Do 11,000 sharks die every hour?

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Hello, and thanks for downloading the More or Less podcast.

We're the programme that looks at the numbers in the news, life, and under the sea.

And I'm Lizzie McNeill.

Thanks to certain Hollywood stereotypes, sharks have something of a scary image problem.

But what if the films have got it wrong?

And it's actually the sharks who should be scared of us.

Imagine a fishy cinema, school of sharks cowering as the young shark family on screen are mercilessly caught in man-made nets and fishing trawlers.

And they've the right to be afraid.

Many sharks are killed this way each year.

The question is, though, how many?

Loyal listener Anne Stewart got in touch to ask about a stat she saw in a newsletter from marine conservation charity Sea Shepherd Global, and the email claimed that every hour 11,000 sharks are killed.

That's over 100 million every year, caught in nets, hunted for fins, or discarded as bycatch.

Those sound like very big and surprisingly round numbers.

So are these numbers fishy or is it true that this many sharks are being killed?

Yes, it's true that the annual mortality of sharks that we estimate globally is at least 100 million sharks, or about 11,000 sharks every hour.

So that's a lot.

This is Dr.

Boris Worm, a professor in marine conservation at Dalhousie University in Canada.

His research is the source of this claim.

Unfortunately, it's more than that very ancient group of species can sustain.

Now, before we start explaining how he worked this number out, you need to know something about sharks.

Loads of them are the complete opposite of massive and terrifying.

There's over 500 species of sharks, from very large, actually the largest fish in the world is the whale shark, to some very tiny ones that are about the size of my thumb and everything in between.

And we mostly think about the larger sharks, say hammerheads, blue sharks, bull sharks, white sharks, and so on.

They are also, mostly, not being hunted on purpose.

Instead, they're getting caught in nets and lines used to hunt other fish.

In most cases, actually, they're being caught as bycatch.

We're not intending to kill them, but unfortunately, we do by fishing for other things.

Now, there's no doubt that fishing has put many species of shark at risk.

Of those 500-plus types of shark, 31% are threatened and 6.5% are critically endangered.

Sharks are being killed in very large numbers by industrial fishing fleets and fishermen and women all around the world.

But how did Boris work out this figure of 100 million for the total shark death toll?

So we did a variety of global analyses to derive that number.

We separated shark mortality to those that live in coastal waters and those that live far from shore on the high seas.

These two data sets posed quite different problems to Boris and his team.

For the coastal fisheries, governments do publish data on the quantities of sharks that are caught in the waters, though the quality of this data is highly variable.

We knew how many sharks were reported to be killed by those countries, but those numbers are underestimated by a factor of three or four.

So we had to go country by country and reconstruct from old catch records and interviews and expert data to estimate the missing sharks, if you will.

At least there's some actual data from these coastal fisheries.

For the open ocean, away from land, the data is even worse.

So Boris divided these waters into a grid and used a machine learning system to extrapolate figures for the sharks killed in each zone using the patchy data at his disposal.

So, it's fair to say that this is not a precise number, and on top of all of this uncertainty, we can add a bit more.

First, as we said, a lot of these sharks are caught accidentally, and some are thrown back into the ocean by the fishes.

And that bycatch is a problem because a lot of those sharks don't survive.

So we also had to estimate the proportion of sharks that, when they get caught, get retained and sold or thrown back in the water.

And then of those, what proportion survives?

And we used all available studies that people have done to get at those proportions in the most accurate way we possibly could.

What's more, much of the data comes in the form of the weight of the shark that's being killed.

But because sharks come in a massive range of sizes, including lots of tiddly little sharks that no one thinks about, you've got to make more estimates about the size of the sharks being killed to work out the numbers of sharks per ton.

On top of this, some of the catch data doesn't record sharks, but only a grouping called Elasma Branch, which includes sawfish, skates, rays, as well as sharks.

So it was very hard to resolve what proportion of those bulk reported elasmobranch fishes are actually sharks.

So that's just one source of error.

They They use this data where the sharks were identified to work out what proportion of the Elasma Branch might be sharks.

And that makes up 20 million of the 100 million figure.

So if you have unreliable government data combined with reconstructed missing data combined with estimates on top of estimates.

Yeah.

So it's really hard to say what the margin of error is.

In this paper, we didn't provide a range of numbers that could be plausible.

In the previous paper, we estimate that the range was between 60 and 270 million sharks killed every year.

I think from our latest paper, I would think that the range is a little lower, but it could be in the order of a couple hundred million sharks.

Boris is very clear that the estimate is, well, an estimate.

Sharks are hard to count.

We did a bunch of expert interviews to back up our analyses and to see whether the trends that we were seeing in the data were matched up by what people who had a lot of knowledge about particular geographies were seeing on the ground.

And then we put all of that together and it matched up near perfectly.

So that gave us a lot of confidence that what people are seeing on the ground who are visiting markets, who know about fishing communities, who are tracking entire fleets is similar to what we found in our global analysis.

So maybe 100 million sharks are being killed a year.

Give or take, tens of millions of sharks.

Either way, it seems the international fishing industry is taking a sizable chunk out of the shark population every year.

In a previous study, we estimated that six to seven percent of all sharks that live in the sea are being killed every year.

So that's the rate of exploitation, which we measured three different ways just to be sure that it's correct.

And I think it's in that ballpark.

A major purpose of Boris's research was to work out whether international efforts to protect shark populations had actually worked to lower the numbers being killed.

Much of this has focused on the practice of so-called finning, which is a horrifically wasteful practice where a shark's fins are cut off for use in shark fin soup and traditional medicine, and the rest of the shark, often still alive, is discarded back into the sea.

That practice had been outlawed in a lot of jurisdictions, and according to our data and our expert interviews, has become much less common, which is a real success.

Unfortunately, it did not prevent fishes from taking and fishing and retaining sharks because they were just not allowed to cut the fins off and sell them separately.

So they had to retain the whole sharks and find markets for shark meat, shark cartilage, shark liver oil, and other products.

And so, in some ways, there was even an incentive to keep the whole shark and develop these new markets, which increases the value, which increases the demand.

This might have caused this spike in shark mortality that we did not anticipate and did not want.

Market economics really can have a nasty bite.

All in all, you can think of 100 million as a plausible estimate, albeit one with a wide margin of error.

Just in case you thought it was safe for sharks to be back in the water.

Our thanks to Dr.

Boris Worm.

And that is all we have time for this week.

If you see any suspicious stats, please do let us know at more or less at bbc.co.uk.

Until next week, goodbye.

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