.
Gauld also took note of the corals he saw. And in doing so, he created the oldest known records of Florida reefs.
“With the early charts you can actually see the reef itself being drawn,” said Loren McClenachan, a marine ecologist at Colby College in Maine. “It matches almost exactly with the satellite data.” In a study published this week in the journal Science Ad- vances, McClenachan and her colleagues compared those 240-yearold observations with present-day satellite images.
A stark picture of shrinking coral emerged: Half of the reefs recorded in the 1770s are missing from the satellite data.
The coral nearest to shore fared the worst, with 88 per cent of the coral that Gauld recorded now gone. At the fore-reef, the coral at the most seaward edge of the reef, there appeared to be no loss between historical coral observations and modern habitat maps.
“It’s a very important study,” said Sam Purkis, a marine geoscientist and conservationist at the University of Miami who was not involved. The maps are old and were “generated with very primitive techniques,” he said.
As for the exact reasons for the disappearance of the Florida corals, McClenachan said “we can’t get at that. All we have is then and now.”
But the marine ecologist offered a few possible explanations, most of them involving humans. Humans built a causeway through the Keys and dredged the Key West harbour. We’ve changed the way fresh water flows through the Everglades. Perhaps agriculture played a part. McClenachan said we might not know how severe other local coral population declines have been. “If something’s not there, you don’t know to look for it,” she said.