At a lonely stretch of Caspian Sea coast east of here, a sailboat with two men aboard silently approaches the shore where others are waiting. Large fish change hands rapidly in the night, and the sailboat sets out to sea again.

In this coastal town 100 miles northeast of Tehran as the crow flies, the owner of a confectionery augments his income from the sale of cookies and cakes by doing a brisk trade on the side in a more expensive product.

At a roadside stall on the way to the capital, a grizzled vendor displays fish that have been poached from the nearby waters of the world's largest inland sea. Caked in ice inside a crate under a canvas cover are tins of its specialty: "The Pearls of the Caspian."

All these Iranians are involved in the smuggling and illegal sale of caviar, the tiny, gray or honey-colored eggs produced by Caspian Sea sturgeon and prized around the world as one of the great gastronomic delicacies. And now the increase in poaching and smuggling is raising fears of a future "caviar crisis" in Iran and, by extension, the rest of the world.

With the breakdown of goverment authority in Iran that accompanied the revolution against the shah, caviar smugglers are doing a booming business - and more openly than ever before. Yet it is a business still fraught with dangers for the buyer and the trafficker. (Recently, according to dealers here, a man was shot by police while driving a truckload of caviar to a distribution point in the nearby town of Sari.)

There are dangers for the buyer, too: Gourmets must be careful lest they get stuck with black-market caviar that has not been processed or stored properly, in which case the little eggs can turn and become poisonous, even lethal, according to Iranian officials.

The central goverment has long maintained a monopoly on the lucrative caviar industry. Thus, it is illegal for ordinary citizens to catch sturgeon in Iran's Caspian Sea waters and sell the caviar or the fish themselves.

The reason for this, Iranian officials say, is to guard against depletion of the sea's sturgeon and protect buyers from contamination by bad eggs. But residents of this and other Caspian towns generally seem to regard the measures as an attempt to deprive fishermen of a decent living.

The world's leading caviar exporter, Iran last year sold 156 tons worth about $10 million. Although the neighboring Soviet Union has a much longer Caspian coastline, Iran's deeper and less polluted water attract proportionately more sturgeon. In addition, the Soviets themselves consume much more caviar than Iranians, even to the point of having to import 50 tons of it a year from Iran.

Swayed by the relative lack of wealthy local gourmets and higher prices abroad, some entrepreneurs have tried to set up international caviar smuggling operations. Little information has been available about them, but the Iranian goverment evidently considered the problem serious enough earlier this month to have alerted western Europen goverments to the arrival of a large illegal shipment.

Recently some nine tons of polluted caviar have been brought as contraband into Hamburg and Copenhagen, and it appears that the aim is to introduce the same for sale in the markets of Western Europe, warned a May 14 letter from an Iranian embassy to one European foreign ministry.

The letter asked the ministry to instruct local customs and health authorities to prevent sales of Iranian caviar lacking proper Iranian goverment certificates.

The letter did not explain how Iranian authorities knew the contraband was unhygienic and polluted. Western sources here believed that Iran was more concerned that the nine tons - amounting to about 6 percent of the total official exports last year - would seriously undercut European sales of the goverment brand.

"We are in trouble," says Ali Rejali, general manager of the states-owned Iranian Fisheries Company. "If this goes on for another two or three years, we are not going to have any more caviar."

He adds, however, that the goverment is resuming patrols to check for poaching and hopes to have the situation under control in another two or three months.

Authorities also are placing their hopes to avert such a crisis in Iran's first sturgeon hatchery near Bandar Anzali, formerly Bandar Pahlavi, the country's "caviar capital." The harchery has been producing 3.5 million baby strugeon a year of which only an estimate 3 percent survive after being released into the sea. The Iranian Fisheries Company plans two more hatcheries to boost production to 10.5 million fish a year.

Complicating the conservation effort is the long lead time before sturgeon can produce caviar - something which in the nature of these things only happen once.

The big Beluga sturgeon, which weighs hundreds of pounds at maturity. only starts producing the priced eggs when it is about 18 years. The Osciera species of sturgeon matures several years sooner, but only grows to about 130 to 150 pounds and thus produce less caviar. The smallest variety, the Sevruga, reaches about 350 pounds and has become fertile in as little as six years.

According to dealers, a sturgeon weighing about 130 pounds normally produces about 18 to 26 pounds of caviar. On the black market here, that caviar sells for 600 to 900 rials per 300-gram can, which works out to $12.92 to $19.38 a pound. The official price for caviar processed and packaged by Iranian Fisheries Company canneries is about 32.30 a pound in the United States. Sevruga caviar retails for about $140 a pound.

At the Isakhani Confectionery in Babolsar, Seed Abdullahzadeh, a schoolteacher in his mid-20s, closed a deal with a couple of visitors for two cans of black market caviar. As he was doing so, two men from the fisheries company walked in and began chatting amicably with him.

"They are only employes, not executives," Abdullahzadeh explained. "They realize that fishermen have to make a living."

Local police and gendarmerie officials, however, have started to crack down. Sturgeon had been sold openly on the streets of Babolsar, but last week authorities began arresting vendors and fining some up to $140.

This has not deterred dealers like Abdullahzadeh. "Nothing can stop us from plying our trade," he said. "The Ministry of Education only pays me 30,000 rials ( $430) a month and I have to sell caviar to make ends meet." CAPTION: Picture, no caption