Set against the sweeping expanse of the Forth estuary, the boats in Port Seton's harbour bob gently up and down with the ebb of the tide. While so many coastal villages around Scotland have become shells of their former selves due to falling fish stocks, Port Seton has stayed alive as a fishing community thanks to one of the few catches which still offer a viable living - prawns.

"The boats fish primarily langoustines - there's hardly any fish to be had," says Daniel Mack, who runs Neptune's Larder on the harbour front, and whose father is a prawn fisherman. "Many moons ago, the firth was full of oyster beds and mussels. There used to be huge numbers of boats - herring fleets, huge amounts of mackerel and cod - but now there's virtually nothing."

The market for Port Seton langoustines, though, is thriving. The catch is valuable and sold not just in the UK, but in Spain and elsewhere in Europe. And while it is currently not possible to fish for mussels in the firth, as they are regarded as unfit for human consumption, says Mack, the fishermen of Port Seton hope that one day that will change. Mack has already seen a massive improvement in water quality in the Forth: "Through the industrial period, there was heavy pollution. I can see a big difference now and it's heading in the right direction," he says.

But there are threats, too. For one thing, the community is concerned about proposals to allow ship-to-ship oil transfers in the Forth, which, if there were an accident, could do great damage to the marine environment.

And then there's the small matter of Seafield sewage works in Edinburgh. Millions of litres of sewage spilled into the Forth over the weekend after a pump failed at the plant, which deals with most of the waste from about 800,000 people in the region, on Friday. The Food Standards Agency Scotland said yesterday that food caught in the area should not be eaten and urged fishermen not to fish in the vicinity. The Scottish Fishermen's Federation said the warning was "premature" as "most shellfish, probably the majority of fish, is caught further out".

"It's a real concern," says Mack. "Everyone is hoping it will be cleaned up as soon as possible."

Among those who have been anxiously watching events unfold are bird and marine conservationists. As a haven for wildlife, the Firth of Forth may not look promising, what with Grangemouth oil refinery, Cockenzie power station and Torness nuclear power plant all lining its banks while a steady traffic of oil tankers and ferries travel up and down. Yet the ecology of the firth is of international importance. It is one of Europe's most important nesting stations, with around 400,000 birds living along its shores, and supports whales and dolphins.

You can tell how important it is by the forest of SPAs, SSSIs, SACs and other conservation acronyms that have sprung up around it. The Firth of Forth Islands Special Protection Area (SPA) covers the likes of Bass Rock, the Isle of May and Inchmickery, near Edinburgh.

Sir David Attenborough has described Bass Rock as "one of the wildlife wonders of the world". A spit of volcanic rock rising 100m into the air, it is home to a staggering 140,000 gannets from January onwards. Two to three dozen film crews a year come to film the birds.

"Bass Rock is the biggest single rock gannet population in the world," says Tom Brock, chief executive of the Scottish Seabird Centre at North Berwick and chairman of the Sustainable Tourism Partnership.

The Isle of May off the Fife coast is another hugely important sanctuary. It has 100,000 puffins and the largest grey seal population on the east coast of Britain.

There's also the Firth of Forth SPA, which covers a broad range of coastal and intertidal habitats stretching 100km from the River Forth at Stirling out to the estuary mouth. Saltmarshes, dunes, grasslands and mud-flats support a wide range of divers, sea-ducks and waders, such as eiders, redshanks, godwits, golden plovers and grebes.

"In terms of seabirds, the Forth is one of the most important areas in Britain," says Lachlan Lamont, a Scottish Natural Heritage area officer for Lothian, Fife and Falkirk.

The water has "definitely got cleaner" over the past 20 years, he says. "A lot of the industrial effluent discharge has been stopped or cleaned up considerably."

Obviously, the Seafield spillage has not helped. But Lamont is optimistic that it will do the bird population no real harm, especially as solids had been removed from the waste that was flowing into the firth. "The liquid effluent gets dispersed quite quickly," he says. "It will be diluted. In terms of the whole ecological system, it's better not to have it, but actually birds can cope quite well with it."

The Marine Conservation Society (MCS) says that some species of filter-feeding shellfish, such as surf clams and razor shells, are harvested for personal consumption in the firth. These can accumulate pathogens from the water and therefore those caught on its shores should not be eaten.

But Calum Duncan, Scottish conservation manager with the MCS, agrees with SNH when it comes to the broader environmental impact. "I think this recent spillage is more of a human health issue than a huge environmental concern," he says. "This is an acute event, but there is drip-drip pollution going on all the time. All over the UK we have sewer overflows in cases of bad weather."

But he supports an investigation into why no back-up system existed to prevent the effluent having to be released into the sea.

After all, the area is becoming popular for spotting porpoise, dolphin and occasionally whales.

Bottle-nosed and common dolphins are not uncommon, especially near the estuary mouth. Killer whales are also spotted occasionally off the Fife and East Lothian coasts, including in January by workmen on the Forth bridge.

A "whale and dolphin hotline" begun by Green MSPs at the end of last year for members of the public to provide details of sightings in the Firth of Forth collated sightings of nearly 100 animals in eight weeks, including killer, minke and pilot whales. There's even been a sighting of a fin whale - the second-largest whale - off Craigleith. Perhaps the most exciting report, however, was of a leatherback turtle as far inland as Alloa, about five years ago.

Against this backdrop, there are acute fears about the possible effects of an oil spillage on the local environment. "We really have a jewel in the crown here in terms of wildlife," says Tom Brock. "We've got fantastic beaches on both sides of the Forth. It's very important for the economy.

"I think we've made tremendous strides forward in terms of trying to improve the environment. But Seafield and the ship-to-ship proposals show that things can happen. It's appalling that raw sewage going into the water is the contingency plan." he says. "My worry is the long-term consequences in terms of lack of contingencies. Our tourism depends on a clean environment. We'd be absolutely crazy to put that under threat."

Oil transfers flowing to disaster
Proposals to allow nearly eight million tonnes of Russian crude oil to be transferred each year between tankers in the Firth of Forth has provoked impassioned opposition from conservationists, councils, fishermen and local communities along the Forth.

Sunderland-based Melbourne Marine Services (MMS) wants to establish an anchorage four miles off the Fife coast at Methil, where small tankers from terminals in the Baltic and Barents seas can pump oil into giant tankers to deliver to the US and Far East. More than 100 transfers are envisaged every year. Such transfers do take place at Scapa Flow in Orkney, Sullom Voe in Shetland and at the Nigg oil terminal in the Cromarty Firth, but conservationists warn that a spillage on the firth's unique ecosystem could be severely damaging. According to RSPB Scotland, the consequences of a small spill could be "catastrophic".

The Scottish Executive has conceded that ministers cannot step in. Forth Ports Authority used to be a publicly-owned company, and that meant it had regulatory powers in the firth. Its privatisation in 1992, making Forth Ports plc, left it with the same powers.