Putting facts in the way of a good story

18 September 2012 by Tom Webb, posted in Uncategorized

Or: 100 cod in the North Sea?

I’ve just had a nice weekend almost completely away from screens of all kinds, and was starting to think ‘perhaps it’s time to curtail some of my online activity <cough> Twitter <cough>’. But a quick scan down my timeline on a Monday morning couldn’t hurt could it?

Best laid plans and all that… One of the first things I saw was this, from @rivercottage: Just 100 cod left in North Sea. Now I’m a big fan of Hugh F-W, I own (too?) many of his cookbooks, have eaten in his canteen, and admire his campaigning work. BUT: Just 100 cod left in North Sea? Shurly shome mishtake?

I kept scanning, and the same figure kept appearing: 100 cod in the North Sea. All of these tweets linked to a Daily Telegraph story which I duly looked up when I got to work and there it was, under the headline “Just 100 cod left in North Sea” and the only marginally less ludicrous subheading “Overfishing has left fewer than 100 adult cod in the North Sea, it was reported”: a summary of a recent Cefas survey of catches at European ports. (To be fair to the Telegraph, an almost identical piece appeared in (hold your nose) the Mail, although the headline there made the made the ‘100 adult cod’ claim more clear.) Now I know people at Cefas, I trust them to do good surveys (although there’s no link to the actual report in either the Telegraph or the Mail piece, and I can't find it on the Cefas website), so could this 100 cod figure possibly be true?

Short answer: No. Of course it couldn’t.

What the survey found was that last year, “fishermen did not catch a single cod over the age of 13”, and Chris Darby of Cefas is quoted as saying that this suggests “there are fewer than 100 such fish in the whole North Sea”. This rather uncontroversial statement from Darby is the source of the headline and thus, the Twitter meme. Anyone reading the article, though, will see that the survey did find 191 million 1 year old cod, 18 million 3 year olds, and 65,300 tons of cod aged 3 or more years. So "Just 100 cod left in North Sea" is, as @BobOHara put it, an impressively wrong headline.

But what about the subheading about adult cod? Do cod mature as late as 13? No. From Fishbase figures, North Sea cod mature at 3-4 years. So a good proportion of those 18 million 3 year olds, and most of the 65,300 tons of 3+ y fish, will be adults. Subheading debunked too then.

As for the wider issue of overfishing and the state of the cod stock in the North sea, there is some information in the article, namely that the 65,300 tons of 3+ fish is a reduction from the 276,000 in 1971, “a peak year for cod population”, although no indication of whether the 2011 figure is any kind of historic low. But the number of 13+ year old fish is presented in glorious isolation: although no fish of that age were caught in 2011, I have no idea from this article when last such a fish was caught. I suspect some time ago, for a couple of reasons.

First, although the article quotes a maximum age for cod of 25y, which is indeed the figure given in Fishbase (taken, if you follow through to the references, from a 1974 Collins Guide to the Sea Fishes of Britain and North Western Europe book), looking at stock-specific data, again from Fishbase, gives one figure for North Sea cod: 11y (and this from a study published in 1959).

Second, an anecdote: way back before I started my PhD I worked for a while at Cefas. I shared an office with a guy who was interested in the age structure of cod stocks, and who used the annual growth rings on cod otoliths (inner ear bones) to get a handle on this. While I was there, someone sent in an otolith with 7 annual growth rings taken from a cod caught in the Irish Sea (as I recall; so a different stock, but still…). He was over the moon, because cod as old as 7 were so scarce in his dataset. That was 15 years ago.

Even if we accept that 13 is an attainable age for North Sea cod, then, it’s certainly a ripe old age, so saying from these data that there are 100 adult cod left in the North Sea is like saying there are 12,640 adults living in the UK based on the number of centurions.

I don’t want to downplay the perilous state of many fish stocks, nor the fact that overfishing is primarily what’s led them there. Another statistic in the Telegraph piece is sobering: the Cefas team estimated a 2011 population of 600 cod aged 12-13, of which 200 were caught. This tallies with another personal anecdote, this time from my undergraduate project working on data obtained from tagging North Sea plaice. These tags were returned to Cefas by fishermen when they caught a tagged plaice. We reckoned on a 30% return rate within a year. So: chuck a commercially important adult fish into the North Sea, and it has about a 1/3 chance of being caught within a year. That’s pretty intensive fishing. Likewise, there’s a load of good work – much of it by Cefas scientists – showing for instance how the biomass of big fish in the North Sea is massively reduced compared to predictions of the situation with no fishing.

Given this background, isn’t it perhaps a good thing to highlight fisheries in crisis? A few people I interacted with on Twitter over this seemed to think so, for example @oceanCO2 said title is certainly misleading but the article itself has context for the numbers. As you say ‘100 cod left’ does catch the eye! and @S_Danielsson wrote situation bad even if tweets are a bit oversimplified, 93% of NS cod are caught before reproductive age. Here and elsewhere both have made reasonable points, but I cannot agree that using misleading numbers to make a point is ever justified. In this specific case, just look at the responses of some in the fishing industry, for instance @LapwingPD972 who’s been fishing the North Sea for 33 years: Who are these people that say there is only 100 mature Cod left in the north sea,OMG invitation to anyone,come away on my boat to see fish to which @AjaxAH32 (skipper with 30y experience) replied I thought it was 1st of April Brian.

This matters because the relationship between fishers, scientists and conservationists in the UK is often fraught; but listening to various talks at the Oceans of Potential meeting in Plymouth last week I really felt that real progress has been made, with a focus on the business case for sustainability which everyone can buy into. Silly headlines – seemingly designed to be picked up and circulated without thought around Twitter – set back these efforts and risk returning us to a harmful new cod war of words. More generally, you really don’t strengthen your case by simply parroting obviously nonsensical numbers.

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Postscript (19/09/12)

Since I wrote the above, Defra have released an official myth bust statement, worth quoting in full here:

The myth: An article in The Sunday Times claimed that ‘fewer than 100 mature cod are left in the North Sea’.

The truth: This is completely wrong, in fact we know there to be around 21 million mature cod in the north sea. Cod start to mature from six months old and are fully mature at age six.  There are a small number of cod over the age of 12 years old which has always been the case in the North Sea even when fished at lower levels in the 1950s and 1960s. Cod older than 15 have never been recorded in the North Sea.

So - the Sunday Times, Telegraph and Mail were out by a cool factor of 210,000…

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