Climate Impact: Threatening Sharks with Coastal Upwelling
As the planet's climate undergoes dramatic shifts, the delicate balance of our ocean ecosystems is being disrupted in ways that threaten even the most resilient marine predators. In a captivating new study published in Nature Climate Change, researchers uncover a chilling reality facing one of the ocean's most iconic inhabitants - the mighty shark.
The story begins on the southeast coast of South Africa, where in 2021, a shocking occurrence played out. Coastal communities were inundated with a startling sight - the washed-up remains of hundreds of marine creatures, spanning 81 different species of fish and invertebrates. This mass die-off coincided with a powerful shift in the Agulhas Current, a major offshore ocean current, which brought with it drastic changes in temperature, currents, and water chemistry.
Intrigued by this phenomenon, the research team, led by Valentina Di Santo, delved deeper, uncovering a troubling connection between climate change, altered ocean circulation patterns, and the plight of one of the ocean's apex predators - the bull shark (Carcharhinus leucas).
Using a multidisciplinary approach that combined environmental data analysis, modeling, and animal tracking, the scientists revealed a disturbing trend. Climate-induced shifts in pressure and ocean circulation systems have intensified the frequency and severity of cold upwelling events along the western boundary currents, like the Agulhas and East Australian Currents. These upwelling events, where cold, deep water is brought to the surface, are proving to be deadly for marine species seeking thermal refuge.
The researchers focused their investigation on the bull shark, a species known for its global distribution and relative tolerance to environmental fluctuations. By tracking the movement patterns and thermal ecology of 40 tagged bull sharks, the scientists uncovered a concerning reality. These sharks, which typically migrate between warm and cold waters to optimize their physiological processes, are now finding themselves trapped in a perilous game of hide-and-seek, desperately seeking refuge from the relentless onslaught of cold upwelling.
The data revealed that bull sharks commonly operate near their maximum thermal thresholds when they encounter these active upwelling cells, often at the far poleward extent of their range. Forced to navigate an increasingly narrow thermal corridor, these sharks are struggling to find the conditions necessary for their survival.
This study serves as a sobering reminder that the impacts of climate change extend far beyond the warming of surface waters. The cascading effects on ocean circulation patterns are creating a perfect storm for marine species, with cold upwelling events proving to be just as deadly as extreme heatwaves.
As the planet continues to warm, the severity of these extreme events is only set to increase, reshaping the migratory routes, movement corridors, and seasonal habitats of a multitude of marine species. For the mighty shark, the future looks uncertain, as it is forced to adapt to an ever-changing seascape, where the very elements that sustain it may one day be its undoing.
Source: https://www.nature.com/articles/s41558-024-01975-7
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