Sharks can be dangerous to us – but actually it’s the other way around. How humans have harmed shark populations can be shown by analyzes of tiny remains of sea predators in sediments, researchers illustrate using the example of the Caribbean. In up to 7000 year old deposits of shark skin teeth there is a reflection of the drastic decline of the population since humans hunted the predatory fish and destroyed their livelihoods. Information about the initial natural state and the background to the losses could benefit efforts to protect the ecologically important predators in the future, say the researchers.
Overfishing, habitat destruction and climate change: It is well known that man’s machinations lead to sometimes large losses of many marine animal species. The predators of the oceans are particularly hard hit: The global shark populations have collapsed dramatically and some species are threatened with extinction. This damage is not only limited to the predatory fish themselves: they play an important ecological role, especially in coral reefs – their loss can thus critically upset the ecological community. In order to counteract this, information about the precise influence of humans is important. “To do this, we need insights into what a healthy shark community in coral reefs looked like before human exploitation,” says Erin Dillon of the University of California at Santa Barbara.
Tiny traces in sediments
She and her colleagues have now explored the extent to which the investigation of shark skin teeth (denticles) in sediments is suitable for obtaining data on the development of the size and diversity of shark populations in coral reefs. These are the tiny scales that cover the body of these fish. They reduce resistance when swimming and protect the animals from harm. “The denticles are the reason the shark’s skin feels smooth in one direction when you stroke it, but as rough as sandpaper in the other. Sharks are covered in millions of these tiny teeth, ”explains Dillon. In the course of their life, the fish constantly shed their denticles through regeneration processes, as a result of which they collect in sediments. Due to different characteristics, they can also be assigned to different representatives of the sharks.
As part of their study, the scientists examined the frequency and composition of fossil shark denticles from marine sediments of an approximately 7,000 year old coral reef on the coast of Panama. The age of the layers was determined by means of the so-called uranium-thorium dating. The results were then compared with findings from shark denticles in modern deposits. The researchers also collected information about the settlement history and fishing practices in the region in order to be able to relate the data obtained to human influence.
Decline by over 70 percent
It became fundamentally clear: The frequency of the denticles and thus the population density of the sharks was more than three times as high before humans began to use the marine resources in the region. The abundances of all denticle types were found to decrease over time, but those derived from commercially fished species decreased the most. The biggest decline was in the fast-swimming species – the so-called pelagic sharks such as hammerheads and requiem sharks. By far the steepest decline in deposits began in the late 20th century. As the researchers explain, increased hunting is emerging on the Panamanian coast, which has been selectively targeting pelagic sharks. However, the decline in nurse sharks, which have rarely been caught, suggests that indirect factors such as habitat degradation or loss of prey are also an important nuisance.
“When the Spaniards arrived in America, they reported seas full of sharks,” says Dillon. “But nowadays we only see very few – we are lucky if we occasionally see a nurse shark. Our data now document how the shark populations were decimated by both intensive fishing and the destruction of the habitat, which began above all with the expansion of banana cultivation and the development of the coast ”. According to the researcher, there is a connection with water pollution: “Today there is so much land runoff from the coast that in some places it is like swimming through lemonade”.
Dillon and her colleagues now see potential in their method for shark research and the protection of predatory fish: “We are planning to extend the process to other locations in order to investigate broader geographical patterns of changes in reef shark communities over long ecological time scales,” so Dillon. Concluding, Senior Author Aaron O’Dea of the Smithsonian Tropical Research Institute in Balboa said, “The study shows that denticle deposits can be used to reconstruct the evolution of shark communities over time. Sharks are an integral part of ocean health and play an important role in the diversity and functioning of coral reefs. Empirical information about past shark abundances and the composition of the species can help us to better assess what is natural in the seas, ”says O’Dea.
Source: Smithsonian Tropical Research Institute, article: PNAS, doi: 10.1073 / pnas.2017735118