Bulgaria has imposed a four-year ban on fishing for sturgeon on the River Danube and in its Black Sea waters, the farming ministry said Tuesday. The restrictions cover four sturgeon species, all threatened with extinction, and was prompted by the need to protect their populations and align Bulgaria\'s rules with a 10-year ban imposed in 2006 across the Danube in Romania, it said. \"In the case of chance catching of sturgeon specimens, these should be immediately returned to the waterway where they were caught,\" the statement added. It also outlawed the transportation and sale of wild sturgeon and sturgeon products in Bulgaria, apart from those breeded artificially for their expensive caviar. The Black Sea is the second largest source of sturgeon in the world after the Caspian Sea, and the Danube upstream is one of the most important spawning habitats for giant Beluga sturgeon. An ancient fish that outlasted the dinosaurs, sturgeon has been critically endangered by overfishing for its caviar, which can cost over 6,000 euros (7,800 dollars) per kilogramme. Bulgaria imposed a first one-year ban on sturgeon fishing last year. Both Sofia and Bucharest were meanwhile urged by the environmental group WWF in November to strengthen their regulations to battle persistent poaching and illegal trade in sturgeon caviar.