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The number of oceanic sharks and rays worldwide has fallen by 71% since 1970, according to a report by a team of scientists from around the world.

Their study finds that more than three-quarters of these oceanic species are now threatened with extinction and over the past 50 years fishing pressure has doubled and shark and ray catches have tripled.

The team assessed the risk of extinction for all 31 species of ocean shark and rays. Of the 31 oceanic species, 24 are now threatened with extinction and three shark species (the oceanic whitetip shark, and the scalloped and great hammerhead sharks) have declined so sharply that they are now classified as critically endangered.

Researchers discovered that the risk of extinction to marine species is primarily caused by overfishing.

Sharks and rays are especially overfished because they tend to grow slowly and produce few young.

They are caught for meat, fins, liver oil, gill plates, and recreation such as fishing and diving.

Overfishing of oceanic sharks and rays threatens "the health of entire ocean ecosystems as well as food security for some of the world's poorest countries", said Dr. Nicholas Dulvy.

Nathan Pacoureau, lead author and postdoctoral Fellow based at Simon Fraser University, spoke about the need for governments to protect threatened species and said "countries should be addressing insufficient progress toward global sustainability goals".

He hopes this report "serves as an urgent wake-up call" and argues that immediate action is needed to prevent species from becoming extinct.

The team are calling on governments to introduce fishing limits to help species recover.

Sonja Fordham, President of Shark Advocates International, said "there is hope to be found in a few shark conservation success stories."

The goal of conservation is to protect species from extinction, and maintain habitats and ecosystems.

Sonja and her team have documented "rebuilding for several Northwest Atlantic species, including Great White Sharks and Hammerheads, achieved through science-based fishing limits."

"Relatively simple safeguards can help to save sharks and rays, but time is running out. We urgently need conservation action across the globe to prevent...negative consequences and secure a brighter future for these extraordinary, irreplaceable animals", she added.