2023-08-10 09:30:28
An international research team of more than 150 researchers has managed to precisely quantify the decline of sharks on coral reefs. Result: 63% of the five main species have disappeared. The authors recommend the establishment of protected areas and to supervise the practice of fishing.
Sharks play a major role in the tropic chain, as they are a so-called “keystone” species for marine ecosystems. They are also considered apex predators, while otherwise presenting great vulnerability. For years, the scientific community has been sounding the alarm, because this population is threatened with extinction, due to the increasing pressure exerted by man. A new study, published in the revue Science, reveals the magnitude of the overall loss across the world. For five years, it monopolized more than 150 researchers from more than 120 institutions located in the four corners of the globe.
The scientists used an easy-to-reproduce process: baited underwater video. This method consists of placing bait in a cage to attract sharks, without them being able to reach it, but under the eye of a camera. It is thus possible to count and identify the different species that approach it. To get a comprehensive understanding of the decline of this population, more than 22,000 underwater video stations were placed in 391 different coral reefs, distributed in 67 countries and territories, equivalent to 90% of the world’s reefs.
The system put in place has made it possible to identify 104 different species of sharks, which represents more than 77% of those known to be present on coral reefs at some point in their lives. First result: a little more than half (53%) of the hundred species of sharks identified were counted less than ten times. A very low figure, which demonstrates the extent of the decline. Then it appears that the five main species of sharks living on coral reefs, i.e. gray reef shark, blacktip shark, whitetip shark, nurse shark and reef shark Caribbean, have declined by an average of 63% globally. This loss was calculated in relation to an estimate made by a numerical model of the expected abundance of sharks at each site, if there had been no anthropogenic pressure.
The decline is greater than expected
According to Colin Simpfendorfer, lead author of this study and professor at James Cook University in Australia, this is one of the best estimates of the decline in the population of shark species, due to the very large number of reefs and countries sampled. “This tells us that the problem of sharks on coral reefs is much worse and more widespread than anyone thought,” he adds. As for the reasons for this extinction, not surprisingly, widespread overfishing is the main culprit. The authors of the study write that “direct and indirect effects of fishing have resulted in shifts in species composition from shark-dominated assemblages to ray-dominated assemblages. Moreover, a complete loss of sharks and rays is even observed in about 7% of the reefs studied.
The results of this research have already been used to place four shark species on the IUCN (International Union for Conservation of Nature) Red List of Most Endangered Species. They were also presented at the latest conference of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), to help governments around the world better regulate trade in these species of sharks.
On a planetary scale, the Indo-Pacific region has globally better preserved shark populations than those of the Atlantic. The places where the decline is least significant are sanctuaries, that is to say places where each species is totally protected from human activities. Although reef sharks are at considerable risk in many places, the study still demonstrates that reef decline has relatively little effect on reefs tens to hundreds of kilometers away. The authors estimate that even in an area threatened with extinction, the potential for replenishing the abundance of shark species is relatively high, but on condition of protecting it and prohibiting fishing for these animals.
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