By merging three recent data sets—from the National Oceanic and Atmospheric Administration and other sources—McClenachan and her collaborators were able to pull together information covering the same region. Then, they matched up the coral on Gauld’s map with the modern reefs. “Some of the reef aligned just about perfectly,” McClenachan says. Since reefs are constructed over thousands of years, in the absence of direct disturbances, they will persist in the same locations for a very long time. Gauld was meticulous in distinguishing between rocks on the seabed and coral, along with many other features, and the researchers had reason to believe in the accuracy of his depiction.

If there was no coral within three-fourths of a kilometer of Gauld’s markings, the researchers declared the patch gone. Many of the reefs, unfortunately, had completely disappeared. More than 50 percent of the area that used to be covered with coral in the Keys was no longer. Corals close to the shore and in the Florida Bay were especially hard-hit. In the Bay, where sediments accumulated after the draining of the Everglades in the early 20th century and the construction of a railway, coral cover declined by 87.5 percent. Close to the shore, where anchors, dredging, and reclamation have affected the sea bottom, it declined by 68.8 percent.

The comparison made it clear that if we are interested in restoring coral reefs to their previous extent, we might have farther to go in getting back to historical levels than estimates based on more recent snapshots.

Still, says McClenachan, the Keys have been leaders in the area of coral restoration, a process of growing corals in aquariums and then planting them in the wild. “Maybe some of these places that used to have reef would be good to have reef again,” she says. Understanding where coral used to thrive might help us see where, with the right preparations, they could live again.

The details Gauld’s chart captured, with its sand banks, mangroves, turtle nests, and coral patches, could easily have gone unnoticed, and today our knowledge of the Keys would be poorer. We have this information “just because because this particular chart maker was paying attention,” McClenachan says. Today, in the face of great challenges, modern ecologists persevere in documenting and studying the great dying of coral reefs—taking notes, taking images, making maps.

“I think it’s necessary work; I don't think it’s necessarily uplifting work,” says McClenachan. “I am hopeful that it’s useful.”

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