A few days ago, a study revealed the existence of an extinction event that killed sharks in the high seas 19 million years ago. To realize this, the authors studied the distribution and abundance in sediments of microfossils called ichthyolites, which are made up of fragments of scales and teeth from sharks and other fish. Today, scientists at the University of California at Santa Barbara apply the same technique to study reef sharks in more recent periods, beginning 7,000 years ago.
Three times fewer sharks than in prehistoric times
They analyzed ichthyolites at an ancient coral reef called Bocas del Toro on the Caribbean coast of Panama, as well as three other modern sites. Their results, published in the journal Proceedings of the National Academy of Science, indicates that shark abundance in the region has declined nearly three-fold since prehistoric times, with fast-swimming species being hard-hit. Much of this reduction has occurred in the last 100 years, and especially in the second half of the 20th century. It is the fast-swimming pelagic shark that appears to have suffered the most from human activities.
The authors also compiled and analyzed archives corresponding to human habitations in the region and found that the fossil rock studied predates the first traces of human settlements in the region. This allows them to claim that it is essentially the effects of marine exploitation by newly settled communities that are responsible for the decline in shark diversity.
False-color scanning electron microscope images of the scales of several species of shark found on Caribbean reefs. Credits: Erin Dillon, George Ceballos, Aaron O’Dea, and Ashley Diedenhofen.
A Downfall That Isn’t Just Linked to Overfishing
The researchers also found that the types of sharks found on these reefs changed between prehistoric times and today. Pelagic species such as sand sharks and hammerhead sharks have declined more than bottom-dwelling bottom-dwelling species, such as nurse sharks. “If you had smelled these reefs a few thousand years ago, not only would sharks be more common, more pelagic sharks would have swam faster.”, study lead author Erin Dillon said in a statement.
She was also amazed by the fact that all kinds of sharks had declined during this period. “We did not expect such a major decline in nurse sharks over time, as they have little commercial value and are rarely targeted by fisheries in the region.This suggests that shark declines are not only related to overfishing, but may also be due to indirect factors such as habitat loss or reduced prey numbers. It now remains whether similar trends are observed elsewhere in the world, a work tackled by Erin Dillon, who collected sediment cores in regions with different human and ecological histories to better understand shark history over the past millennia. has worked to do.