Warmer Seas Kills Sharks' Eggs

07 July, 2024

Warmer seawater combined with acidification could pose a major threat to ocean sharks. Their eggs develop much more poorly, leading to the death of the embryo, according to a new study.

Small spotted catshark in the marine fish laboratory in Lysekil. File photo. Photo Björn Larsson Rosvall/TT

The study, published recently at a scientific conference in Prague, is based on experiments with eggs from the small spotted catshark (Scyliorhinus canicula), a small species common along the coasts of Europe, including the Swedish west coast.

The researchers, led by biologist Noémie Coulon at the Marine Biology Laboratory in Dinard, Brittany, France, tested how the eggs would cope with increases in temperature under two of the scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). In one case, the test assumed a rise of two degrees Celsius by 2100, in the other a rise of four degrees. The latter could be the case if greenhouse gas emissions are not significantly reduced.

Egg capsule with an almost fully formed embryo of a small spotted catshark. Photo: Noémi Coulon/TT

Increased mortality

At the same time, it was recognised that the oceans are becoming more acidic as they warm. Consequently, the researchers lowered the pH of the water in both tests.

It must also be taken into account that seawater, in this case off Brittany, is significantly colder in winter than in summer. The water is warmest in August.

The results are telling. Egg development was normal and 83 per cent of all embryos survived when the temperature rise was moderate. Spiny little sharks hatched into the world.

However, when temperatures rose to well above normal, things got worse. Mortality increased dramatically, especially in August, and only 11 per cent of all embryos survived to hatch. Clearly, their growth was slower. In most, the gills were not fully developed either.

Great concern

The scientists themselves say they were shocked by the mortality rate. There is evidence that the world’s oceans have been getting warmer since at least the 1980s. The rise in surface water temperature is now about 0.8 degrees above the 20th century average, which has caused great concern among scientists. It is well known that coral reefs are suffering, but there are concerns that many other organisms will also be affected.

At the same time, Noémie Coulon points out that the fact that 11 per cent of the sharks survived gives some hope. “It shows that there is a lot of individual variation, which means that natural selection has a lot of material to work with. If so, the species may be able to survive when the environment changes.

The small-spotted catshark (Scyliorhinus canicula) is a small species, no more than one metre long. It is found from Norway down to the Mediterranean and Senegal, and in Sweden it is common along the west coast. Its diet consists of fish, crustaceans and molluscs. The species has no special spawning season but mates throughout the year. The eggs, or rather the egg capsules, are anchored with threads to suitable objects on solid surfaces. They hatch after eight to nine months. The species is protected, just like other sharks.

Source: Swedish Agency for Marine and Water Management.

Text: Roland Johansson/TT
Photo: Björn Larsson Rosvall/TT, Noémie Coulon/TT

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