They are the staple starter in upmarket restaurants – fried with garlic, chilli and chorizo, perhaps, and washed down with champagne or a white wine.

Adored by chefs for their versatility, meaty texture and delicate taste, scallops are increasingly popular on domestic menus, too. But in the freezing waters of the Irish Sea, a dispute is raging over the rights to fish them when they are young and succulent.

Such is the severity of the problem that the Isle of Man authorities have introduced a bylaw preventing fishing for scallops amid protests from Scottish fishermen. The Scots claim they are victims of discrimination.

More than 100,000 tonnes of scallops were landed in 2008, worth £25.1m, to the Scottish economy, according to Seafood Scotland. But overfishing by massive trawlers around the Isle of Man is threatening the marine ecosystem. A flotilla of huge vessels sails in for two weeks every November, takes the cream of the crop and then disappears for 50 weeks until the start of the next season.

The island's authorities are keen to conserve and protect its scallop beds to ensure they are not overfished to disastrously low levels. It is claimed that so many young specimens have been taken during the key November period that the population is being put at risk. So many are being harvested, say campaigners, that huge amounts are being discarded as the processing companies can't cope.

The island's bylaw bars from its territorial seas all vessels over 300hp that have not fished at least 50 days in the area over the past 18 months. The Scottish fisheries minister, Richard Lochhead, has called it a "war" against Scots. But environmentalists regard it as a simple choice between sustainable and non-sustainable fishing.

Lochhead described the Manx bylaw as "unnecessary and unwarranted". He said: "While some Scottish vessels can still fish those waters, others are excluded because of the size of the vessel, but the vessels that are still able to fish can fish as much as they want, and we don't feel that that's a conservation measure. There's been no scientific case to back up the bylaw that's been put in place, which blatantly just excludes Scottish vessels."

But Andy Read, director of fisheries in the Isle of Man's government, has said that more of the scallops being caught are small and are of little commercial value. In 2009, half were undersized, Read said, adding: "Last year the largest crews were using heavy, metal gear bouncing across the seabed and smashing rocks." Many of the juvenile scallops were thrown back, but they died. Read says the vast majority of the young scallops were "wiped out in a fortnight last November".

He added that the problem was the sheer number of vessels: last year more than 40 were fishing in one square mile. He says the island's move might bring short-term pain for the fishermen, but there is scope for long-term gain. It was, he said, possible for the interests of conservationists and fishermen to co-exist.

Conservationists largely back the Manx government's stance. Dr Jean-Luc Solandt, the biodiversity policy officer at the Marine Conservation Society, welcomed the Isle of Man's marine protected area, which he said would benefit local fishermen, help to secure livelihoods and the regeneration of seabed wildlife.

"The Isle of Man was the first location in the British Isles where a zone was set up to scientifically study the effect of a closure on scallop fishing," he said. "This came at the end of the 1980s, when scallop catches were down and fishermen and local scientists agreed that an experimental protection area could provide answers.

"Other areas around the world which have been set aside as marine protected areas have shown a tremendous response to such measures. On the whole, the weight of marine life recovers over fivefold, and the diversity increases by over 100%. Fish, and shellfish, grow large inside these reserves, and go on to breed, and therefore supply surrounding waters with greater catches of fish. Our seas are exhausted. But marine reserves and protection have incredible value."

Chef Paul Heathcote of the Heathcote Grill and Bar, a new restaurant in Clitheroe, Lancashire, says the demand for scallops is unlikely to drop: "You can guarantee the other half of the menu won't get touched when scallops are on.

"From a chef's point of view it is versatile and very quick and easy to cook. It is utopia on a plate."