More than 100 shark species may face major population declines by 2100

Egg-laying sharks around the globe could take a big hit to their population by the end of the century as increasing ocean warming and acidification destroy their embryos. This could affect more than 100 shark species.

The finding is based on a study of the small-spotted catshark (Scyliorhinus canicula), which is found in the Mediterranean Sea and the north-east Atlantic. It is among the roughly 40 per cent of sharks that reproduce by laying a tough, leather egg case that contains an embryo. These shark embryos are highly sensitive to changing ocean conditions, such as temperature and pH. As the ocean soaks up excess carbon dioxide from the atmosphere, it becomes warmer and more acidic.

Noémie Coulon at the French National Museum of Natural History subjected catshark eggs to various ocean conditions, including monthly temperature changes, in tanks in the lab. Coulon and her colleagues chose this species because it is one of Europe’s most abundant sharks.

The first test created water conditions that would be seen in a “middle-of-the-road” climate scenario with a temperature rise of 2.7°C above pre-industrial levels, and an associated drop in pH of 0.2 by the year 2100. The second scenario – in which the world continues to rely heavily on fossil fuels – predicts a temperature rise of 4.4°C and a drop in pH of 0.4 by the end of the century. The third was a historical baseline, recreating the water temperature and pH in the shark’s habitats from 1995 to 2014.

They simulated the conditions over the next four months as the embryos developed and found dramatic differences in embryo hatch success depending on experimental conditions. In the baseline scenarios and the middle-of-the-road scenario, around 82 per cent of the eggs successfully hatched. But in the warmest scenario, only five out of 45 embryos survived – a nearly 90 per cent loss.

“We were deeply shocked by the high mortality rate,” says Coulon. “It would probably cause population collapse.”

Even relatively short periods of warmth – such as an especially warm August – were enough to cause hatching failure. Based on these results, Coulon expects other egg-laying sharks, including endangered and vulnerable species like nursehounds, would be similarly devastated.

But their demise is not set in stone, says Coulon. “If we make an effort to keep the temperature increase to only about 2 degrees… then the species could survive.”