A new survey that included more than 400 coral reefs around the world found that species of sharks once common in the waters around the reef are disappearing with the findings noting that sharks are at a greater risk of going extinct than previous studies suggested.

The study, published in the journal Science said that the five most common reef shark species have experienced a decline of up to 73%, adding that “As shark species decline on coral reefs, ray species increase, indicating a community-wide shift.”

The study found that coral reef ecosystems are facing increasing pressure from human activities, such as overfishing, pollution, and climate change. These activities are threatening the survival of many species that play important roles in the ecosystem.

According to the journal Science:

Our species-level analysis revealed global declines of 60 to 73% for five common resident reef shark species and that individual shark species were not detected at 34 to 47% of surveyed reefs.

As reefs become more shark-depleted, rays begin to dominate assemblages. Shark-dominated assemblages persist in wealthy nations with strong governance and in highly protected areas, whereas poverty, weak governance, and a lack of shark management are associated with depauperate assemblages mainly composed of rays.

Without action to address these diversity deficits, loss of ecological function and ecosystem services will increasingly affect human communities.

The study added that the decline in shark populations was most severe in poorer countries with weak regulations. In contrast, shark populations were much healthier in wealthier countries and in protected areas.

Fewer Sharks than Expected

To conduct the survey, researchers used thousands of underwater cameras to monitor shark and ray prevalence at 391 coral reefs in 67 nations.

The troubling results found far fewer sharks than would be expected, with the complete disappearance from reefs in some places of some populations of species including blacktip reef sharks, whitetip reef sharks, nurse sharks, and grey reef shark.

The decline in shark populations was most severe in poorer countries with weak regulations. In contrast, shark populations were much healthier in wealthier countries and in protected areas.

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.

“It’s absolutely jaw-dropping,” David Shiffman, a marine biologist at Arizona State University not involved in the survey work, told The Washington Post.

“And it’s telling that if you look at all the reef sharks at all the reefs everywhere, you get a very similar pattern.”

The Rays Rise

The survey added that the disappearance of shark populations has given way to rays to fill the absence.

According to the findings, the four key ray species (yellow and southern stingrays in the Atlantic; blue-spotted mask and blue-spotted ribbontail rays in the Indo-Pacific) increased only with the depletion of one or more resident reef shark species, with rays dominating in the most shark-depleted areas.

“These predictable changes in assemblage provide the ability to infer the status of reef shark populations, and the level of human pressure they are experiencing, in future surveys,” the report said.

You can read more about the findings in “Widespread diversity deficits of coral reef sharks and rays” here.