Friday, December 11, 2009

The Numbers - revisited

Ever heard of the Monty Hall problem?

Me neither, until the silly con-troversy about the correct Shark numbers exploded in the blogosphere.
I stumbled across this prime example of counter-intuitive numbers crunching (try it, you'll be surprised!) after researching the Bayesian Inference, a method of data analysis that is based on Baye's Theorem and promises to deliver better results than its more conventional counterpart. Having once studied the latter, I can now attest to the fact that they will both trigger equally severe migraines.

The reason for my digression into the arcane realm of advanced statistics is that I've finally managed to read the relevant paper dealing with those controversial numbers, and that the researchers pride themselves in having applied Bayesian statistical methods when analyzing their data.
Its title is Global estimates of shark catches using trade records from commercial markets and you can download it right here.

In a nutshell, it comes to the conclusion that based on the Shark fin market, as of 2000, the global annual number of Sharks killed was between 26 and 73 million, with a median value of 38 million (you can find this at the bottom of page 1119).

That's certainly useful information as it attributes an order of magnitude, tens of millions, to that specific problem.

What I found more interesting are the numbers on page 1121 that attribute orders of magnitude, and consequently, relative values, to the trade in fins of the 11 identifiable species, i.e. Blue, Silky, Oceanic Whitetip, Sandbar, Shortfin Mako, Great Hammerhead, Bull, Dusky and Tiger , plus unspecified Hammerheads and the Threshers - which if you run the numbers, incidentally means that approx. half of the inspected fins were of unknown origin.

Still, and I'm repeating myself, the whole thing is nothing more than an erudite crapshoot.
You may want to scroll to page 1117 and the chapter Extrapolation to global trade volumes where an attempt is made to estimate the global trade figures based on the hard data available, i.e. the number of fins traded in Hong Kong. You will undoubtedly notice that there's a formula which is undoubtedly theoretically impeccable and, one would think, should thus yield impeccable results.

But only in theory.
The practical problem is of course that every single one of its variables is based on assumptions.
Plausible as those may be, that sure is a lot of cumulative speculation and the end result is accordingly rather fuzzy, a fact that the researchers themselves are more than willing to concede.

Also, please keep in mind that the paper only investigates the number of Sharks killed and then traded for fins.Granted, those may indeed be the majority (or maybe not if one considers the waste of discarded bycatch) and thus a strong indication for the scope of all global catches and granted, the latter numbers would be obviously higher - but claiming anything beyond that would just be utter speculation and of little help to the cause of Shark conservation where we need to advance credible, fact-based and verifiable arguments.

I've said it before, the numbers per se are rather irrelevant.
To be of any use, they must be put into context, by comparing them to what is known about the status of individual species in individual habitats - as in page 1122 where the researchers have applied their findings to the situation of the Blue Sharks, the species which is yielding the largest amount of fins.
These are their conclusions.

Acknowledging the margins of error, and the likely downward bias of trade-based estimates, our evaluation,using a Pacific numbers-based reference, suggests that blue sharks globally are being harvested at levels close to or possibly exceeding MSY (Maximum Sustainable Yield).
In contrast, our comparison with an Atlantic, biomass-based MSY reference point suggests catch levels may be less problematic.

But then, this.

Given that we have no population estimate, we are not able to evaluate the actual sustainability of our estimated catch levels. The MSY reference point is the highest possible catch that could theoretically be sustainable, and thus any catch that approaches or exceeds this level is of concern.

And this.

As a result of the global nature of our assessment we cannot evaluate the exploitation status of individual populations.
Furthermore, the blue shark is one of the most prolific and resilient of shark species (Smith et al. 1998; Corte´s 2002) and thus our blue shark results cannot be used to make inferences about other shark species. Conclusions regarding the sustainable or unsustainable use of other species, and thus the shark fin trade as a whole, will require more detailed species-based stock assessment reference points.
However, given the lower productivity of the other species common in the fin trade (Smith et al. 1998; Corte´s 2002), the large difference between trade-derived estimates of exploitation and the catch estimates reported to the FAO adds to
growing concerns about the overexploitation of sharks.

Alas, we don't seem to have those data.
Plus, and that's one of the recurring threads of this blog, time is running out and we must stop procrastinating.
Collecting and analyzing the required data is difficult and tedious, sometimes impossible, and whilst this happens, the stocks may well decline below their Minimum Viable Population size and be irrevocably doomed for extinction.

Until we know more about the parameters that determine the MSY, and thus, the extent at which each species can be harvested sustainably given its current numbers, its life history and the attributes of its individual habitat, it is imperative that we apply the precautionary principle and pre-emptively halt, or at least severely curtail any further harvesting whenever, and as soon (!) as we perceive that the animals are being overexploited.
On a happy note, that's what I understand just happened in Florida, which is great!

With that in mind, the current debate may be intellectually stimulating - but other than generating a lot of incestuous and superfluous sound bites about the pathetically trivial imperative that we need to get the facts right before opening our mouth, it is of little practical use.

May I thus humbly suggest that we, yours truly included, stop vociferating and revert back to the mission at hand, to try and protect the animals we all profess to love.


Anonymous said...

And a volcano erupted in Fiji this week...actually it was Mikes brain exploding.

Damn my ADD would not have gotten me past page 8 so that for that!

DaShark said...


Yup that's precisely how I feel!

The synopsis for your ADD being that:

- tens of millions of Sharks are being killed for their fins every year

- that's just too many to be sustainable