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Shark embryo mortality expected to rise by 71 percent

New experimental research has revealed that the combined effects of ocean warming and acidification could dramatically reduce the survival rate of embryonic sharks by 2100. This study is also the first to show that monthly temperature variation significantly impacts shark embryo mortality.

Ocean warming and acidification result from increased CO2 levels dissolving into marine environments, leading to rising water temperatures and lower pH levels. 

Hatching success of shark embryos 

“The embryos of egg-laying species are especially sensitive to environmental conditions,” said Noémie Coulon, a PhD student at the Laboratory of Biology of Aquatic Organisms and Ecosystems in France. 

“The hatching success of embryos is a crucial factor for population dynamics. In the case of skates and sharks, which have a slow pace of life, low hatching rates could be critical for population renewal.”

Small-spotted catshark embryo survival 

The small-spotted catshark (Scyliorhinus canicula) is one of the most abundant shark species in Europe, yet it is already experiencing habitat loss in coastal areas, particularly during the summer months when egg-laying is at its peak. 

Coulon and her team compared embryo survival under three scenarios: a control scenario using baseline temperature and pH data from 1995 to 2014, and two predicted climate scenarios for 2100 based on the UN’s Intergovernmental Panel on Climate Change’s Sixth Assessment Report from 2021.

Predicted climate scenarios 

The SSP2 scenario, known as the “Middle of the Road” scenario, represents a future where slow and non-uniform progress towards tackling climate change is present but net-zero is not reached, predicting a temperature rise of 2.7°C and a pH drop of 0.2 by 2100. 

The SSP5 scenario, called the “Fossil-fueled Development” scenario, represents a future where fossil fuel resources continue to be more rapidly exploited, predicting a temperature rise of 4.4°C and a pH drop of 0.4 by 2100.

Heat linked to shocking embryo mortality rate

The researchers measured embryo growth and yolk consumption weekly over a four-month period and recorded hatching success and the growth of surviving baby sharks for six months post-hatching. 

They found a high survival rate for both the control and SSP2 scenarios (81% and 83% respectively). However, survival plummeted under the SSP5 scenario, with only 11% of embryos hatching. 

“We were shocked by the low survival rate observed in the SSP5 scenario,” Coulon said. The team linked this mortality to lower yolk consumption rates, lower growth rates, and failure to transition to internal gills.

Temperature variation and embryo mortality 

The study also highlighted the significant impact of seasonal temperature variation on embryo mortality, particularly during critical egg-laying periods. “This mortality was most pronounced in August, coinciding with the highest temperatures (reaching 23.1°C),” she explained.

The surviving 11% of embryos in the SSP5 scenario did not exhibit typical growth patterns, which might have contributed to their resilience to environmental changes. 

“What makes these individuals special remains unclear,” said Coulon. Understanding inter-individual variation among young marine organisms could improve predictions of future ecological success.

This research serves as both a warning and a call to action. “It serves as a warning about the responses of other species that may be even more sensitive to environmental change,” she said. “Our findings demonstrate that the more moderate SSP2 scenario can limit the damage inflicted on species like the small-spotted catshark.”

Supported by the Save Our Seas Foundation, this study will be presented at the Society for Experimental Biology Annual Conference in Prague in July 2024. 


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