An Oceanic Whitetip Shark. Credit: Alex Mustard

Ocean sharks and rays threatened with extinction

Three quarters of ocean shark and ray species face an elevated risk of extinction, according to new research.

The study reveals an alarming 71% decline in shark and ray populations over the last 50 years, primarily due to overfishing.

Since 1970, "relative fishing pressure" (exploitation of fish stocks relative to the number of fish left) has increased 18-fold – and the researchers say catch limits are now urgently needed to "avert population collapse".

The research team, led by Simon Fraser University (Canada) and including the University of Exeter, warn that extinctions among these species would jeopardise the health of ocean ecosystems and food security in many poor and developing nations.

"The species that we studied are some of the ocean’s apex predators," said Dr Richard Sherley, of the Centre for Ecology and Conservation on Exeter's Penryn Campus in Cornwall.

"They roam far from land and so might seem immune to the direct impacts of humans on our planet.

"Not so. Our global analysis points to some staggering declines.

"It highlights the very real risks these species face if we do not act now – and act decisively – to limit the pressures fishing exerts on their populations.

"But there is hope.

"A few bright spots in the data demonstrate that even these long-lived animals can recover when science-based fishing restrictions are enacted and enforced."

The research is based on two "biodiversity indicators": The Living Planet Index (LPI) on global population changes since 1970 and the Red List Index (RLI), which tracks changes in relative extinction risk.

The study finds:

• All the oceanic shark and ray species, except for the Smooth Hammerhead, decreased in abundance over the last half-century.
  
  
• 24 of the world's 31 oceanic shark and ray species are now classified as vulnerable, endangered or critically endangered on the International Union for Conservation of Nature (IUCN) Red List. These categories mean a "high", "very high" or "extremely high" risk of extinction in the wild. Some oceanic sharks and rays were moved last year to new categories based on the analysis carried out by this research team, published today in this paper.
  
  
• Species in tropical areas are declining more steeply than those elsewhere. In the Indian Ocean, shark and ray abundance has declined continually since 1970 – falling by 84.7% in total.
  
  
• The longest lived, late-maturing species initially declined faster than those with shorter generation times, but two of these species – including the "Great" White Shark – have shown signs of regional rebuilding since the early 2000s.
  
  
• Some formerly abundant, wide-ranging sharks – including the Oceanic Whitetip and Great Hammerhead – have declined so steeply that they are now classified as critically endangered.

The paper highlights some positive changes, including the recovery of White Sharks in several regions, and signs of population growth among Northwest Atlantic Hammerheads.

These improvements appear to have been caused by strictly enforced fishing rules – and the researchers say further science-based catch limits and landing bans are needed immediately.

They conclude: "These steps are imperative for long-term sustainability, including potentially increased catch once populations are rebuilt, and a brighter future for some of the most iconic and functionally important animals in our oceans."

The study is a project of the Global Shark Trends Project (GSTP), a collaboration of the IUCN Shark Specialist Group, Simon Fraser University, James Cook University and the Georgia Aquarium, established with support from the Shark Conservation Fund to assess the extinction risk for chondrichthyan fishes (sharks, rays and chimaeras).

The paper, published in the journal Nature, is entitled: "Half a century of global decline in oceanic sharks and rays".

Date: 27 January 2021