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A fascinating document has fallen into my lap from now going on in Madrid looking at tuna stocks and catches.

Tuna in fish market

The question it asks - perhaps without intending to - is this: can the Mediterranean bluefin industry ever be properly monitored?

With the bluefin , it's a question with major ramifications politically, commercially and ecologically.

To begin at the beginning: this week's meeting brings together members of the .

It advises the government representatives who make decisions within the .

Anyone familiar with the field will know that the Atlantic bluefin is in trouble, having undergone - largely because of the recent dramatic expansion of fleets in the Mediterranean, which has led to sustained increases in legal and illegal catches.

The big players now are , which use nets to encircle and then scoop up whole groups of bluefin as they spawn. Many of the fish are transferred to ranches and farms, where they're kept in cages until they're plump enough to command maximum profit.

One of the biggest problems identified by Iccat is how to keep track of the fish during this chain of events.

How many fish are actually caught by purse seiners, and how much does the catch weigh? How much goes to ranchers, and is the process traceable?

The 2010 fishing season saw initial operations of the , designed to monitor the biological production line, and operated by consultants and .

It is their draft report back to Iccat that has now fallen into my lap; and very revealing it is.

To the very basic question of all - how much the purse-seiners caught - there are two sets of answers. One consists of data submitted by vessel captains - the other, estimates made by observers on board those same vessels.

And they differ - sometimes hugely.

Using captains' records, the total catch for the 2010 season was 3,829 tonnes; but the observers' estimates tot up to a mere 2,367 tonnes.

Some of the national figures are even more out of whack. French vessels reported a catch double the observers' estimate: Greek captains, even more remarkably, reported a haul of 37 tonnes, while observers on Greek boats saw them catch not a single fish.

How can this happen? The consultants pull no punches:

"The principle reason cited by observers was that there was no reliable means to accurately estimate the number and weight of tuna caught."

Some observers discussed this with the vessels' crews, and tried to use the insights gained to make more accurate estimates; but this appears, if anything, to have muddied the already turbid waters:

"One observer consulted the master on interpreting sonar images which resulted in catch estimations 20%, 25%, 50% and 60% greater by weight than the vessel declaration."

Adding to the confusion is the practice of joint fishing operations, where a group of vessels working together can decide to share the catch, even though only one of them may have physically caught the fish.

This, presumably, explains the Greek situation. It certainly presented a challenge to observers, with the consultants' report noting:

"...no fishing operation was conducted, yet (documents) were generated, which the observer was obliged to 'verify', 'certify' and to countersign."

The observer programme was designed to follow the fish through the chain, monitoring the transfers to towed cages and thence to farms.

Here, another curiosity arises: the weight of tuna registered as having been transferred, at 4,136 tonnes from vessels' records, is considerably larger than the amount that was supposed to have been caught in the first place.

Purse seine net

And when the fish were deposited at farms, the figure swelled again, to a declared weight of 10,188 tonnes.

(The latter figure is, I'm told, under review, with records being checked to eliminate any double-counting; however, I'm also told that a purse-seine owner admitted at the Madrid meeting that catches had been a lot higher than the figures submitted by fleets.)

The report contains a lot more in this vein, with some passages hinting at ways in which illegally caught tuna could be introduced to the supply chain.

The consultants do have suggestions for improving the process; but even so, their report provides on-the-ground evidence of just how hard it is to monitor the number and weight of bluefin being extracted by this most extractive of fishing methods, especially when not all players in the lucrative industry appear to have welcomed the monitoring.

This week's SCRS meeting will conclude by making recommendations on the catch quota for next year, which covers all fishing methods rather than just purse-seiners; they're to be published on Friday.

The recommendation, I gather, will be that anywhere in the range between 0 and 13,500 tonnes for 2011 is consistent with - namely, to set quotas that give at least a 60% chance of restoring the Mediterranean bluefin fishery to health by 2022. (This year's total was 13,500 tonnes.)

However, the scientists are also likely to note that somewhere closer to the 0 figure would be precautionary given the uncertainties noted by this consultants' report.

The size of the discrepancies is worth emphasising. There's more than a fourfold difference between the lowest and highest indicators of catch (the observers' reports from purse seine vessels and the weights arriving at farms, respectively).

Getting a quota wrong by a factor of four would be enough to take a fish population to commercial extinction. It's that important; and it's with that caveat that the SCRS recommendations will go forward to the Iccat meeting in Paris at the end of November, when national delegates will decide how precautionary they want to be for next year.

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