When Tom Reddoch was growing up in southern Louisiana, on a pencil-thin strip of land flanked on one side by the Mississippi and on the other by a marshy inland waterway, he and his friends used to brag to each other that this was the greatest place on Earth. Children had different expectations back in the 1950s: what they meant was that they would never go hungry.

"The one thing we could be certain of is that we would never starve down here. In those days, this was one great protein factory."

Fishing and oyster harvesting was easy, as the nutrient-rich swirl of the freshwater Mississippi and the sea spawned wildlife in endless abundance. But over the course of his lifetime, Reddoch has seen about 70% of that extraordinary biodiversity fade away through the combined onslaught of overfishing, the laying of oil pipelines and man-made diversions to the Mississippi.

Now the region – which includes nearly half of America's wetlands – faces its greatest threat of all, one which, Reddoch fears, could kill off what little environmental riches are left.

"This could be the coup de grace. It could be the last blow," he says.

All along this levee-lined spit of land that stretches from just south of New Orleans down to Venice – a fishing and oil town which, as the name implies, stands on the seafront – people are bracing themselves for the arrival of what could become the worst environmental disaster the US has ever seen. About 50 miles offshore from Venice is the site of Deepwater Horizon, the oil drilling rig operated by BP that exploded on 20 April, leading to the disappearance and presumed death of 11 workers and the spewing of up to 5,000 barrels of oil a day into the sea.

Unless BP succeeds in plugging three leaks some 5,000ft (1,500 metres) undersea, the disaster will within a matter of weeks exceed even the 1989 Exxon Valdez catastrophe off the coast of Alaska.

With the oil being pushed into fragile marshland around Venice by strong winds this morning, Louisiana declared a state of emergency and the Obama administration declared it a spill of "national significance".

For the residents of Empire, a small fishing town about two-thirds of the way down the spit, this is a spill of personal significance. "I'll show you what this means for us," says Clark Fontaine, the owner of a wooden seafood shack on the side of the main road that has a dilapidated hoarding outside advertising "L VE CRAWF H".

"Look at these guys!" He holds up a handful of plump shrimp, about four inches in length. The creatures are grey in colour, and several have an orange stripe along their backs. "That's the eggs they will lay in the marshes that will produce our next crop in August. If we lose these shrimp, then we lose our living for the rest of the year."

Mark Franobich has already lost his main income. He works as a diver mechanic in the oilfields in the Gulf of Mexico, and has just been told that all operations have been shut down until further notice in the wake of the Deepwater Horizon explosion. He now relies on selling timber for firewood from his backyard.

"It was just a matter of time before something like this happened. Everyone around here is dependent on fishing: generations upon generations of them. It's all they know."

Nobody knows precisely when the oil slick will strike them hard. Surface deposits have already been spotted being driven into the grassy marshes along the coast, but how quickly it will move up the Mississippi and spread through the inland waterway is unclear. In the docks at Empire a row of oyster boats sit idle, unable to go to sea because of the high winds that are whipping up 8ft waves, thus hampering the oil clean-up. A Mexican crew is doing odd jobs on board the Lady Marija.

"We don't know when the oil is going to arrive," says Silvestre Frias, speaking in Spanish. "What we do know is that when it does, it will shut down everything."

The Lady Marija brings in about $10,000 (£6,500) worth of oysters a week, supporting the five crew and the ship's owner. "Yes, we're scared. There will be no work for any of us," he says.

Reddoch estimates the oil will reach disaster levels as far up the spit as Empire by Sunday at the latest. And that, he says, will have huge consequences for himself and the wider community.

Reddoch's small business, Down South Services, has two main contracts. The first is to do maintenance work on a nearby fish factory called Menhadden Fisheries, where he employs about 50 workers through the summer.

The factory processes small sardines known locally as Menhadden, which produce some of the highest-grade fish oil in the world. The oil finds its way into Omega-3 vitamin supplements, paint, cosmetics, and even lubricant for the space shuttle.

The plant opened its doors just last week for the 2010 season, and receives up to eight boats a day, each laden with sardine catches worth up to $1m. "Think about that for a second – when it's cooking, that's a lot of business; and all of that will be lost," Reddoch says.

Down South Services' other main contract is, fortuitously enough, to provide labour in cases of oil spillages. He is laying on men to work the booms that are being used to try to prevent the oil coming on land, and once it does his employees will be cleaning up the beaches, marshes and ships.

So what he's lost with one hand, he's gained with the other. "That's what you call diversification," he says, adding that it gives him no pleasure to make money out of the final destruction of his childhood paradise.