In a Science paper, researchers at the Smithsonian Tropical Research Institute (STRI) in Panama answer the question using a remarkable indicator the team jovially coined shark "dandruff."

Sharks are covered in dermal denticles, tiny tooth-like scales that are shed continuously throughout life and settle into reef sediments, where they can survive for millions of years. Count the denticles in a well-dated slice of sediment and you have an estimate of how many sharks swam overhead.

The team applied this approach on both sides of the Isthmus of Panama, comparing sediments laid down between 7,000 and 3,000 years ago, long before intensive fishing, with sediments from the past century. In total, they picked 3,497 denticles from 157 samples.

Hints at the results came early in the study. "In Bocas del Toro, on the Caribbean coast of Panama, we were finding around 50 shark denticles in 10 kilograms (22 pounds) of reef sediment. When we took our first, much smaller sample from a Pacific reef, we were shocked to find 200 denticles in just one kilogram (2.2 pounds)," said Aaron O'Dea, STRI staff scientist and co-corresponding author.

Two coasts, starkly different baselines

As samples came in and data were modeled properly to account for variation in dating and effort, that first impression continued to hold. Before humans began fishing in earnest, reefs in the Gulf of Panama on the Pacific coast supported roughly 20 times more sharks than reefs in the Caribbean, only a hundred kilometers away and sharing many of the same shark species.