Two oceans, one country—and 20 times more sharks in the prehistoric Pacific

August 13, 2026 Two oceans, one country—and 20 times more sharks in the prehistoric Pacific by Smithsonian Tropical Research Institute edited by Sadie Harley, reviewed by Robert Egan Sadie Harley Scientific Editor Meet our editorial team Behind our editorial process Robert Egan Senior Editor Meet our editorial team Behind our editorial process Editors' notes This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: fact-checked peer-reviewed publication trusted source proofread The GIST Add as preferred source Shark dermal denticles. . Lauder How many sharks should a coral reef have?

It sounds like a simple question, but almost nowhere on Earth is there a reef that has not at some time been fished or otherwise affected by humans, so there is no yardstick against which to measure what has been lost or what recovery would look like. 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. Fossil denticles reveal reef shark baselines in Panama. o coasts have diverged further. Caribbean denticle accumulation fell by about 75%, while Pacific rates today are statistically unchanged from their prehistoric baseline.

The upshot is that reefs on Panama's Pacific coast now yield about 100 times more denticles than those on its Caribbean coast. What makes this particularly puzzling is that the Pacific coast has taken by far the heavier fishing pressure. More than 98% of Panama's fishing happens there.