Scientists have known for decades that individual shark species are declining, but a new study drawing on 57 global datasets underscores just how dramatically worldwide populations have collapsed in the past half century. Globally, the abundance of oceanic sharks and rays dropped more than 70% between 1970 and 2018, according to a study published Wednesday in the journal Nature. And 24 of the 31 species of sharks and rays are threatened with extinction, while three species—oceanic whitetip sharks, scalloped hammerhead sharks, and great hammerhead sharks—are considered critically endangered, the AP reports. "The last 50 years have been pretty devastating for global shark populations,” says study co-author Nathan Pacoureau, a biologist at Simon Fraser University.
Sometimes sharks are intentionally caught by fishing fleets, but more often they are reeled in incidentally as "bycatch," in the course of fishing for other species such as tuna and swordfish. Sharks and rays are both fish with skeletons made of cartilage, not bone. In contrast to most other kinds of fish, they generally take several years to reach sexual maturity, and they produce fewer offspring. "In terms of timing, they reproduce more like mammals—and that makes them especially vulnerable," says Pacoureau. "Their populations cannot replenish as quickly as many other kinds of fish." The number of fishing vessels trolling the open ocean has risen steeply since the 1950s, as engine power expanded ships' range. And while climate change and pollution also imperil shark survival, increased fishing pressure is the greatest threat for every oceanic shark species.
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