March 8 (Bloomberg) -- One day in February, I scored some of the last beluga caviar left in America. Caviar prices have soared since the U.S. halted imports of this wild roe in 2005 to conserve the fast-dwindling populations of sturgeon fish. Beluga -- the rarest and most prized roe -- now fetches $250 an ounce, about half the price of gold. Supplies are drying up.

With that in mind, I went to see David Magnotta, the owner of Caviar Russe in New York, in hopes of finagling a taste. ``What makes caviar caviar is its rarity'' Magnotta says. In front of him are eight mother-of-pearl spoons, each piled high with roe that glisten like dark jewels.

The spoons hold beluga as well as hard-to-get wild osetra and sevruga from Caspian sturgeon. Within the next 15 months, it will be impossible to taste beluga in the U.S., at least legally.

In addition, the United Nations has temporarily halted exports of all Caspian Sea roe to protect that sturgeon, whose stocks have tumbled 90 percent since the 1970s because of overfishing, poaching and pollution. The UN is expected to reinstate exports later this year -- although at much lower levels.

All of this leaves caviar lovers hunting for a substitute to this salty delicacy once reserved for czars and kings -- although Louis XV was said to have disliked caviar so much on first tasting that he spit it out on the carpet at Versailles.

Nature's Viagra?

Caviar has been around a long time. In order to classify as caviar, the roe must come from sturgeon, a fish that has been in existence for more than 250 million years. Sturgeon eggs are thought to have all sorts of restorative powers. The name caviar comes from the Persian khavyar, meaning cake of strength, because the Persians believed it had medicinal benefits. Also known as Aphrodite's eggs, caviar is said to increase sexual vigor. That's probably true, given that the roe contains 47 vitamins and minerals as well as arginine, an amino acid that helps increase blood flow. Think nature's Viagra.

No one knows who first salted fish eggs. Roe kept the ancient Phoenicians alive during times of war and famine, and Ovid mentioned caviar in his writings. Batu Khan, grandson of Genghis Khan, called it a ``black delicacy'' in the 13th century, and it was the Mongols who first created an international caviar trade by sending the sturgeon eggs to Italy. Modern, lightly salted caviar wasn't widely available until the 1920s, when refrigerated transportation was developed.

Picasso's Favorite

Caviar is generally associated with three species of sturgeon: beluga, osetra and sevruga. Beluga is the rarest because the female must be 18-20 years old before she produces eggs. The largest type of sturgeon, beluga can reach 20 feet (6 meters) in length and weigh as much as 1,800 pounds (800 kilograms).

Beluga was the favorite caviar of Pablo Picasso, who used to pay for his roe by sending cash wrapped in a signed sketch. Its eggs are the largest and the most delicate in flavor, with a slight buttery taste. The Black Sea beluga produces the more subtle flavor, while the roe of the Caspian beluga is a bit more assertive. Its color varies from light to dark gray, and the eggs melt in your mouth.

Osetra, the favorite caviar of James Bond author Ian Fleming, is slightly less rare because the osetra sturgeon matures at 12-15 years of age, making the product less expensive than beluga. People generally prefer its slightly nuttier, more complex taste, and it's the bestseller of the three, according to Magnotta. It has a smaller grain than beluga and varies in color from golden yellow to brown.

De Gaulle's Choice

The sevruga sturgeon, which produces the roe that was French President Charles de Gaulle's favorite, is the most abundant species in the Caspian Sea, and it matures the fastest, within five to seven years.

Its caviar is the smallest in size and, like beluga, has a buttery flavor, although it's saltier and more intense than its rarer cousin. Its eggs, which range in color from light to dark gray, almost pop in your mouth.

White sturgeon, the second-largest species, comes from the Pacific Northwest and is the lesser-known fourth caviar producer. While this sturgeon is no longer caught wild for caviar production, it's farmed both in the U.S. and in Europe. Its dark- brown roe has a flavor that's close to osetra.

In the late 1800s, white sturgeon caviar was served in bars to make patrons thirsty, and the species was almost wiped out from overfishing.

Caviar is made by adding salt, and the best products are made under the malossol method, which adds less than 5 percent salt content. Its color is designated by 000 for light caviar, 00 for medium and 0 for dark. Generally, the lighter the caviar, the older the fish. Very light or golden roe is also designated imperial or royal and was once reserved for royalty.

Farm-Grown Caviar

Dale Sherrow, head of Seattle Caviar Co., says he expects the caviar business to change dramatically as export quotas from the Caspian become more strict. ``We are going to have a lot more aquaculture in our future and much less caviar from the Caspian Sea,'' he says.

Sherrow, 55, who had sold almost all of his beluga caviar by January, says caviar from wild sturgeon will become harder to find and more expensive, with prices of wild osetra and sevruga possibly topping $250 an ounce.

And farm-grown caviar will never taste as good as wild caviar. ``Wild osetra is more complex and has more depth,'' Sherrow says. ``It lingers on the palate for minutes. You don't get that with farm raised.''

Siberian Standout

Even so, there are some fine farmed caviars available, Sherrow and Magnotta both say. The standout is Siberian osetra farmed in Germany or France, which has a nutty, complex flavor. White sturgeon roe is less multidimensional and has a milder taste.

Magnotta suggests sitting down and tasting a wide variety of roe. ``Caviar is like a diamond: You have to put them next to each other to see which shines brightest,'' he says. The best way to taste caviar is off a pearl or glass spoon (silver gives it a metallic taste), with no condiments or toast. Wash it down with a glass of champagne or ice-cold vodka.