When researchers from the University of Miami published a study in 2022 showing that long-term ocean warming was driving the Atlantic tiger shark population northwards, the news was understandably met with alarm by people living along the densely populated east coast of the US. But in the case of tiger sharks, this concern may be justified, as this apex predator has been implicated in scores of unprovoked attacks on humans, according to the International Shark Attack File. One particularly horrific attack occurred in June 2023, when a 23-year-old Russian man died after being mauled by a tiger shark off an Egyptian Red Sea resort.
The fear that sharks inspire in us helps to explain our fascination with these ancient creatures from an early age. As every child learns at school, sharks have to keep moving to stay alive. While this may sound like a myth, it is true for some shark species, such as great whites, hammerheads and whale sharks, which lack the muscles needed to pull water through their gills.
But from a broader perspective, sharks do indeed need to keep moving if they are to survive, as rapidly warming oceans have triggered an unprecedented mass migration of marine life. It is estimated that more than 80% of the planet’s marine species are migrating away from warming waters and moving towards the poles in search of cooler temperatures. Sharks too are moving out of their traditional waters, following their prey and the range of water temperatures in which they thrive.
The Miami scientists found that the Atlantic population of tiger sharks, which typically winter around The Bahamas, is now venturing many hundreds of kilometres further north during the warmer months, reaching waters off the coast of New York and New England that were previously too chilly for this cold-blooded species. Researchers warn that the growing presence of this apex predator in these waters, coupled with its indiscriminate palate, “have implications for fisheries management, human-wildlife conflict, and ecosystem functioning.”
In fact, some of these fears are already being realised, as once-rare blue sharks, great hammerheads and great whites are increasingly being spotted off the coast of New York State, and the number of shark bites around Long Island has risen sharply in 2023, although they are not believed to be caused by tiger sharks.
A similar story of migration is unfolding off the coast of California, where scientists report that since 2014, juvenile great white sharks have moved 600 kilometres north and their range has shrunk significantly, an indication of how rapidly the ocean is warming in this region of the North Pacific. Many of these sharks have taken up residence in Monterey Bay, where they are killing endangered sea otters and hindering their recovery. These mammals, in turn, keep kelp forests healthy and contribute to a thriving underwater ecosystem, illustrating how apex predators can impact ecosystems in complex ways.
And while people are understandably concerned about the growing presence of sharks in new waters, which is a global phenomenon, the biggest threat is to the animals themselves. Shark numbers have plummeted worldwide in recent decades due to overfishing, but now warming oceans are making it even harder to protect sharks and other marine life. In the case of the tiger shark, they are now venturing beyond designated management zones where commercial longline fishing is banned. How can regulators establish marine protected areas in the best place to protect biodiversity in the midst of a massive underwater migration that scientists have barely begun to understand?
The threat of a mass ocean extinction
Against this backdrop, the alarming news that global average ocean surface temperatures in 2023 have suddenly spiked, surpassing anything seen in four decades of satellite observations and sparking heated debate among scientists about the cause. The US National Oceanic and Atmospheric Administration reported that in June 2023, about 40% of the world’s oceans were experiencing marine heat wave conditions, the largest area on record, and could reach 50% by September.
But global warming is not the only stressor facing our oceans. Acidification, overfishing, invasive species, plastic pollution, oxygen depletion and underwater noise pollution are all wreaking havoc on our planet’s seas and the countless creatures that call them home. Scientists warn of a looming mass extinction in the oceans. But unless there is a viral video of a dramatic shark attack, news of orcas sinking another yacht or a missing deep-sea submersible, little attention is paid to what is happening beneath the waves, forgetting that we depend on our oceans for the very air we breathe. Sadly, it is a case of “out of sight, out of mind”.