Deadly upwellings of cold water pose threat to migratory sharks

Upwellings of cold water from the deep sea to the surface can be deadly for marine animals and such events are increasing in frequency due to climate change.

In March 2021, hundreds of fish and shellfish, as well as squid, octopuses, manta rays and bull sharks, washed up dead on the coast of South Africa.

The animals were fleeing high water temperatures from a marine heatwave gripping South Africa’s coastal waters.

But in their escape, they were caught in a sudden cold water upwelling from the region’s Agulhas current, prompting ocean temperatures to plummet.

“These upwelling events that are happening on the Agulhas bank, the temperature can suddenly drop by about 10°C [18°F] in 24 hours,” says Zoe Jacobs at the National Oceanography Centre in the UK. “It’s a very, very intense, short-lived event.”

Nicolas Lubitz at James Cook University, Australia, and his colleagues studied 41 years of sea surface temperature data and 33 years of wind records to assess cold water upwelling in the two regions affected by the Agulhas current and the Australian current, which snakes down Australia’s east coast.

They conclude that stronger ocean currents and changes in wind patterns, both occurring with climate change, are driving an increase in both the frequency and intensity of cold water upwelling in both regions.

Most marine life living near these currents is adapted to sudden fluctuations in water temperatures and so can cope with these changes.

But the study warns that migratory species such as bull sharks, which pass through these waters and are unprepared for sharp shifts in temperature, are at risk.

Bull sharks struggle to survive when water temperatures drop below 19°C (66°F) for a sustained period. Lubitz and his colleagues used data from 41 tagged bull sharks in southern Africa and Australia to study their migratory patterns.

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The sharks move to warmer tropical waters as soon as water temperatures start to drop after summer ends. During their migration, they appear to take steps to avoid cold water upwellings, by moving to warmer surface waters when swimming through upwelling zones and taking refuge in estuaries and bays during their journey.

But as upwelling events increase in frequency and intensity, it will become ever harder for bull sharks – and other migratory species – to avoid them, the researchers warn.

However, the impacts may be limited to the two regions studied, says Jacobs, who wasn’t involved in the research. “These two particular regions are quite special cases, because the upwelling that occurs there are quite short, intense events,” she says. Other global upwelling systems are more permanent or seasonal fixtures, she says, where marine species are better adapted to withstand or avoid changes in water temperature.