Oxford Dictionary definition: large edible fish yielding caviar.
Our definition: Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America. They are distinctive for their elongated bodies, lack of scales, and occasional great size: sturgeons ranging from 7–12 feet (2-3½ m) in length are common, and some species grow up to 18 feet (5.5 m). Most sturgeons are anadromous (migrating up rivers to spawn) bottom-feeders, spawning upstream and feeding in river deltas and estuaries. While some are entirely freshwater, a very few venture into the open ocean beyond near coastal areas.
Sturgeon are primarily benthic feeders (feeding on the river bed or ocean floor), with a diet of shells, crustaceans and small fish. They feed by extending their syphon-like mouths to suck food from the benthos. Having no teeth, they are unable to seize prey, though larger individuals can swallow very large prey items, including whole salmon. Sturgeons feed non-visually. They are believed to use a combination of sensors, including olfactory sensors, tactile chemosensory cues on the 4 barbules, and passive electroreceptors (ampullae of Lorenzini).
Many sturgeon leap completely out the water, usually making a loud splash which can be heard half a mile away on the surface and probably further under water. It is not known why they do this, but suggested functions include group communication to maintain group cohesion, catching airborne prey, nuptial behaviour, or to help shed eggs during spawning. Other plausible explanations include escape from predators, shedding parasites, or to gulp or expel air. Another explanation is that it “simply feels good”.
Sturgeon can live 100 years and have been around since the dinosaurs. Because of their long reproductive cycles, long migrations, and sensitivity to environmental conditions, many species are under severe threat from overfishing, poaching, water pollution, and damming of rivers. There is also a noticeable decline in sturgeon populations as the demand for caviar increases (see Roe on the R page). According to the IUCN, over 85% of sturgeon species are classified as at risk of extinction, making them more critically endangered than any other group of species.