Just how many great white sharks are there?

Just how many great white sharks are there? Photo: Getty Images

It is understandable that calls to cull great white sharks have surfaced once again in the wake of Chris Boyd's tragic and incomprehensible death while surfing at Gracetown. How else should we respond to an unseen predator as inscrutable as it is terrifying? A reckless motorist can be educated not to kill people; a shark can't.

But in calling for sharks to be proactively killed at our most popular beaches, our Premier is doing more than striking an aggressive stance (and risking earning the nickname Cullin' Barnett).

How widely do great white sharks range?

How widely do great white sharks range?

The cull (and despite his protestations, this would be a site-specific cull) would risk serious harm to a species that, by some estimates, is more endangered than tigers.

No one wants sharks to kill people. But before we press ahead with killing them first, here are ten questions that need answers. Without them, we don't even know whether culling sharks would achieve anything.

How many sharks are out there?

At the moment we don't know. The International Union for Conservation of Nature officially lists great whites as 'vulnerable'. There is no official population estimate of numbers, although some analyses suggest there are as few as 3000 worldwide.

What is the population trend?

"Unknown," is the IUCN's one-word verdict on whether great whites are indeed swelling in number as a result of conservation measures. It's a similar story for the local population, for which data are equally scarce.

Are fatal shark attacks really becoming more common?

It certainly seems so, on the face of it. Mr Boyd's death was the third deadly shark attack at Gracetown in the past decade. In 2011-12 there were five shark fatalities in just 10 months along the WA coast. But drawing conclusions from just a handful of statistically rare events is surprisingly difficult.

How can we prove, for instance, that the spate of deaths is not due to increased human presence in the ocean? Or that it's not simply a tragic anomaly in the numbers?

How widely do great white sharks range?

Another tricky question, although not as difficult as estimating the population. The Department of Fisheries has tagged some 140 great whites in a bid to track where they go – there is tentative evidence that the local WA population roams all the way from northern WA to the Bass Strait.

Tony Cappelluti, of the department's Shark Response Unit, said that the shark that attacked Mr Boyd probably left the Gracetown area within a few hours. Culling a species that roams over such huge distances may well be pointless, given the likelihood that more sharks will arrive from elsewhere.

Would removing sharks from a given area keep those areas shark-free?

Say the government decides to kill any large shark guilty of "lingering" (whatever that means) near Cottesloe Beach. Does that guarantee that Cottesloe Beach will have fewer shark visits in future?

It seems unlikely, meaning that the current system of aerial patrols and evacuations will have to continue. In which case, why bother culling?

Can sharks learn to avoid humans?

It has been claimed that sharks have become 'habituated' to humans because, since the advent of conservation measures, people have turned from hunter to hunted. Given the rarity of shark-human encounters, this seems unlikely. But in truth, this is yet another scientific question to which we don't know the answer.

Is a cull worth the money?

The idea of a hard-headed cost-benefit analysis sounds glib in the aftermath of tragedy. But a cull would cost real taxpayer dollars. In those terms, it's reasonable to ask for a clear projection of the benefit to public safety.

In an ideal world, no one would die in the ocean, for any reason. But if we want to reduce deaths, there are easier gains to be made by focusing on preventable deaths like drowning, or (cliche alert) the road toll.

Will it harm WA's tourism reputation?

Calls for a cull are obviously being made with one eye on WA's status as a fantastic place for a beach holiday. Nothing harms tourism like a shark scare. But tourists might not be reassured by a cull, for two reasons.

First, a cull is a tacit acknowledgement that we have a problem with deadly sharks. And second, eco-conscious holidaymakers may choose not to visit a state that puts human recreation ahead of conservation.

Is it better and cheaper just to learn to live with the danger?

To borrow a line from the climate debate, should we be mitigating or adapting? Mitigation by killing sharks seems more problematic than adaptation with the help of anti-shark suits, electronic deterrents, avoiding surfing at certain times, or simply accepting that the ocean can be a dangerous place.

What are the knock-on effects for the ecosystem?

As apex predators, sharks sit at the top of a bafflingly complex food chain. Targeting large, mature sharks will remove the breeding members of a population we still don't understand. As seen with other overfished species, the damage often only becomes apparent after the fact.

It's a daunting list of questions, and we don't really have the answers to any of them. Until we do, it seems more sensible to leave the grandstanding about killing sharks as exactly that.