Caviar is the prepared roe of different types of sturgeon, including the Beluga, Oscietre and Sevruga, large shark-like fish found especially in the estuaries of rivers flowing into the Caspian and Black Seas, though they also exist to a lesser extent in China and in the USA. The eggs of other types of fish may be sold with the word caviar in their description, but only the roe of the sturgeon is held to be the real thing.
Since the collapse of the USSR and the end of state-controlled fishing, the sturgeon is being decimated there by pollution and unrestricted netting, while a booming black market has further damaged the quality of the former Republic’s caviar. Unless a total ban is imposed on sturgeon fishing now, they will become extinct there. We may think of it as quintessentialy Russian, but the majority of caviar legally imported into Britain today comes from Iran, which has a long history as a quality producer and where the same families who were involved in the business a century ago still run it today.