Fossil evidence shows long-term decline of sharks on Caribbean reefs

In a time before cities, sonar and sportfishing boats, Caribbean reefs pulsed with life; sharks cruising the shallows, snappers gliding through coral thickets. Today, their echoes remain not in photographs or surveys, but in the sediment itself, tiny bones and shark scales, buried beneath centuries of sand and sea.

A new study published in PNAS unearths these ancient biological breadcrumbs to reconstruct an ecological story that spans millennia. The research reveals that over the past 7,000 years, human activity has dramatically reshaped reef ecosystems, chiefly through the slow but steady disappearance of top predators like sharks. The cascading effects, rarely demonstrated at such a scale, ripple through the food web, giving rise to prey species that grow larger and more abundant.

For co-author Katie Cramer, a marine conservation ecologist and research professor at Arizona State University’s School of Life Sciences, this kind of work is about “creating a continuous time series of how humans have impacted these ecosystems over thousands of years to the present.” Her aim is to understand not just how reefs have changed, but why and how those insights might shape future conservation.

Cramer’s scientific toolkit is as eclectic as her timeline. She collects sediment cores from reefs in Panamá and Belize, sometimes drilling into living coral colonies to extract their layered skeletons, which serve as archives of environmental history. These sediment tubes are filled with what she calls “fossils like fish teeth, coral skeletons, mollusk shells and urchin spines,” a dense record of a reef’s life, death and everything in between.

What makes this study stand out is its integration of prehistoric, archaeological, and modern data. Using over 6,000 fish remains including otoliths (fish ear bones) and dermal denticles (tiny, scale-like structures shed by sharks), the team compared reef communities from the mid-Holocene to those of today. The results showed a more than 75% decline in shark presence and a dramatic rise in both the size and abundance of prey species such as anchovies and damselfish.

“These reefs used to be full of large predatory fish,” Cramer explains. “What we’re seeing now isn’t just a phase, it’s a shifted ecosystem, one that’s been altered by thousands of years of human fishing.”

But not all reef inhabitants reacted to predator loss. The smallest species, cryptobenthic fish like gobies and cardinalfish, remained largely unchanged in both size and abundance over time. According to Cramer, this is because “they’re tiny, camouflaged and reproduce quickly,” making them naturally resilient to both predators and environmental change.

This contrast between exposed prey fish and hidden reef dwellers highlights the uneven ways ecosystems respond to top-down pressure. It also challenges the long-standing ecological debate between bottom-up (environment-driven) and top-down (predator-driven) control. For Cramer, this paper contributes important evidence to the latter.

The study’s timeline stretches across major cultural shifts, from Indigenous subsistence fishing to European colonization and commercial export markets. "We’re talking about a period that transcends different types of resource use," Cramer says. "And the reef fish populations really only started to decline in a serious way very recently.”

Still, she’s quick to emphasize that this isn't a condemnation of local communities. “It’s not subsistence fishing that’s the issue,” she notes. “It’s more the commercial fishing pressure, which is often at unsustainable levels of extraction, that's caused this long-term shift.”

Despite the gravity of the findings, Cramer doesn’t see the story as purely bleak. “Yes, climate change is the dominant signal now, and it’s accelerating,” she says. “But that doesn’t mean we’re powerless.”

The key, she explains, is to boost reef resilience. Healthy herbivorous fish populations, for example, help prevent algae from overtaking coral and giving new coral polyps space to grow after bleaching events. “If algae takes over, there’s no room for baby corals to settle and grow back,” she says. “But if fish are keeping the algae in check, reefs have a better shot at recovery.”

She also points to encouraging examples of species rebounding after protection. “Whales came back after commercial whaling ended. Sharks are starting to recover in places where finning has been banned. These things work,” she says. “We’ve seen it happen.”

One major contribution of this study, she adds, is the establishment of historical baselines. “If a country wants to set a restoration target for shark abundance, we now have a chart for that. We know what ‘natural’ looked like.”

For Cramer, who grew up in Phoenix and now works remotely from San Diego, handling ancient coral and fish remains is a deeply reflective experience. “It’s neat to think about what these corals lived through… all the cultural changes, all the environmental shifts,” she says. “Some of the materials we studied predate European contact. That’s powerful.”

And while she admits that the reefs we know today may never return to their prehistoric vibrancy, that doesn’t mean the story is over. “There still are parts of the ocean with beautiful, healthy reefs,” she says. “People don’t realize that. The Great Barrier Reef makes headlines, but there are still places holding on.”

What she hopes, more than anything, is that people begin to expect more from reefs—not settle for today’s degraded systems as the new normal. “It’s powerful to realize that what you're seeing now might not be natural,” she says. “And it’s even more powerful to imagine that we can bring some of that abundance back.”