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. Alarming decline in sharks. Overfishing has wiped out over 70% of some shark and ray populations in the past 50 years, leaving a "gaping, growing hole" in ocean life: studyhttps://t.co/Nt4abhEhVgpic.twitter.com/diLp...