South Africa Shark Diving Guide: Two Oceans and 20x the Sharks

Fossil shark scales discovered in reef sediments provide a new baseline for measuring historical predator populations, according to a study published in Science by researchers at the Smithsonian Tropical Research Institute (STRI) in Panama. By counting shed dermal denticles—jovially termed shark “dandruff”—scientists can estimate historical shark abundances on coral reefs that have experienced centuries of human fishing pressure.

Counting Shark Dandruff in Reef Sediments

Sharks continuously shed tiny tooth-like scales called dermal denticles throughout their lives. According to the research team, these scales settle into reef sediments and remain preserved for millions of years. By extracting and counting these denticles from well-dated sediment cores, scientists reconstruct past predator populations. The research team analyzed 3,497 denticles across 157 samples, comparing prehistoric sediments laid down between 7,000 and 3,000 years ago with samples from the past century.

Ocean Productivity Versus Fishing Pressure

The study reveals a stark contrast between Panama’s two coastlines, which are separated by a single strip of land but exhibit vastly different marine environments. According to Aaron O’Dea, STRI staff scientist and co-corresponding author, prehistoric reefs in the Pacific Gulf of Panama supported roughly 20 times more sharks than Caribbean reefs. Today, Pacific reefs yield about 100 times more denticles than Caribbean sites, as Caribbean accumulation fell by roughly 75 percent while Pacific rates remained statistically unchanged from baseline levels.

This divergence persists despite the Pacific coast absorbing more than 98 percent of Panama’s modern fishing pressure. Lead author Erin Dillon explains that the Gulf of Panama benefits from seasonal upwelling that brings deep nutrients to the surface, supporting large predator populations. The Caribbean, conversely, is nutrient-poor, meaning its energy is held tightly in reef biomass, leaving local shark populations far more vulnerable to overfishing.

Implications for Modern Marine Conservation

Conservation managers routinely gauge the health of a depleted reef by looking at the nearest healthy ecosystem. However, applying Pacific baselines to the Caribbean—or assuming both coastlines suffered identical impacts—would miscalculate conservation targets by an order of magnitude or more, according to the findings. The study argues that ocean productivity metrics require as much weight as historical fishing data when setting reef recovery goals.

At the same time, Pacific sharks face mounting modern threats. O’Dea notes that targeted shark fishing ramped up in the region during the 1980s, and recent declines may be masked when averaged over a full century.

Frequently Asked Questions

What are shark dermal denticles?

Dermal denticles are tiny tooth-like scales that cover a shark and are shed continuously through life.

How do scientists use shark scales to count past populations?

Sharks shed these scales continuously through 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.

Why do Pacific reefs have more sharks than Caribbean reefs?

The Pacific coast of Panama experiences seasonal upwelling that delivers deep nutrients to the surface, fueling a highly productive food web that sustains larger predator populations compared to the nutrient-poor Caribbean.