The number of rays is growing in coral reef ecosystems around the world as overfishing obliterates shark populations.
The findings come from a global study using thousands of underwater cameras to survey shark and ray prevalence at 391 coral reefs in 67 nations.
It found far fewer sharks than would be expected, with populations of species including blacktip reef sharks, whitetip reef sharks, nurse sharks and grey reef sharks having completely disappeared from reefs in some places.
The populations of five of the most common reef shark species were 60 to 75 per cent below their expected abundance, based on a modelled scenario with no human pressures, the study found.
Colin Simpfendorfer at James Cook University in Townsville, Australia, who led the research, says the declines were probably due to overfishing. In remote reefs, or in areas where reefs were inside effective marine protected areas (MPAs), the sharks are far more abundant, he says – and this was more often the case in higher-income nations.
In many reefs where shark numbers have plummeted, ray numbers have increased, the study found. Numbers of yellow and southern stingrays in the Atlantic, and blue-spotted maskrays and blue-spotted ribbontail rays in the Indo-Pacific, were higher in places where one or more reef shark species were depleted. “We go from very shark-dominated communities to ray-dominated communities,” says Simpfendorfer.
These trends have a “cascading” effect throughout the entire reef ecology, he says. For example, without sharks to prey on them, the numbers of herbivorous fish soar. “They eat algae, and so we’ve actually seen on coral reefs that lose reef sharks, a lot less carbon is sequestered in the algae,” says Simpfendorfer.
As a result of the reef surveys, at least one species – the grey reef shark – has been reclassified by the International Union for Conservation of Nature as endangered.
Simpfendorfer says countries with coral reefs should be given more support to enforce MPAs. “One of the things that comes through very clearly is that MPAs which are well enforced are able to actually recover reef shark populations fairly quickly,” he says.