Ray numbers are growing in coral reef ecosystems around the world as overfishing wipes out shark populations.
The findings come from a global study that used thousands of underwater cameras to study the prevalence of sharks and rays in 391 coral reefs in 67 countries.
It found far fewer sharks than expected, with populations of species including blacktip reef sharks, whitetip reef sharks, nurse sharks, and gray reef sharks disappearing from the reefs entirely in some locations.
Populations of five of the most common reef shark species were 60 to 75 percent below their expected abundance, based on a scenario modeled without human pressures, the study found.
Colin Simfendorfer at James Cook University in Townsville, Australia, who led the research, says the declines were likely due to overfishing. On remote reefs, or in areas where the reefs were within effective marine protected areas (MPAs), sharks are much more abundant, he says, and this was more often the case in higher-income nations.
On many reefs where shark numbers have plummeted, ray numbers have increased, the study found. Numbers of yellow and southern rays in the Atlantic, and blue-spotted mask rays and blue-spotted ribbontail rays in the Indo-Pacific, were highest where one or more reef shark species were depleted. “We go from highly shark-dominated communities to ray-dominated communities,” says Simpfendorfer.
These trends have a “cascading” effect on the entire reef ecology, he says. For example, without sharks to prey on them, the number of herbivorous fish skyrockets. “They feed on algae, so we’ve seen that in coral reefs that lose reef sharks, the algae sequester much less carbon,” says Simpfendorfer.
As a result of reef surveys, at least one species, the gray reef shark, has been reclassified by the International Union for Conservation of Nature as endangered.
Simpfendorfer says that countries with coral reefs should receive more support to enforce MPAs. “One of the things that is seen very clearly is that MPAs that are applied well can rebuild reef shark populations quite quickly,” she says.