Sharks are missing from most coral reefs near where humans are fishing – even in protected areas.
This stark finding is revealed by one of the World’s largest ever studies of coral reef fish.
Sharks, and other reef predators such as large snappers, were found in just over a quarter – 28 per cent – of the scientists’ observations and were hardly seen at all at reefs where human pressure, through fishing or pollution, is high.
The research, which involved an international team of 37 scientists and which looked at almost 1,800 tropical coral reefs around the world, provides vital evidence on the most effective ways to protect fish populations.
Researchers used a new way of measuring the effects people are having on fish on the world’s reefs. The ‘human gravity’ scale calculates factors such as human population size, distance to reefs, and the transport infrastructure on land – which can determine reefs’ accessibility to fishermen and markets.
Where human gravity was high, the probability of encountering a top predator dropped to almost zero (less than 0.005). This scarcity is regardless of whether there are protections in place, such as ‘no-take’ marine reserves or restrictions on fishing equipment.
However, there is some good news for sharks. The study showed that the number of top predators in large remote marine reserves in areas with very low human pressures are much higher – more than quadruple the numbers found in remote lightly fished unprotected areas.
Lancaster University’s Professor Nick Graham, who surveyed reefs around the Chagos Islands, Maldives, Seychelles, Papua New Guinea, and the Great Barrier Reef said: “Large predatory fish such as sharks were extremely rare under any form of management where human influences were high.
“However, it is in protected areas located where human pressures are generally low that sharks are most commonly found. Clearly these remote protected areas are important for shark conservation.”
The research was not able to pinpoint why sharks do better in remote reserves, however the market value of fins make sharks particularly vulnerable to fishing. In addition, scientists believe the size of reserves in heavily fished areas are likely to be too small to protect sharks as they have large hunting ranges that likely expose them to fishing when they stray outside reserves.
The study also reveals that people are profoundly degrading communities of fish on coral reefs. Marine reserves located in remote areas with little human pressure have more than four times as many fish compared to reserves near highly fished areas.
However, the research shows that although those reserves within areas of high human pressure are relatively depleted, they play a vital conservation role, containing around five times as many fish as openly fished areas under similar human pressure.
“We showed that reef fish communities are greatly diminished inside and outside protected areas where human influences are greatest. Although somewhat depleted relative to pristine reefs, marine reserves in high human pressure areas can have a lot more fish than openly fished reefs,” said Professor Graham.
Professor Josh Cinner, lead author from the ARC Centre of Excellence for Coral Reef Studies, in Australia, said: “We found the greatest difference in fish biomass between openly fished areas and marine reserves was in locations with medium to high human pressure. This means that, for most fisheries species, marine reserves have the biggest bang where human pressures are medium to high.”
Political, cultural and ethical contexts limit the kinds of protection strategies that can be used on different coral reefs. Although the study reveals no-take reserves to be the best form of protection, other measures are also found to be beneficial.
Dr Christina Hicks, of Lancaster University and co-author of the study, said: “Other forms of fisheries management also had a role to play, for example restrictions on the types of fishing gears permitted, or controls on who could fish where. The amount of fish in these areas was greater than openly fished areas, though not as high as protected areas.”
“The study highlights how a diversity of conservation and management approaches are necessary under differing human pressures to maintain fish communities and key species.”
The research is detailed in the paper ‘The gravity of human impacts mediates coral reef conservation gains’, which is published by the Proceedings of the National Academy of Sciences of the United States of America (PNAS).