The Dangers of Deadly Cold Water Gush for Migratory Sharks

The upwelling of cold water from the deep ocean to the surface can be deadly to marine animals, and such events are becoming more frequent due to climate change.

In March 2021, hundreds of dead seafood, squid, octopus, manta rays, and bull sharks washed up on South African beaches.

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

However, during their escape, they were caught in a sudden burst of cold water from the region's Agulhas Current, causing ocean temperatures to plummet.

“These upwelling events occurring on the banks of the Agulhas River could cause temperatures to suddenly drop by about 10 degrees Celsius.” [18°F] “within 24 hours” Zoe Jacobs At the UK National Marine Centre. “This is a very intense, short-term event.”

Nicholas Lubitz Professors at Australia's James Cook University used 41 years of sea surface temperature data and 33 years of sea surface temperature data to assess cold water upwelling in two regions affected by the Agulhas Current and the Australian Current, which meanders along Australia's east coast. I studied wind records.

They conclude that stronger ocean currents and changes in wind patterns associated with climate change are increasing both the frequency and strength of cold water upwelling in both regions.

Most marine organisms that live near these currents are adapted to sudden fluctuations in water temperature and can therefore cope with these changes.

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

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

As soon as summer ends and water temperatures begin to drop, sharks migrate to warmer tropical waters. During migration, they appear to take steps to avoid cold water upwelling by moving to warmer surface waters as they swim through upwelling zones, or by seeking refuge in estuaries and bays during migration. .

But as upwelling events increase in frequency and intensity, researchers warn that it will become increasingly difficult for bull sharks and other migratory species to avoid them.

But Jacobs, who was not involved in the study, said the effects may be limited to the two areas studied. “These two particular regions of hers are very special cases because the upwelling that occurs there is a very short and intense phenomenon,” she says. Other global upwelling systems are more permanent or seasonal, and marine species are better adapted to withstand or avoid changes in water temperature, she says.