By Angus Roxburgh BBC News Online, on the Dana fisheries research vessel, in the Baltic Sea

Every December the European Commission makes proposals for fishing quotas in EU waters, based on the advice of international scientists.

This year the scientists have advised that in order to save the endangered cod stocks, there should be a total ban next year on fishing for cod and associated species in waters around Britain.

The scientists trawl in poor as well as rich fishing grounds

The proposal will be haggled over by ministers at their December council in Brussels. But already fishermen have denounced it as unrealistic and unnecessary.

The dispute between scientists and fishermen over the state of cod stocks has become an annual ritual.

Every year the fishermen say the scientists are exaggerating the danger to the stocks, while the scientists say the fishermen are threatening their own long-term livelihoods by ignoring their warnings of an imminent collapse of cod in the North Sea.

So who is right? Two days spent aboard the biggest Danish fisheries research vessel, the Dana, reveal a team of dedicated scientists who feel much maligned in the annual battle over quotas.

"We feel sorry for the fishing communities," says the team leader, Henrik Degel, of the Danish Institute for Fisheries Research. "But our job is to give unbiased scientific advice. It is for the politicians to take decisions on the social consequences."

The fishermen go to parts of the sea where there are a lot of fish, so they get the impression of abundance David Griffith International Council for the Exploration of the Seas

The scientists believe they have a broader view of the situation in the sea than the fishermen.

"Because they are good at their jobs," says David Griffith of the International Council for the Exploration of the Seas (Ices), "the fishermen go to parts of the sea where there are a lot of fish, so they get the impression of abundance. We have to study the whole sea, including the gaps."

On the Dana, the crew do five or six daily trawls at designated spots on the sea, each one lasting exactly 30 minutes.

Henrik Degel says politicians must decide what to do with research

They are not interested in the size of the catch, but in getting a representative haul, which is analysed to determine the age-groups and species caught.

The fish are measured, weighed and dissected in a wet lab on board the ship. Crucial to the investigation are six little bony "otoliths" which are taken from the fish's inner ear.

The otoliths are the fish's sensory mechanism, which allow it to balance - but for the scientists they represent the best way of establishing the fish's age, for the otolith grows rings, like a tree.

The data collected on the Dana is collated with data from many other research ships around Europe.

Mathematical model

By working out the numbers of fish in each annual age-group, the researchers draw up a mathematical model which enables them to calculate what proportion of the fish stock disappears each year, either by natural causes or by being caught in fishermen's nets.

They then apply the model to the catches landed in ports around Europe, and work out how many fish from each species they believe are left alive.

The graphs they produce to show the decline in cod stocks are alarming.

This year's Ices reported that in the North Sea and Skaggerak there are only 52,000 tons of cod - one-third of the minimum stock size which they believe is necessary to ensure the species' survival.

The Dana's voyages have confirmed a huge crisis in cod stocks

In the waters west of Scotland they say there are only 2,500 tons of cod left - compared to the 22,000 tons they say are needed.

Based on these figures, they advise that no more cod should be caught until the stock recovers.

That could happen at any time, because certain years - inexplicably - produce large numbers of juvenile fish.

But there is no point in hoping for this to happen, they say: only a moratorium will give the fish a chance.

The scientists believe their estimates are correct to within about 10-15%.

But while they are dedicated to producing objective data, the decisions on how much fish will be caught are ultimately political.

At the December fisheries council ministers try to strike a balance between the European Commission - whose proposals invariably follow the scientific advice - and the fishermen, desperately trying to save their communities.

Ways forward

This year, Scottish fishermen have come up with two ideas which they hope will help to avert a head-on collision with the commission and the scientists.

Information from the fishing trips is used to compute the scale of the problem

One is to persuade the commission that they can trawl for haddock - the staple Scottish catch - without taking out too much cod as a by-catch. (The scientists argue that the two species swim largely together.)

The other is an invitation to the scientists to put far more observers on board the fishing vessels.

Mike Park, of the Scottish White Fish Producers Association, says scientists treat the landings reported by fishermen with scepticism, so having them aboard "on a continuous basis" would increase trust.

"We would get a real picture, not just an anecdotal one," he says.

In the meantime, December's fisheries council, when ministers meet to hammer out next year's quotas, is likely to be as difficult as ever.