Sharks, one of the most resilient animal species, are also suffering the negative impacts of climate change. Having so far survived five mass extinctions, they are now facing an unprecedented decline due to the warming, hypoxia and acidification of the ocean caused by climate change, along with changes in the abundance and diversity of their prey and overfishing.
Sharks are known as one of the most resilient marine animals. In fact, they emerged more than 400 million years ago and have survived profound transformations of the Earth, including five mass extinctions. Despite their diversity and excellent adaptation to the marine environment, shark populations are currently facing an unprecedented decline. Climate change is one of the main drivers. The ocean, the largest habitat on the planet, where more than 500 species of sharks live, is suffering the impacts of the changes imposed by climate change. It is estimated that the ocean has absorbed more than 90% of the heat generated by climate change and around 30% of the CO2 resulting from anthropogenic emissions, acting as a real buffer for the planet. As a result, the ocean’s surface has experienced a continuous increase in its average global temperature since the beginning of the Industrial Revolution. Scientists have observed an opposite trend in the dissolved oxygen content of the oceans, a phenomenon known as deoxygenation. The higher the water temperature, the lower its capacity to retain oxygen.The higher the water temperature, the lower its capacity to retain oxygen. In addition, the pH (a measure of how acidic or basic a liquid is) has decreased globally since the Industrial Revolution, meaning that the ocean has been experiencing an increase in acidity. These phenomena – warming, deoxygenation and ocean acidification – are known as the “deadly trio” and pose a serious threat to ecosystems and marine life.
Studies show that sharks respond mainly in two ways to climate change, either by shifting their latitudinal range or by choosing colder, deeper waters to improve their physiological processes. Research led by an American scientist1 used a combined analysis of satellite tracking, environmental variables, habitat modeling and catch data to investigate the impacts of climate variability and change on the migration and distribution of the tiger shark Galeocerdo cuvier in the North Atlantic. They found that tiger sharks extended their migration areas, arriving earlier at northern latitudes in response to anomalously high sea surface temperatures. In the same study, they analyzed around four decades of tiger shark catches in the region and found that the areas of highest catch density have steadily increased northwards and that catches are occurring earlier. This is not the only study that has shown that climate change affects shark migration by “pushing” them further and further into overfishing.
In another study, researchers2, using satellite tagging of blue sharks Prionace glauca and environmental modeling in the oxygen minimum zone in the Northeast Atlantic Ocean, in Cape Verde’s Exclusive Economic Zone, showed that sharks are pushed to the surface, thus potentially increasing the possibility of interaction with industrial fishing and their capture. This represents a major challenge to the survival of the species, given that industrial fishing is the biggest threat sharks face. Scientists3 have estimated that 100 million sharks are caught every year, around 70 million of them to supply the million-dollar shark finning market for the production of shark fin soup, mainly in Asian countries. Some oceanic species have seen their populations reduced by more than 70% in the last 50 years.
Other species are affected indirectly through impacts on their prey or the habitats where they live. The thresher shark Ginglymostoma cirratum (also commonly known as the cat shark), an abundant species off the coast of Cape Verde, and other species that live associated with coral reefs, are impacted by the bleaching and destruction of these habitats caused by the warming and acidification of the ocean.
From the point of view of climate change, the impacts could be even more problematic for oviparous species that lay their eggs in coastal habitats. The embryonic stage of oviparous species is immobile and therefore more vulnerable to changes in the properties of seawater. Research carried out in the laboratory shows that oviparous shark species will suffer high mortality if the current trend of greenhouse gas emissions continues until the end of this century.
Evidence of the impacts of climate change on sharks is growing, demonstrated in various scientific studies using different approaches. Stopping climate change is therefore imperative to ensure the survival of humans and other species, such as sharks, which play a key role in the health and balance of the ocean.
[1] Hammerschlag N, McDonnell LH, Rider MJ, et al. Ocean warming alters the distributional range, migratory timing, and spatial protections of an apex predator, the tiger shark (Galeocerdo cuvier). Glob Change Biol 2022; 28: 1990–2005. doi:10.1111/gcb.16045
[2] Vedor M, Queiroz N, Mucientes G, et al. Climate-driven deoxygenation elevates fishing vulnerability for the ocean’s widest ranging shark. eLife 2021; 10: e62508. doi:10.7554/eLife.62508
[3] Pacoureau N, Rigby CL, Kyne PM, et al. Half a century of global decline in oceanic sharks and rays. Nature 2021; 589: 567–571. doi:10.1038/s41586-020-03173-9
Author
Jaquelino Varela
Biologist and PhD candidate in Climate Change and Sustainable Development Policies