white bell. The dark dorsum contrasts strongly against the rest of the body; Very old specimens are stocky, with a large head and an enormous mouth. Size Among all extant bony fishes, the beluga sturgeon rivals the ocean sunfish (Mola sp.) as the most massive fish and is the second-longest bony fish after the giant oarfish (Regalecus glesne). It is the largest freshwater fish in the world. The beluga also rivals the great white shark (Carcharodon carcharias) and the greenland shark (Somniosus microcephalus) for the title of largest actively predatory fish. The largest accepted record is of a female taken in 1827 in the Volga estuary at and . Another specimen reportedly weighed and measured in length. Claims about bigger length (, or even ); and weight (, or , or even ) are disputed and unconfirmed; but they are not impossible. Several other records of aged sturgeon exceed . Among sturgeons, only the closely related Kaluga (Huso dauricus) can attain similar size, with a maximum weight of . Beluga of such great sizes are very old (continuing to grow throughout life) and have become increasingly rare in recent decades because of heavy fishing of the species. Today, mature belugas that are caught are generally long and weigh . The female beluga is typically 20% larger than the male. An exceptionally large beluga recently caught weighed and measured . Biology Spawning Like most sturgeons, the beluga is anadromous, migrating upstream in rivers to spawn on clean, hard substrate, which offers both support and cover to their sticky and adhesive eggs. Spawning biology and development of larval stages of the sturgeon, the most ancient fish of the Danube, co-evolved with the formation of the Danube valley, resulting in very different survival strategies in its early life stages. This appears to explain why different individuals of the same long-migratory species spawn as far upstream as 1,700 km upstream, while others spawn just 100 km. To make the long journey to very distant spawning grounds, the sturgeon adapted a two-stage migration strategy, beginning in the fall when they enter the Danube River overwinter in the river and the second stage is their spawning which takes place in the spring the river in fall and staying over winter in reaches of the river offering adequate substrate and water-flow resting conditions. Very few locations of existing wintering and spawning grounds for sturgeon are presently known