in the lower Danube, and none are known to exist in the river's upper reaches. The same situation concerns nursery sites upon which young sturgeon depend during their journey to the Black Sea. Males attain sexual maturity at 12–16 years of age, whereas females do so at 16–22 years. They will spawn every four to seven years. At one time, beluga sturgeons could migrate up to upriver to spawn, but dams in almost every major tributary that they utilize have impeded historic spawning routes. The female lays her eggs on gravel from underwater. Upon hatching, the embryo are long, and 10–14 days later when they absorb their yolk sack, the length is . Thereafter, the larvae usually subsist on benthic invertebrates, but when reaching at least in length, they will switch to a fish-based diet. While swimming back to the ocean, the young sturgeon may cover up to a day. Diet Huso huso is a pelagic predator whose local distribution is not influenced by the nature of the substrates, unlike with most of the sturgeons that show demersal attitude. The prey is sucked into the mouth opening extremely quickly. Juveniles feed on benthic invertebrates in rivers and shallow coastal waters, where they grow quickly. At the length of 8–10 cm, they become largely piscivorous. Different diets have been observed throughout the distribution range of beluga sturgeon, as well as according to spawners' migration stage. Adults mainly eat a great diversity of large fish (73% of the diet). Additional food items may include molluscs and crustaceans, aquatic birds and young seals (Caspian seals, Pusa caspica). The piscivorous diet of beluga sturgeon tends to change with age: in the Caspian Sea, it mainly consists of Clupeonella sp. for juveniles smaller than 40 cm, different species of Gobiidae for fish ranging between 40 and 280 cm and then mullets, Alosa sp. and other sturgeons for the largest. In brackish environments of the Ponto-Caspian basin, the genera Alosa, Aspius and Engraulis are the preferred prey. In estuaries and rivers of the same area, migrating spawners eat various cyprinids, mainly Cyprinus carpio and Rutilus rutilus, Sander lucioperca and, among sturgeons, Acipenser ruthenus is the main prey. Little is known about the diet of the Adriatic population. It has been reported that in marine and brackish environments, adult H. huso forage primarily on molluscs (Cephalopoda, of which common cuttlefish, Sepia officinalis, and European squid, Loligo vulgaris,