Human fishing reshaped Caribbean reef food webs, 7000-year old exposed fossilized reefs reveal

When we think of fossils, giant prehistoric creatures like dinosaurs may come to mind. But the fossil record also holds the remains of smaller organisms, such as fish and corals, that tell us about our oceans’ past.

Scientists at the Smithsonian Tropical Research Institute (STRI) recently studied exposed fossilized coral reefs from Panama's Bocas del Toro Province and the Dominican Republic, comparing them with nearby modern reefs. These exceptionally well-preserved reefs date back 7,000 years, offering a unique window into what Caribbean reefs looked like before human impact. Within the fine sediments of these ancient reefs, the team discovered thousands of tiny fish ear bones and shark scales, allowing them to reconstruct entire ancient fish communities.

The results revealed a dramatic shift in fish communities over time: sharks have declined by 75% and human-targeted fish have become 22% smaller. But the real surprise came from the prey fish species — those eaten by predators like sharks. These have doubled in abundance and grown 17% larger on modern reefs. This study provides the first historical evidence for the "predator release effect" — where removing top predators allows their prey to flourish. Whilst scientists have long predicted such an effect, evidence for it was scarce without knowing what reefs looked like before human impact. Remarkably, the tiniest reef fish that shelter in coral crevices, showed no change in size or abundance over millennia. Their stability suggests a remarkable resilience to the multitude of changes occurring on reefs at higher layers of the food chain.

To compare fossilized and modern reefs, scientists collected, quantified and measured thousands of skeletal remains, including the tiny tooth-like scales that give shark skin a sandpapery texture, called dermal denticles.

To study the abundance and size of prey fish and small coral reef-sheltered fish (also known as cryptobenthic fishes), they also examined fish otoliths — the calcium carbonate structures found in fishes' inner ears. Because otoliths grow in layers, scientists can estimate a fish’s size at death. In total, the team examined 807 denticles and 5,724 otoliths.

The behavior of some organisms can also leave a fossil record. In this study, scientists measured the frequency and size of damselfish bite marks on coral branches from both fossilized and modern reefs. They found that the number of bites has increased in modern reefs — also indicating the rise in prey fish populations.

These results illustrate an important change in food webs of modern Caribbean reefs: with fewer sharks and other predatory fish to control the population of exposed prey fishes, they have become bigger and more abundant, reflecting release from predation. On the other hand, small reef-sheltered fish remained unchanged in size and abundance over thousands of years, suggesting that the degradation of water quality and habitat in the region did not drive the changes in community structure.

This study demonstrates the power of the fossil record for future conservation. By revealing what reefs looked like before intensive human fishing, these 7,000-year-old fossils provide the missing baseline critical to understand the food webs of pre-human coral reefs, and document which elements of reefs changed and which are resilient.

This research, published in the Proceedings of the National Academy of Sciences, PNAS, was a collaboration among scientists from the Smithsonian Tropical Research Institute (STRI), the Sistema Nacional de Investigación (SENACYT) in Panama, the Marine Science Institute at the University of Texas at Austin, the Center for Biodiversity Outcomes at Arizona State University, the Graduate School of Oceanography at the University of Rhode Island, The Nature Conservancy, the Biodiversity Research Center at Academia Sinica in Taiwan, the Department of Earth & Environmental Sciences at Boston College, and the Cotsen Institute of Archaeology and Department of Anthropology at the University of California, Los Angeles.

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