A recently published study of great white shark populations in South Africa has sparked widespread reports that the country's iconic ocean predators are "on the brink of dying out". Peer behind the dramatic headlines, however, and the situation in South Africa begins to look a little different. 

The study, led by researchers from Stellenbosch University and published last month, suggests that just 350 to 520 great whites remain in South African waters. These low numbers are jarring, and they've resulted in dozens of articles (VICE NewsReutersGristBBCThe Guardian, and others) warning of the imminent demise of the species in the region. But not everyone is convinced the figures are accurate. 

The unfolding dispute among experts underscores something that's largely overlooked by the media: counting sharks is extremely difficult.

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Image: Shutterstock

The counting conundrum

"The first thing to remember when looking at population studies of large, free-ranging animals is that a population estimate is just that – an estimate," notes shark biologist Dr Austin Gallagher. "Enumerating fish is insanely challenging, especially for large species, which are ironically often the most cryptic and difficult to [work with]." 

When scientists want to gauge the size of an animal population on land, they can turn to camera traps, helicopter surveys, scat counts and DNA analysis to get the job done. But when the study subjects are deep-diving, open-ocean-dwelling sharks, things get a bit more complicated. "None of these are viable options for sharks," explains Gallagher. "Except DNA approaches, which are still in their infancy, and are harder for large sharks that [often] require capture for blood samples."

Because the whereabouts of their study animals are often a mystery, marine biologists must rely on tagging and tracking (telemetry) or photo identification (mark-recapture) techniques, combined with statistical modelling to infer the big picture. This is exactly what the team at Stellenbosch University, led by Dr Sara Andreotti, did in South Africa.

Cameras in hand, the researchers set sail in Gansbaai, a white shark hotspot some 100 miles off the coast of Cape Town. Like our fingerprints, the sharks' fins posses unique notches and irregularities that can be used for identification. Over the course of two years, the team managed to pinpoint a total of 426 individual white sharks. That data was then plugged into a modelling programme that produced the national population estimate of 350 to 520. 

A selection of white shark dorsal fins showing unique notches and scars. Image: Andreotti et al.  

According to Andreotti, these figures represent "52% fewer great whites than estimated in previous studies", but it's important to note that this data was collected from 2009 to 2011. And although media reports have since claimed that the Stellenbosch study will "form the first ever database of the country's shark population", scientists have actually been collecting population data sets from the region since 1996. In fact, in the same period that the Stellenbosch researchers were at work, another Gansbaai study was also going on. Conducted over four years and including 532 individual sharks, that research resulted in a population estimate edging on 1,000

It might seem odd that two studies in the same region performed during overlapping years and using similar methods could produce different results, but such outcomes do happen, and not just in South Africa. Back in 2011, an alarming study claimed there were only 219 great whites left in central California. Three years later, a re-evaluation (based on the same data set) found that some 2,000 great whites likely reside near the state. 

Moving targets

Counting great whites is made all the more tricky because these fish are highly migratory. Past research in nearby False Bay indicates that new sharks are constantly shifting in and out of monitored populations. The sharks in South Africa don't always stay in South Africa, nor does every shark frequent every hotspot. 

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Great white shark 'hotspots' along the South African coast.

"One of the assumptions made in the [Stellenbosch] study is that the Gansbaai aggregation site represents the entire South African white shark population," says shark ecologist Dr Alison Kock, who specialises in the area's white sharks. "And we're just not sure this is true. Overwhelming scientific evidence shows that white sharks are separated by size and sex during parts of their lives, and not all white sharks visit Gansbaai." 

This sentiment is echoed by NOAA shark scientist Tobey Curtis, who has done extensive work in population dynamics.

"One of the biggest challenges with shark population estimates using these types of methods is figuring out what percentage of the population is actually being sampled," he says. "There's no way to know for sure, and those assumptions have a huge impact on conclusions. In this case, the researchers' assumption that they sampled most of the population, over just three years in a portion of the coast, is not well supported."

We're still in the early stages of tracing their migratory patterns, but we do know these animals move around. Females are especially good at hiding from researchers, and some individuals have disappeared from an area for years before showing up again. In fact, a white shark tagged in Gansbaai back in 2003 took an impressive 11,000-kilometre cruise to Western Australia. Others have made the journey since.

"Nicole" the white shark swam nearly 7,000 miles from Gansbaai to Western Australia. Image: White Shark Trust

The Stellenbosch study also contained a genetic component, which concluded that the white shark population in South Africa contains just 333 breeding adults. Such a low figure would imply a genetic bottleneck suggesting problems with inbreeding that could prevent these sharks from bouncing back. But the researchers’ model and calculations were based on an assumption that the shark population is "closed" with no individuals moving in or out of it. Given the sharks’ migratory habits, we know this is not the case in reality. And while assuming a "closed" population certainly makes genetics modelling simpler, animals from different regions can be overlooked which can drastically lower the estimates you end up with.

Further still, despite estimating such a small group of viable breeders, none of the identified white sharks appeared to be related. Experts suspect this might indicate a problem with the genetics methods used

Teaming up

So what's the best way of estimating population size? In light of all the obstacles, it makes sense to go broad: look at all of the estimates obtained for a variety of approaches and locations, and then make the most informed judgement. This is precisely what Kock and her colleagues are planning to do. 

Scientists from all major aggregation sites in South Africa have agreed to pool their data in the hope of producing what they believe will be a more accurate national estimate of great white numbers. Dr Andreotti did not respond to requests for comment about her potential involvement in this collective effort and the criticisms levelled at her research.

"Although the recent study led by Stellenbosch University is a genuine attempt to estimate the population, there is reason to believe that this contribution needs rigorous examination and testing with further work," says Kock. "The study has not provided evidence on the current trend of the population, whether it is decreasing, increasing or stable."

Of course, just because we don't have a reliable population estimate doesn't mean South Africa's great whites are doing fine the tough reality is that we simply don't know. What this dispute does illustrate, however, is how worthy of our attention these animals are. We're talking about some of the largest predators on the planet, and yet we don't have a firm grip on their global numbers, where they give birth, their migratory routes, and most importantly, how humans are affecting them.

Anglers are prohibited from targeting white sharks in South Africa, and fisheries law states that any animals caught incidentally must be released. But illegal activity, human encroachment and accidental bycatch continue to threaten these sharks. 

Renowned white shark photographer and researcher Chris Fallows has spent nearly two decades observing South Africa's white sharks near Seal Island, (home of the famous Air Jaws), and he's noted a dip in recent years. 

"In the early days we would commonly have 15 and even 20 sharks seen at our boat during a morning's outing," he says. "Now we get excited when we see five and ten is a real season's high. Up until 2010, the longest we ever went without a sighting was five days, even in our low season. In 2012, 2015 and 2016, we had periods of 50 days or more during our high season."

But is this the natural ebb and flow of the population from Seal Island to other locations? Are environmental factors at play? Or are we seeing a decline that will continue? These are the kinds of questions Kock and her colleagues hope to answer. 

"White sharks are low in abundance and vulnerable to human impacts and every effort should be made to ensure that the population is conserved," she says. "But future management and conservation actions need to be based on the best available numbers and further research is needed to resolve outstanding questions."

In the end, if we want to be effective guardians of South Africa's great white sharks, it is accurate knowledge, not sensational headlines, that we need most.

Editor's note: An earlier version of this article stated that a 2011 study estimated there were 219 white sharks in California. "Central" California has been added and a quote was removed for clarity. 


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