Long before intensive human fishing began, Pacific reefs naturally supported far more sharks than Caribbean reefs, according to a new study. The findings, based on an analysis of fossilized shark scales, suggest that differences in ocean productivity – and not fishing alone – have shaped shark populations for thousands of years, information that could guide future conservation efforts. Shark populations have declined worldwide, especially on coral reefs, where many species are now at high risk of extinction. However, conservation efforts have been hindered by a lack of historical baselines showing what healthy shark populations looked like before widespread human impacts, making it difficult to set realistic conservation and recovery goals. To address this gap, Erin Dillon and colleagues reconstructed reef shark communities spanning the past 7,000 years by analyzing fossilized shark dermal denticles – tiny tooth-like scales preserved in coral reef sediments. Denticle abundance in reef sediments closely reflects shark populations.

Dillon et al. collected fossil denticles from the Caribbean and Pacific coasts of Panama, two neighboring regions with contrasting ocean conditions and levels of human activity but many of the same shark species. By comparing ancient (~7 to 3 ka) and recent (past ~100 years) sediments, the authors were able to determine natural baselines for reef shark populations and evaluate how environmental conditions and human pressures have shaped their decline over time. The findings revealed that healthy reef shark populations naturally differed between Pacific and Caribbean reefs. Pacific reefs supported about 20 times more sharks than Caribbean reefs thousands of years before intensive fishing. Since then, Caribbean shark populations, particularly fast-swimming pelagic species, have declined by roughly 75%, while Pacific populations have remained comparatively stable, despite experiencing higher historical fishing pressure.  As a result, the gap between the two regions has widened dramatically, with modern Pacific reefs now supporting about 100 times more sharks than comparable Caribbean reefs. Dillon et al. conclude that differences in ocean productivity have long played a crucial role in determining how many sharks a reef can naturally support. The Pacific coast of Panama, fueled by nutrient-rich seasonal upwelling, has historically sustained much larger shark populations and appears more resilient to human impacts than the less productive Caribbean, where shark populations have declined far more dramatically. These findings suggest that conservation goals should be tailored to regional ecological conditions rather than applying a single baseline across different ecosystems. In a related Perspective, Peter Roopnarine discusses the study in greater detail.