Science | biology
Pressure from overfishing, aggravated by water pollution and climate change, have put two-thirds of these animal species in serious jeopardy.
Coral reefs are one of the most diverse ecosystems on the planet (a third of the ocean's fish species live there), but they are also one of the most threatened. In recent decades, intensive fishing, compounded by the impact of climate change, habitat deterioration, water pollution, and coastal development, has pushed many of its animals and plants to the limit. Among them, sharks and manta rays are in a particularly critical situation, according to research published Tuesday in the journal Nature Communications.
To quantify the status, evolution and threats facing reef sharks and rays around the world, the scientists used the International Union for Conservation of Nature (IUCN) Red List as a reference. Of the 134 studied species that live in this ecosystem, 79 are threatened, according to the IUCN, 14 of them in a very critical condition, 24 in a critical condition and 41 in a vulnerable situation. In addition, regardless of their degree of threat, a total of 94 have a decreasing population. The only species with a growing population is the bluespotted skate.
In other words, almost two thirds (59%) of sharks and rays in coral reefs are in danger of extinction, a figure that doubles the threat to which the 1,199 known species of sharks and rays are exposed. In addition, the researchers also compared their status with all other coral reef species and the results confirmed that these animals are the second most threatened group in the ecosystem, after marine mammals, which include eight species of dolphins and manatees.
Blue-spotted manta ray, the only coral reef species that is increasing in population. /
"The main threat, by far, is intensive fishing," says Samantha Sherman, a marine biologist and research fellow at Simon Fraser University in Canada, and lead author of the study. “Every coral reef shark and ray species listed in an IUCN Red List threatened category has 'fishing' as one of its causes. This is due to the proximity of the reefs to the large coastal communities that depend on the ocean for food and income,” she maintains.
Thus, they highlight that the risk of extinction of these animals is higher in nations with more intense fishing pressure and weaker governance, such as Brazil, Tanzania and Indonesia. "The greatest risk to sharks is in the western Atlantic and parts of the Indian Ocean. For rays, in the Indian Ocean and Southeast Asia. These regions are highly exploited and currently do not have the necessary management to reduce the impact of fishing on these species," laments Sherman.
This situation is mainly affected by fourteen species (five sharks and nine rays) listed as 'Critically Endangered' (the highest threat category) on the IUCN Red List. Among them, the gray shark and the 'rhinoceros ray' species, named for their similarity to sharks. The risk is higher for larger-bodied species, which are the most widely distributed in the world, as they tend to have lower population growth rates, less ability to withstand fishing mortality, and generally are the most commercially valuable, according to the study.
Two shovelnose rays, a type of 'rhinoceros ray', off the Australian coast. /
Apart from these animals, Sherman warns that pelagic species (those that live in mid-water or near the surface and have practically no contact with the seabed or the coast) are also highly threatened. 77% of the 31 species are in a risk category, according to the IUCN Red List. This is because "many reef species are not found in deeper water, which means they are more accessible to a wide variety of fishermen and fishing gear, without shelter in the depths," the biologist says.
Both sharks and manta rays are caught in large-scale and small-scale industrial fisheries. Coral reef fisheries directly support the livelihoods and food security of more than 500 million people. However, the human footprint left on these ecosystems far exceeds the productivity of reefs in many parts of the world. Sharks are traded internationally mainly for their fins and meat; and the rays for their meat and their skins, which become leather. Other common uses include display in aquariums (27%), use as animal feed (19%), and drug development (11%).
As a consequence, the ecological pyramids are destabilized. "Sharks and rays play key ecological roles on reefs that cannot be filled by other species. For example, gray reef sharks eat mostly pelagic (not reef) fish, but then transfer those nutrients to the reef, and they are key to coral growth," Sherman says. In other words, healthy populations of reef sharks and rays help to maintain these ecosystems in good condition, while their absence affects both the abundance and the behavior of other species.
A porcupine ray near a jetty. /
If urgent action is not taken, the researchers predict there will be escalating ecological consequences for coral reefs, many of which will be difficult or impossible to reverse. “We are already seeing functional extinctions of reef sharks globally. Functional extinctions mean that there are not enough individuals of the species to perform its ecological role. We need to act now to ensure that these functional extinctions do not spread to more reefs," Sherman asks.
So far, the measures that have been taken are few. "Many reef species are found in different countries, which means that conservation efforts require global cooperation," says the scientist. “In November 2022, governments around the world voted to apply international trade regulations for 104 species of sharks and rays. More recently, 21 coral reef species were listed, in addition to the 18 species already 'protected' by these regulations. This is a step in the right direction, they still need to be implemented and it doesn't stop the problem of mortality related to (involuntary) bycatch."
Although the plight of sharks was already widely known, this study shows that rays are in greater danger on coral reefs. Despite everything, there are very few limitations on catching rays. "Solutions to this problem are similar for both animals: fishing limits, well-located and properly implemented Marine Protected Areas, and alternative livelihood solutions to reduce the number of fishermen on coral reefs," he concludes. Sherman.