Stocks of cod in the North Sea have increased by 5 per cent from last year according to scientists, prompting elation in the fishing industry but further warnings from conservationists.

Annual stock assessments released today by the International Council for the Exploration of the Sea (ICES) show that the number of spawning cod has risen slightly but the number of fish discarded at sea is still equal to the amount that reaches the shore.

Seafish, a body representing the seafood industry, points to a 40 per cent increase in the number of fish that have reach reproductive age, compared with the average between 2000 and 2008.

“Do not underestimate the significance of this announcement,” says John Rutherford, the Seafish chief executive. “This is the first time that ICES scientists have felt confident to recommend a substantial increase in target catch since calling for a full closure to cod fishing in 2003.”

But marine conservationists warn that much of the rise is due to a single good year for breeding in 2005 for cod, which take three or four years to mature. “Those fish started entering the fishery last year, but the majority are entering this year. That’s why we are seeing this increase,” said Sam Wilding, of the Marine Conservation Society.

“But last year’s spawning was one of the lowest we’ve seen, so we’ll be relying on the 2005 stock to see us through. If we take them all, the fishery won’t recover. In three years’ time, when we’re relying on the 2008 class, there’s not going to be enough.”

Younger fish reproduce less successfully than older fish and dependence on fish spawning for the first time is thought to be a factor in the low numbers of newborn cod in recent years. A major problem with fisheries is the number of fish that are caught but discarded because they are too young.

Efforts to control discards have been implemented by several nations including Britain, where a “conservation credits scheme” is being tried out in Scotland. Among other things, this allows fishermen to catch more in return for discarding less. But cod discard rates increased substantially last year, exceeding the number landed at the quay. Several nations failed to report on discard levels by their own vessels. The other unknown is whether European fisheries ministers will follow their scientists’ advice.

In every year except 2007, ICES has advocated a zero catch, but was ignored by the ministers, who each time pushed the total allowable catch up to 20,000 tonnes or above. Conservationists fear that similar inflation will occur this year.

Charles Clover, author of End of the Line, points out that ICES has never estimated the pre-industrial biomass of the cod, unlike colleagues on the other side of the Atlantic. “We therefore aren’t looking at the big picture, which is probably that the North Sea cod has declined by around 97 per cent since 1850, which is the case on the Scotia Bank and the Grand Banks. Would it not be better to aim for recovery to something like that abundance, rather than this continued bumping along the bottom?”

If continuing surveys indicate significant changes, ICES will issue new advice in November, before the European Commission’s quota allocation for 2010.

Times recommends

• Energy-saving ideas from switched-on professor
• Rooks are as clever with tools as chimpanzees
• Burping of the lambs blows roast off menu