The number of cod in the North Sea has fallen to a critical low point, a senior fisheries scientist says.
By Alex Kirby
BBC News Online environment correspondent
Unless the surviving fish can reproduce successfully, he believes, the area's cod fishery will soon be a memory.
A good catch of cod
The cod are now at the lowest level ever seen, according to his observations.
But Scottish fishermen say their catches prove there are ample fish left.
The scientist is Dr Robin Cook, head of the Scottish Fisheries Research Service, based in Aberdeen, in north-east Scotland.
He told BBC Radio Four's environment programme Costing the Earth: "There's been a small increase in the cod stock over the last year, but we don't expect that to be a long-term change unless successful management measures are introduced.
"The change over the last 25 years has been an almost continuous decrease - it's gone down by a very substantial amount.
"The stock is currently at about the lowest level it's been observed, but it has recovered from levels almost as low, so the important thing is to make sure it doesn't go any lower.
"The remaining fish have to produce young, and if they can't, then we're lost. We've certainly reached a critical point... if we continue to fish in the same way as we fished a couple of years ago, then the prospects are not at all good."
Dr Cook is one of the UK's two scientific delegates to the International Council for the Exploration of the Sea (Ices).
Busy day at Peterhead
The European Union fishery ministers use Ices' advice when deciding their hotly-contested measures to control fishing by member states, including catch quotas and the permitted number of days at sea.
Dr Cook said Ices obtained its data from survey vessesls, trips by its scientists on commercial fishing boats, and catch landing figures recorded at harbours.
New look wanted
But many fishermen sailing from Scottish ports dispute its findings. Robert Mitchell, now directing a flotilla of trawlers from his base in the small town of MacDuff, spent 25 years at sea.
He told Costing the Earth: "Given that the Ices scientists have had over 20 years to get this fisheries management right, and with a total failure to do so, I think it's time now to look elsewhere for guidance.
"We believe the evidence we're getting has been so badly mismanaged that we need a proper investigation into the state of our stocks."
Mr Mitchell said the Faroese had ignored Ices' advice to cut their cod catches. Instead, they had followed advice from Icelandic fisheries scientists, who had correctly said they could safely maintain existing catch levels.
Carol MacDonald and Morag Ritchie of Peterhead founded the Cod Crusaders, campaigning against plans to reduce catches, and have collected 45,000 signatures supporting their petition.
Mrs Ritchie told the programme: "We know the true scientists of the sea are the fishermen, our husbands who are out there. We know the fish are there, we see what's landed every week when they come home."
MacDuff fears the future
The Crusaders campaign for unrestricted catches of cod, haddock and whiting, the three species for which Ices is urging a complete suspension of fishing.
A skipper who had just returned to Peterhead from a week at sea with a mixed catch including cod said he had no doubt there were plenty of fish available.
But he, like others interviewed by the programme, said he thought warmer water in the North Sea was driving the cod northwards.
Ann Bell, fishing industry co-ordinator for Aberdeenshire council, says the cod are in crisis, and wants more co-operation between scientists and fishermen.
She told the programme: "There is a need for fishermen to be more involved in decision-making.
"If they were they might realise why they need to conserve the fish. But I think a lot of them are aware of the need for conservation."
Costing the Earth is broadcast on BBC Radio 4 at 2100 BST on 28 August.