Carcharodon carcharias, otherwise known as the great white shark, strikes fear into the hearts of many, conjuring up wild images of a ferocious ocean predator patrolling the vast ocean waves. But could the reign of this fearsome fish soon be over?
Humankind has been exploiting our seas for centuries, triggering catastrophic ecological changes within the marine environment, such as through trophic downgrading. Sharks in particular are one of the most heavily exploited groups of marine organisms, with as many as 273 million estimated to be killed annually. Many populations have dwindled by more than 90% - a study published in Communications Biology last December unveiled that hammerhead and white shark populations had undergone staggering declines of 92% around the Queensland coastline in Australia.
But what happens when the top dog moves out? The latest research from Dr. Neil Hammerschlag – marine ecologist and Research Associate Professor (at the Miami Rosenstiel School of Marine & Atmospheric Science and Abess Center for Ecosystem Science & Policy) and colleagues conducted over a period of 18 years discovered that the next in-line enters the scene.
During the study, researchers monitored white shark abundance and predatory behaviour at Seal Island, False Bay in South Africa; an area that a large Cape fur seal colony calls home – the preferential dinner item of the white shark.
At a recent press conference, Dr. Hammerschlag said “In 2017 and 2018, their numbers reached an all-time low, with great whites completely disappearing from our surveys for weeks and months at a time. [This study] provided a truly unique opportunity for us to see what happens to an ocean ecosystem following the loss of an apex predator”.
Seal Island has been experiencing a decline in its white shark population since 2015, but why have white shark numbers been declining? It’s unknown exactly, but there are some hypotheses.
Potentially there is scope for any of the following to be the prime culprit - overfishing, habitat loss or a shift in environmental conditions and/or prey – or more likely, an amalgamation of several of these factors. Back in 2017 five white sharks washed up dead further east down the Western Cape coastline in Gansbaai after suspected orca attacks, resulting in a 6 month dry spell of white shark sightings; with some researchers postulating that this could be the reason for their disappearance as another apex predator asserts its dominance.
But what species made way into the white sharks prior stomping ground? Following the disappearance of great whites, the emergence of the elusive Sevengill sharks took place. Sevengills are so named for possessing seven gill slits like their ancient ancestors, rather than the five gill slits that modern shark species exhibit. Sevengill sharks do not occupy the same geographical areas as the white sharks, having previously only been spotted 11 miles from Seal Island, so their greater presence in white shark foraging sites after their departure is fascinating.
Despite the long hours spent on the water monitoring these mysterious animals by researchers, it is integral to remember the limitations. With all the sweet-smelling chum in the world not all sharks are attracted to research boats, nor are they obliged to swim to the surface for a better viewing by intrigued humans. Larger sharks also pose dominance over smaller individuals, having the potential to bias results as well. White sharks are also thought to live more than 70 years, so studies such as this only provide a mere snapshot into their secretive lives.
What is clear, however, is that white sharks are growing ever more vulnerable to mounting pressures from both the fishing industry – especially as bycatch in demersal shark longlines and gill nets – and culling via shark nets, alongside a reduction in prey throughout their range. However, the demise of one predator leads to the arise of another, with opportunistic species, like the sevengill, being prepped and ready to move in when the white sharks fade away.
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