Reconstructing Millennial-Scale Variability in Reef Shark Communities Across The Isthmus of Panama

Abstract

Shark populations have declined over the last half century, but the patterns of change vary across space. Long-term records of shark abundance are limited, making it challenging to determine how local environmental conditions influence pre-exploitation shark baselines and their susceptibility to human impacts. We use shark scales (dermal denticles) preserved in coral reef sediments to reconstruct shark communities during the mid-Holocene and today across the Isthmus of Panama. We interpret these data alongside records of primary productivity, habitat, and fish abundance to explore energy flow to higher trophic levels on each coast. The Tropical Eastern Pacific is a productive system driven by seasonal upwelling with a long history of shark exploitation. The Caribbean coast, on the other hand, is oligotrophic and environmentally stable, with lower rates of harvesting. We find that denticle accumulation rates, a proxy for shark abundance, are an order of magnitude greater in Pacific Panama (Gulf of Panama) than in Caribbean Panama (Bocas del Toro). Primary productivity and fish abundance are also higher in Pacific Panama, helping to support these large predator populations. Denticle accumulation rates declined by 71% since the mid-Holocene in Caribbean Panama, including a selective loss of pelagic sharks. In contrast, modern denticle accumulation rates in Pacific Panama are comparable to their range of variability during the mid-Holocene, and the composition of denticle assemblages remained similar through time—suggesting that sharks in the Gulf of Panama have persisted despite intensive fishing. We postulate that the region’s high productivity might underlie its high shark abundance and apparent resilience by increasing available resources for predators. Our findings shed light on the role of energy in shaping natural variability in shark baselines and recovery potential. They also highlight the importance of incorporating oceanographic context into shark management.