BURNABY, CANADA — Scientists say more than 70 percent of the world's sharks and rays disappeared from the ocean between 1970 and 2018, in large part due to overfishing — and more than three quarters of shark species are now threatened with extinction.

According to a study published in the journal Nature, there are 31 species of oceanic sharks and rays. Of these, three are critically endangered and 13 are endangered. Eight are listed as vulnerable and three as near threatened. Only six are classified as of least concern.

The study in Nature attributes the declines to decades of overfishing, notably longline fishing. Longlines can stretch to 100 km and catch fish and other wildlife indiscriminately.