Big Sharks Keep Disappearing From Shorelines, and That's a Problem

"I think it is fair to say that humans are now a much greater threat to sharks than sharks are to humans."

shark

For all of our fear of and obsession with shark attacks, it’s really the sharks that should be afraid. While killer whales are no friends to sharks, it’s humans who are their greatest threat. It isn’t just that we kill them: A study released Tuesday in PLOS Biology demonstrates that human activity, more so than any ecological factor, is the biggest factor in deciding where they live and which species disappear.

“Humans kill about 100 million sharks per year and in contrast about five humans are killed by sharks per year,” lead author Tom Letessier, Ph.D., tells Inverse. “I think it is fair to say that humans are now a much greater threat to sharks than sharks are to humans.”

Letessier is a marine ecologist and research fellow at the Zoological Society of London, an international conservation charity. He and an international team of scientists used 1,041 midwater baited videos, taken across the Pacific and Indian oceans, to survey sharks and other predators like the black marlin and mahi-mahi. In the process, they measured the size and abundance of 841 individual sharks, comprising 19 different species, across nine different regions.

Doing so revealed a troubling trend: The closer sharks are in proximity to cities with more than 10,000 people, the smaller they are physically, and the fewer of them there are.

Silver-tip sharks and manta rays were caught on camera during the study. 

The minimum distance where there was no measurable effect on the sharks from the influence of people and fishing was 932 miles away from shore. The team notes that this is much farther than the minimum distance established in other studies, suggesting that larger sharks are moving farther and farther away from the more coastal regions they once frequented.

Overall, they didn’t even really see too many sharks, observing them only at 12 percent of the sites they monitored.

They also noticed that sharks were markedly smaller in size in areas that were warmer than 82 degrees Fahrenheit. They say this isn’t too surprising, considering that smaller sharks are known to live in tropical waters. This could, however, be a problem for bigger sharks as the oceans continue to warm.

“There are now fewer sharks in proximity to human cities and markets,” Letessier explains. “The average size of the predator community, both sharks and other fish, has declined in proximities with cities and markets as well.”

shark fins
Seized dried shark fins displayed during a Hong Kong Customs and Excise Department presentation. 

Since the ‘50s, industrial fishing has expanded globally in physical range and in haul. Sharks are especially targeted because of the popularity of shark fin soup. Today, catches amount to 1.4 million tonnes per year — more than double the amount caught 60 years ago.

Sixty percent of shark species are threatened as a result of this overexploitation, a 2018 study published in Marine Policy showed. Given that only 12 percent of shark fisheries are considered sustainable and legal, that means 25,000 tonnes of dried fins likely come from illegal fisheries each year.

Shark finning is increasingly ongoing in the open seas, which could explain why larger sharks aren’t observed as often in the regions far from shore that they used to frequent. Just 13.2 percent of Earth’s oceans are considered “wilderness refuges” — areas beyond the detectable range of local human pressures.

Letessier hopes that, this research results in decisive action and policy, “including the implementation of large marine protected areas where no fishing is allowed, in order to protect the last refuges and remaining coastal hotspots.”

Healthy oceans need sharks. Their presence maintains the balance of ocean ecosystems because as top predators, they dictate the population numbers of other sea creatures in reef food chains. Without them there’s a fundamental change in the balance.

“If we don’t ensure sustainable management and effective conservation measures,” Letessier warns, “sharks risk becoming functionally extinct in our oceans, with likely severe and unforeseeable consequences for global food security and future generations.”

Abstract:

Since the 1950s, industrial fisheries have expanded globally, as fishing vessels are required to travel further afield for fishing opportunities. Technological advancements and fishery subsidies have granted ever-increasing access to populations of sharks, tunas, billfishes, and other predators. Wilderness refuges, defined here as areas beyond the detectable range of human influence, are therefore increasingly rare. In order to achieve marine resources sustainability, large no-take marine protected areas (MPAs) with pelagic components are being implemented. However, such conservation efforts require knowledge of the critical habitats for predators, both across shallow reefs and the deeper ocean. Here, we fill this gap in knowledge across the Indo-Pacific by using 1,041 midwater baited videos to survey sharks and other pelagic predators such as rainbow runner (Elagatis bipinnulata), mahi-mahi (Coryphaena hippurus), and black marlin (Istiompax indica). We modeled three key predator community attributes: vertebrate species richness, mean maximum body size, and shark abundance as a function of geomorphology, environmental conditions, and human pressures. All attributes were primarily driven by geomorphology (35%−62% variance explained) and environmental conditions (14%−49%). While human pressures had no influence on species richness, both body size and shark abundance responded strongly to distance to human markets (12%−20%). Refuges were identified at more than 1,250 km from human markets for body size and for shark abundance. These refuges were identified as remote and shallow seabed features, such as seamounts, submerged banks, and reefs. Worryingly, hotpots of large individuals and of shark abundance are presently under-represented within no-take MPAs that aim to effectively protect marine predators, such as the British Indian Ocean Territory. Population recovery of predators is unlikely to occur without strategic placement and effective enforcement of large no-take MPAs in both coastal and remote locations.