It’s easy to wonder how many sharks there were before humans began fishing intensively in our oceans. A few dozen more? Hundreds? But how could we possibly know? In most marine ecosystems, there is no historical record to tell us what an intact shark population actually looked like. And if we don’t know what the ecosystem’s “health” was like before major human impacts, how can we know how much populations have declined or what their recovery should look like? A new study conducted in Panama offers an unusual way to answer this question: by looking down.
A biological archive on the seafloor
Instead of relying solely on modern studies, the researchers examined the tiny fossilized dermal denticles that cover the bodies of sharks; like fingerprints, these microscopic scales can contain information about the species that inhabited an ecosystem. When sharks shed these scales, they can accumulate in sediments on the seafloor. Some remain preserved for thousands of years, creating a kind of biological archive.
Thus, in this study, the researchers analyzed 3,497 denticles from 157 sediment samples collected on both sides of the Isthmus of Panama—regions that are geographically close but environmentally very different (Bocas del Toro, on Panama’s Caribbean coast, and the Gulf of Panama, on the Pacific side). They studied sediments from the Middle Holocene (approximately 7,000 to 3,000 years ago, before intensive human exploitation) and compared them with sediments representative of the last century or so. This allowed them to establish a baseline that extends beyond the earliest fishing records or scientific studies, going back thousands of years.
What they discovered was striking: over the course of those millennia, the two regions evolved in very different directions. In the Caribbean, the accumulation of shark denticles declined by about 75% from the mid-Holocene baseline, meaning that sharks are estimated to have been about four times more abundant before intensive fishing began. The decline was particularly pronounced among pelagic sharks, including groups of species such as requiem sharks and hammerhead sharks, which are commonly targeted by fisheries. Demersal sharks also came to represent a larger proportion of the remaining community, but that does not mean their populations thrived: their absolute abundance also declined substantially.
A very different picture in the Pacific
The Panamanian Pacific, by contrast, tells a very different story. Before significant human impact, the reefs of the Panamanian Pacific were home to approximately 20 times more sharks than those of the Caribbean, and denticle accumulation remained comparable to levels seen in the Middle Holocene. Pelagic sharks continued to account for more than two-thirds of the total denticles, and the overall structure of the shark community did not change significantly over the millennia reflected in the sedimentary record; a particularly striking finding given that the Panamanian Pacific has historically borne more than 95% of the country’s fishing pressure.
How were sharks able to show relative resilience in the area subject to the highest fishing pressure, while those in the Caribbean declined despite facing lower fishing levels? Researchers point to oceanography as part of the answer. The Panamanian Pacific experiences seasonal upwelling, a phenomenon in which deep, cold, nutrient-rich waters rise to the surface. These nutrients fuel the base of the food web, promoting greater phytoplankton abundance, more fish, and, ultimately, more energy available to large predators. The Caribbean, by contrast, is more oligotrophic—that is, its waters contain fewer nutrients. This difference between the two regions amplifies as energy moves up the food web, resulting in substantially greater fish biomass in the Pacific. In other words, the Pacific may simply have had a much larger ecological “budget” to support sharks.
Rethinking Conservation Goals
The team argues that these new findings should change the way we set conservation goals. Assuming that two regions with similar shark species have the same carrying capacity could lead us to underestimate what an ecosystem is actually capable of sustaining, in addition to being biologically unrealistic. Therefore, the researchers argue that the environmental context must be considered alongside the sharks’ life history, fishing pressure, habitat availability, and connectivity.
Furthermore, the authors emphasize that “resilient” does not mean—nor should it mean—“safe,” and they are careful not to suggest that Pacific shark populations are immune to human impacts. The fossil record offers a long-term perspective, but since the most recent samples span roughly from the 20th century to the present, declines that occurred during the most intense period of modern shark fishing may be difficult to detect. The broad ecological categories used to classify denticles may also mask changes in specific species; in fact, fisheries data indicate that some Pacific shark populations have become increasingly unstable since the early 2000s. The long-term record appears to show that the Panamanian Pacific has historically had a high capacity to support large shark populations, and that this capacity may have protected them from past pressures; however, this does not guarantee that they will continue to withstand increased fishing pressure and other environmental changes.
It is worth remembering that today’s ecosystems are snapshots—and, at times, snapshots of ecosystems that have already been profoundly altered. The fossil record offers us another perspective on this moment captured in the present, revealing ecological changes that occurred long before scientists began counting sharks. But what happens when the environmental conditions that once protected a population also begin to change? Climate change is projected to reduce ocean productivity in some areas of the Eastern Tropical Pacific, which could weaken the very ecological conditions that have sustained its sharks for millennia. If productivity declines while fishing pressure remains high, could a population that has so far seemed resilient cross a threshold leading to rapid decline?
These are difficult questions, but they are precisely the kinds of questions that long-term ecological records allow us to ask. A tiny shark scale buried in ancient reef sediment may seem insignificant, but thousands of such scales, layered over time, can tell us not only how many sharks we’ve lost, but also what the ocean was capable of sustaining before we transformed it. Perhaps that is the baseline we need to build a smarter future for sharks. This new study is a strong case for it.
