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Sturgeon

Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. Their evolution dates back to the Triassic some 245 to 208 million years ago.Both sturgeons and paddlefish have been referred to as "primitive fishes" because their morphological characteristics have remained relatively unchanged since the earliest fossil record. Sturgeons are native to subtropical, temperate and sub-Arctic rivers, lakes and coastlines of Eurasia and North America.

Sturgeons are long-lived, late-maturing fishes with distinctive characteristics, such as a heterocercal caudal fin similar to that of sharks, and an elongated spindle-like body that is smooth-skinned, scaleless and armored with 5 lateral rows of bony plates. Several species can grow quite large, typically ranging 7–12 feet (2-3½ m) in length. The largest sturgeon on record was a Beluga female captured in the Volga estuary in 1827, weighing 1,571 kg (3,463 lb) and 7.2 m (24 ft) long. Most sturgeons are anadromous bottom-feeders which migrate upstream to spawn but spend most of their lives feeding in river deltas and estuaries. Some species inhabit freshwater environments exclusively while others primarily inhabit marine environments near coastal areas, and are known to venture into open ocean.

Several species of sturgeon are harvested for their roe which is processed into caviar—a luxury food and the reason why caviar producing sturgeons are among the most valuable of all wildlife resources. They are particularly vulnerable to overexploitation and other threats, including pollution and habitat fragmentation. Most species of sturgeon are considered to be at risk of extinction, making them more critically endangered than any other group of species.

Sturgeons retain several primitive characters among the bony fishes. Along with other members of the subclass Chondrostei, they are unique among bony fishes because the skeleton is almost entirely cartilaginous. have four barbels—sensory organs that precede their wide, toothless mouths. They navigate their riverine habitats traveling just off the bottom with their barbels dragging along gravel, or murky substrate. Sturgeon are recognizable for their elongated bodies, flattened rostra, distinctive scutes and barbels, and elongated upper tail lobes. They are also among the longest-lived of the fishes, some living well over 100 years and attaining sexual maturity at 20 years or more. The combination of slow growth and reproductive rates and the extremely high value placed on mature, egg-bearing females make sturgeon particularly vulnerable to overfishing

Sturgeons are long-lived, late maturing fishes. Their average lifespan is 50 to 60 years, and their first spawn does not occur until they are around 15 to 20 years old. Sturgeons are broadcast spawners, and do not spawn every year because they require specific conditions. Those requirements may or may not be met every year due to varying environmental conditions, such as the proper photoperiod in Spring, clear water with shallow rock or gravel substrate where the eggs can adhere, and proper water temperature and flow for oxygenation of the eggs. A single female may release 100,000 to 3 million eggs but not all will be fertilized. The fertilized eggs become sticky and will adhere to the bottom substrate upon contact. It takes 8–15 days for the embryos to mature into larval fish. During that time, they are dependent on their yolk saks for nourishment. River currents carry the larvae downstream into backwater areas, such as oxbows and sloughs where the free-swimming fry will spend their first year feeding on insect larvae and crustacea. During their first year of growth, they will reach 18 to 20 cm (7.1 to 7.9 in) in length and migrate back into the swift-flowing currents in the main stem river.

Sturgeon are primarily benthic feeders, 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, tactile and chemosensory cues detected by the four barbels. 

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, courtship display, 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". Leaping sturgeon are known to occasionally cause injuries to humans in boats; in 2015, a 5-year-old girl died after a sturgeon leapt from the Suwannee River and struck her

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