Fossil Shark Scales Reveal How Ocean Productivity Shapes Shark Resilience

 

A new international study published in Science has revealed how differences in ocean productivity can shape shark abundance and resilience to human pressure. Dr Jonathan CYBULSKI, currently with the School of Biological Sciences and the Swire Institute of Marine Science (SWIMS) at The University of Hong Kong (HKU), was among the researchers involved in the study.

The researchers reconstructed thousands of years of shark history on the Pacific and Caribbean sides of the Isthmus of Panama using dermal denticles — tiny, tooth-like scales shed naturally by sharks and preserved in seafloor sediments. Because different types of sharks produce denticles with different forms, these microscopic fossils provide scientists with a record of past shark communities.

The method itself took more than a decade to build. “This project began 12 years ago with a wild idea to search for shark scales in reef sediments. Finding the first denticle was exhilarating,” said Dr. Erin Dillon (lead author) and collaborator of Dr. Cybulski at the Smithsonian Tropical Research Institute (STRI). “We had to work out what denticles could tell us, how to get them out of reef sediments and how to read a fossil assemblage. Panama represented the perfect place for applying the approach, because it let us compare shark baselines across two very different oceans separated by a single strip of land.”

 

By comparing denticles dating from several thousand years ago with those from the past century, the team was able to examine shark abundance before and after intensive human fishing.

The results revealed a striking natural difference between the two oceans. Even before significant human exploitation, the Pacific side supported around 20 times more sharks than the Caribbean side, despite their geographical proximity and similarities in shark communities.

Their responses to human pressure were also very different. Shark abundance in the Caribbean declined by around 75%, while no comparable decline was detected on the Pacific side even though it experienced >95% of the historical fishing pressure.

The researchers attribute this contrast largely to differences in ocean productivity. Nutrient-rich Pacific waters support greater primary productivity and higher fish biomass, providing more food for sharks and potentially buffering their populations against fishing pressure. The less productive Caribbean naturally supports lower shark biomass, making its populations more vulnerable to additional human impacts.

“This study shows the difference that environmental factors can play in controlling biomass in our oceans,” said Dr Cybulski. “In the Caribbean, with less nutrients and overall productivity, we have less shark biomass at the top of the food chain. Conversely, in the nutrient-rich Pacific with higher primary productivity, we have greater shark biomass through time.”

During his postdoctoral research in Panama at STRI, following his PhD at HKU, Dr. Cybulski was involved in collecting reef-matrix cores from which Pacific shark denticles were recovered, and subsequently participated in the analysis and interpretation of the data.

The findings are not, however, a clean bill of health for Pacific sharks. “Our samples from the recent time point are not capturing the whole story for modern shark populations, because our sampling methods averaged several hundreds of years’ worth of data,” said Dr. Cybulski. “We know that fishing in the Gulf of Panama, and globally, has increased in the last several decades. As a result of increased fishing effort, nearly 1/3 of all sharks and rays are now critically threatened. Additionally, human-induced climate change is altering known ocean productivity processes, such as upwelling in the Gulf of Panama. So, although we found the sharks in the Pacific may have a high capacity to recover, modern impacts from fishing and climate change may push them to a limit they have never experienced."

 

The findings highlight the importance of establishing historical ecological baselines when assessing changes in marine populations. Understanding how environmental conditions naturally influence shark abundance can help inform more realistic and locally appropriate conservation and recovery targets.

 

To access the paper, please visit: https://www.science.org/doi/10.1126/science.aec2144