Steve Connor: Quotas are the penalty for exceeding safe, sustainable limits for the past 25 years

The scientific consensus is that cod stocks in the North Sea have been in decline almost continuously since the early 1970s and we are now close to total collapse unless something drastic is done soon.

Each year government scientists from around Europe meet under the auspices of the International Council for the Exploration of the Sea (Ices) to assess the state of cod stocks in terms of the number of breeding adults. This number – it is actually expressed in terms of weight – is known as the "spawning stock biomass" and scientists estimate that it needs to be around 150,000 tons to ensure a healthy cod population.

However, the actual amount of adult cod has fallen below this critical level every year for the past 20 years – and the decline continues to get worse each breeding season. Current research suggests that the spawning stock biomass for cod in the North Sea is about 38,000 tons, which is a third lower than it was two years ago and almost three times lower than in 1988 when it was 100,000 tons – two thirds of the sustainable level.

As breeding adults have declined, the rate at which fish are being caught has risen steadily. In fact the Ices scientists believe that "fishing mortality" has exceeded the safe, sustainable limit for the past 25 years or so.

There is one complication in the otherwise simple equation suggesting that too many fish are being caught to maintain a viable breeding population. This is something called "recruitment", which is the measure of how many newly hatched fish survive the first year to become potential breeders.

Recruitment can vary widely from one year to the next – some scientists say it can easily be five times bigger than expected – because of unpredictable factors such as the weather and passing predators. In 2000, for instance, marine scientists found that young cod less than a year old were highly sensitive to rising sea temperatures. Eggs and the juvenile stages are easily killed by a sea that is just a degree or two warmer than normal.

Measurements taken over the past few years clearly indicate that fewer and fewer very young cod are celebrating their first birthday. This could indicate a smaller breeding population, a greater death rate in the first year or both. Whatever is the cause, the signs of an imminent collapse are writ large, which is why the Ices scientists believe that something radical has to be done. In the past, control has centred on "total allowable catches", or quotas. Some scientists say that this policy is a failure because the quotas have been set too high – many countries have not been able to find enough cod to fulfil their allowances.

One suggestion, supported by scientists such as John Shepherd, professor of marine science at Southampton University, favours a more direct control of individual trawlers whose catches and movements could be monitored by satellite.

It may be the last chance of saving the better half of Britain's national dish.