What do warmer seas mean for shark pups?

‘I aim to convey a message of hope,’ begins Noémie Coulon. ‘Hope’ is a word we hardly hear connected to climate change and yet, as eco-anxiety grips many of us who care about our collective future, hopeful stories are exactly the kind of stories we need to hear. As Jane Goodall writes in The Book of Hope: A Survival Guide for Trying Times, ‘Hope is often misunderstood. People tend to think that it is simply passive wishful thinking: I hope something will happen but I’m not going to do anything about it. This is indeed the opposite of real hope, which requires action and engagement.’

And this sense of being able to act with concrete, considered steps to tackle climate change is what Noémie, a Save Our Seas Foundation project leader and a PhD candidate at Laboratoire de Biologie des Organismes et Ecosystèmes Aquatiques (BOREA), wants her work to contribute to. Noémie is the lead author of a new paper, ‘Shark critical life stage vulnerability to monthly temperature variations under climate change’, published in the journal Marine Environmental Research. At first glance, the results of her study are frightening. Her study subject, the small-spotted catshark, will be unable to rear new generations of pups by 2100 if we follow our ‘business-as-usual’ approach to fossil fuels. But Noémie’s research looks at other possibilities and scenarios, and those results are encouraging. ‘If we implement mitigation measures that allow us to achieve controlled warming,’ she says, ‘then it is possible for us to limit the damage.’

An innocuous little nocturnal shark that hugs the sea floor, the small-spotted catshark is common and abundant in the waters off coastal Europe. It produces eggs year-round, tenderly tethering growing pups in their protective purses to algal anchors on the sea floor, where they remain fixed in the current like miniature mermaid balloons. Egg-laying peaks just before summer. Both adult and juvenile catsharks stay very close to home, unlikely to move far. These traits make the small-spotted catshark particularly well suited to studies on how sharks will respond to our changing climate; they can’t, effectively, escape the heat. And although this catshark is common and abundant right now, conducting research on how its ability to breed and rear young might change can help us to also understand the consequences for potentially rarer species. Without these future projections, we cannot adapt now our monitoring and management of sharks to anticipate the impact of climate change.

Noémie looked at the effects of a warmer and more acidic ocean on developing small- spotted catshark pups over the course of their first 10 months of life. Her work considered two different scenarios that are outlined by the Intergovernmental Panel on Climate Change (IPCC). The first was ‘Shared Socio-economic Pathway’ (SSP) 2, also dubbed ‘middle of the road’, where carbon dioxide emissions remain at current levels until they fall after 2050, but do not reach net zero. The second was SSP5, also called ‘fossil-fuelled development’, where carbon dioxide emissions triple by 2075.

Labelled shark eggs in the experimental tank. Photo © Noémie Coulon

In the SSP2 scenario, the ocean warms by 2 ˚C with an ocean pH measure of -0.2. In the SSP5 scenario, the ocean warms by 4 ˚C with an ocean pH measure of -0.4. The growth, behaviour and development of eggs and shark pups in observation tanks under these different conditions were compared to the same in a control group of shark pups treated under current climate circumstances (the scientists used the median water temperatures and pH levels measured from 1995 to 2014 to set up these observations).

If we use the information readily available from the IPCC to cut our fossil fuel reliance and curb the rise of carbon dioxide levels, there is a future for sharks and for healthy oceans – and for us. As the results of this study showed, the success of the pups’ hatching and healthy development depended on the climate scenario; where temperatures rose too high (above the threshold of 3 ˚C to 5 ˚C that has been shown for other species like Port Jackson and epaulette sharks), the survival of shark pups was slim. The hatching success of eggs in the current climate scenario (control group) and the SSP2 ‘middle of the road’ group was 82%. However, this dropped to a worrying 11% of successful hatchlings in the SSP5 group. ‘We were not anticipating such a high mortality rate in the fossil-fuel development scenario and we were deeply shocked,’ says Noémie of the 89% of eggs that didn’t hatch.

Fluctuations in the monthly temperatures also had a bearing on the hatching and successful development of these shark pups. ‘The results from this study absolutely reaffirmed the importance of doing climate-projection research for sharks,’ confirms Noémie. ‘By mimicking seasonal temperature variations that were previously overlooked, we have demonstrated that they are a crucial factor in the success of embryonic development.’ Ultimately, Noémie believes that gradual temperature increases and small variations around an average 20 ˚C ocean temperature may be less harmful to the small-spotted catshark’s growing embryos than prolonged exposure to high temperatures.

By doing the right thing, right now, it appears we still have a chance to ensure that sharks like the small-spotted catshark will survive. On the other hand, the devastating results of a fossil-fuelled development projection will have consequences not only for this little shark, but potentially catastrophic ones for more rare or constrained species that cannot easily move place or adapt. For Noémie, taking action is an act of radical hope. Her research has shown that we need to be thinking of the future, taking into account the very complex nuances of seasonal variation and monthly fluctuations, and how individual animals as well as the entire group respond to change at every stage of their lives.

**Reference: Coulon N, Pilet S, Lizé A, Lacoue-Labarthe T, Sturbois A, Toussaint A, Feunteun E and Carpentier A. 2024. Shark critical life stage vulnerability to monthly temperature variations under climate change. Marine Environmental Research 106531.