Herring from the Thames estuary are expected to become the first fish in the world to be certified as ecologically correct. That's because the men who catch it are, essentially, peasants of the sea.

In an age when oceans and continental shelves are being pillaged by hi- tech catchers, they fish a couple of miles off Essex on day-long trips, using methods which have changed little in the past 100 years: small boats, large-mesh nets and a lot of manual labour.

It is cold, back- straining, often meagrely rewarded work, especially this winter, when unusually warm water has delayed the shoaling of the fish. But the West Mersea fishermen and a couple of other small harbours near by have rules and methods which safeguard the distinctive stock of local herring in the long term. (It is a sub-species with one vertebrae less than those farther out into the North Sea.)

That is why the new Marine Stewardship Council, an international organisation set up by the World Wide Fund for Nature and the food and fish multinational Unilever, hopes to grant them its first certificate for sustainable fishing.

Whoever sells their herring will be able to label the packaging boasting of this, easing the corporate conscience and probably allowing a premium price to be charged. ``If someone in Islington wants to pay extra for our herrings, that's just fine,'' said John Jowers, a West Mersea fish merchant and chairman of the local fishermen's co-operative.

"But we're as interested in conserving local jobs and communities as in conserving the fish.'' The boats use a long line of curtain-shaped drift nets with a diagonal mesh at least 54mm across.

They are left hanging in the water for several hours: tiddlers swim through, while big fish find their way barred and bounce off. But mature herring, aged two years or more, are trapped.

Once their heads get through the mesh the rest is too big to pass through, but they cannot swim back because their gill covers snag the filaments. Unlike a lot of North Sea fishing, theirs is targeted: nearly everything in the nets is adult herring. They catch none of the juvenile fish so vital to the stocks' future. Next to nothing gets chucked over the side. There are 14 boats involved and in summer they catch bass and Dover sole. They only go after herring during the season, from October to March, sticking to rules written and theoretically enforced by the Ministry of Agriculture and Fisheries for the northern side of the Thames estuary. The boats must be under 17m long, can only use drift nets and the total quota for all of them is 131 tonnes a year, which one big trawler could catch in less than an hour.

Mr Jowers says it was the fishermen who demanded the constraints to conserve stocks, who stuck to them for years before the ministry made them law and who enforce them among themselves. He reckons 100 local jobs rely on the fishery, with their herring trucked to markets around Britain and on the Continent. Some families from West Mersea, near Colchester, have been catching herring since the Middle Ages. ``It provides a good living for kids who are never going to get lots of exams at school. If a place likes this loses its locally working community it just becomes a dormitory suburb ... It's no longer a pleasant place to live.''

In the harbour, skipper Chrissy Mole and his crewman were shaking about 30 stone of herring out of his nets. That would fetch about pounds 70, not much for catching the tide at 3am and not getting back until 4pm. ``It's not an easy job and a lot of people don't like doing it,'' he said. He and Matthew Howard, another West Mersea fishermen, enthused about their methods being certified by the Marine Stewardship Council. ``When we tell people how we catch herring, they look at us as if we're mad,'' said Mr Howard. Certification, which, it is hoped, will be completed by October, will not dramatically hike the price of their fish but could guarantee a decent market throughout the season, avoiding the occasional collapses to 50p a stone, which make it not worth their while to go to sea.