HOW DOES PRODUCTIVITY SHAPE PRE-EXPLOITATION SHARK BASELINES AND RESILIENCE ON CORAL REEFS OVER MILLENNIA? A PRELIMINARY EXPLORATION

Shark populations worldwide have declined steeply over the last half century, but the patterns of change vary across space. Long-term records of shark abundance are limited, so it is challenging to quantify sharks’ natural spatiotemporal variability and the extent to which they have shifted from pre-exploitation baselines in regions with different environmental conditions. Here, we use shark scales (dermal denticles) accumulating in coral reef sediments to reconstruct shark communities during the mid-Holocene (~4-7ka) and today on both coasts of the Isthmus of Panama. The Tropical Eastern Pacific is a highly productive, dynamic system driven by seasonal upwelling with a long history of shark exploitation that continues today. The Caribbean coast, on the other hand, is oligotrophic and environmentally stable, with much lower rates of harvesting. We find that denticle accumulation rates, a proxy for shark abundance, are an order of magnitude greater on reefs in Pacific Panama (Gulf of Panama) than those in Caribbean Panama (Bocas del Toro). Comparing patterns over time, denticle accumulation rates declined by 71% since the mid-Holocene on reefs in Caribbean Panama, including a selective loss of pelagic sharks. In sharp contrast, modern denticle accumulation rates in Pacific Panama are comparable to their range of variability during the mid-Holocene, and the functional composition of denticle assemblages remained similar through time—suggesting that the shark community in the Gulf of Panama has persisted despite intensive fishing activity. We postulate that the region’s high productivity might underlie its high shark abundance and apparent resilience, although we remain unsure whether they are manifested through resource availability and/or life history traits. Our preliminary findings, albeit geographically restricted, provide insight into the drivers of variability in reef shark baselines and recovery potential in the face of ongoing overfishing.