Since the second half of the twentieth century the species has undergone a rapid decline. The disappearance of this apex predator will have negative effects on the entire ecosystem of the basin.
The majority of the planet's apex predators are declining rapidly and risk disappearing, triggering serious cascade effects that will have repercussions across entire ecosystems. This phenomenon affects both terrestrial predators, such as big cats, and marine ones, such as sharks. The future of the most feared (undeservedly so) marine carnivore, the great white shark (Carcharodon carcharias), is also uncertain. The species is indeed at risk of extinction in the Mediterranean.
This was revealed by the study Abundance and distribution of the white shark in the Mediterranean Sea, conducted by a group of researchers from the Sapienza University of Rome, in collaboration with Stanford University and the University of Virginia, and with the support of the Institute for Biological Resources and Marine Biotechnologies of the Cnr, the Anton Dohrn Zoological Station of Naples, and Ispra. For the first time, the scientists estimated the distribution of the great white shark in the Mare Nostrum over the past 160 years.
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We are accustomed to associating this large pelagic shark, capable of exceeding six metres in length, with oceanic environments, but the great white shark, the Cnr noted, has inhabited the Mediterranean for centuries, and historical records of sightings date back as far as the Middle Ages.
In the basin the species is classified as "critically endangered", but a lack of information makes it impossible to draw up targeted conservation and management plans. The aim of the study is precisely to gather as much data as possible in order to gain a clearer picture of the distribution of great white sharks, their habitat use and population fluctuations.
The researchers collected 773 observations relating to the presence of the great white shark in the Mediterranean between 1860 and 2016 and used them to develop spatial and temporal models. The findings of the research, as stated in the study, "highlighted a complex trajectory, characterised by a progressive increase in the number of sharks followed by a rapid decline that began in the second half of the twentieth century."
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The decline in populations was not uniform, but was particularly severe in certain areas. "In the central Mediterranean a reduction of 52 per cent was recorded, while in the Sea of Marmara it reached 96 per cent," explained Giovanna Jona-Lasinio, professor in the Department of Statistical Sciences at La Sapienza. "The decrease is, moreover, often accompanied by a reduction in the areas occupied — a signal associated with at-risk populations."
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The great white shark is an apex predator, at the very top of the food chain; its disappearance would therefore disrupt the entire marine trophic chain, causing, for example, a population explosion among its prey. "It has been demonstrated that the removal of super predators from marine ecosystems leads to disastrous top-down effects that reverberate throughout the entire food chain," said Stefano Moro, of the Department of Environmental Biology at the Rome university. "The Mediterranean, from this point of view, holds a negative record, with more than 50 per cent of shark species classified by the Iucn as regionally threatened."
The actual impacts of the disappearance of these ancient and fascinating fish are, however, still poorly understood. What is certain is that of the forty-seven species of shark living in the Mediterranean, twenty of which are large predators at the apex of the food pyramid, the majority are declining due to anthropogenic impact. Sharks, in particular, are victims of accidental bycatch in deep-sea fishing, targeted fishing and human pressure on coastal areas. As well as declining in number, Mediterranean sharks are increasingly smaller — a pale shadow of the "monsters" that swam in our seas until just a few centuries ago, and that now live on only in fantastical tales.