Sturgeon

Sturgeon are large, slow-maturing, long-lived, and primitive fish found in large inland and coastal rivers, as well as in some lakes. They are contemporary species of ancient lineages; fossil remains of sturgeon and related paddlefish have been dated to early in the Triassic period of the Mesozoic kala (230 to 265 million years ago), making them contemporaries of dinosaurs and causing them to be referred to as “living fossils.”

Best known for the black caviar made from their eggs, sturgeon and paddlefish are members of the order Acipenseriformes, but at some distant point they separated from a common ancestor. As a result, sturgeon are members of the family Acipenseridae, and paddlefish are members of Polyodontidae. Both are considered bony fish; however, they have a mostly cartilaginous skeleton. Their closest living relatives are gar and bowfin.

Like paddlefish, sturgeon are distinctive in appearance. Each species possesses a heterocercal tail (the upper lobe is larger than the lower), a spiral valve intestine, a spiracle (aperture for breathing), an upper jaw that is not fused with the cranium, and a cartilaginous backbone as an adult. The sturgeon have five rows of bony scutes (scalelike plates), a bottom-oriented, extendible, hoselike mouth with fleshy lips; four barbels; an extended snout; and a teardrop-shaped body.

Species

In North America, there are nine recognized species. White sturgeon (Acipenser transmontanus) and green sturgeon (A. medirostris) occur on the West Coast of North America. White sturgeon occur in lower and upper waters, sometimes hundreds of miles inland.

Green sturgeon are usually found in the lower areas of estuaries. Atlantic sturgeon (A. oxyrinchus oxyrinchus) and shortnose sturgeon (A. brevirostrum) live on the East Coast. The lake sturgeon (A. fulvescens) occurs in the Great Lakes and the upper Mississippi river system.

Shovelnose sturgeon (Scaphirhynchus platorhynchus) and pallid sturgeon (S. albus) are found in the Mississippi River system. The Alabama sturgeon (S. suttkusi) is endemic to the Mobile River drainage in Alabama. The gulf sturgeon (A. oxyrinchus desotoi), a subspecies of the Atlantic sturgeon, occurs frequently in all Gulf drainages from Tampa Bay, Florida, west to Mermantau River, Louisiana.

Life history

Members of the genus Scaphirhynchus, as well as the lake sturgeon, are potamodromous. They live in rivers or lakes, respectively, and migrate upstream into smaller tributaries or rivers to spawn. Their migratory patterns are similar to those of paddlefish.

Adult sturgeon of the genus Acipenser, with the lone exception of lake sturgeon, are anadromous. They typically winter in the ocean, migrating into coastal rivers as the water warms above 54°F. Sturgeon also use peak river discharge in the spring as a cue for migratory behavior. Most sturgeon stage in brackish water for a few days before migrating upstream or out to the ocean.

They then migrate hundreds of miles upstream to reach gravel bars and spawn in high-velocity currents. Several males spawn with each female, and the eggs adhere to the gravel. The eggs hatch, and the fry are carried downstream to areas with slower water velocity. Adults then move downstream to summer habitats, where they remain until the fall.

Early growth is rapid, and juveniles may reach their adult size in as few as 3 years. Sturgeon often do not mature until 6 years of age, and in some areas they do not mature until age 10 or 12. Sturgeon spawn intermittently, every 2 to 6 years, depending on the species.

Most sturgeon are opportunistic feeders. Juveniles primarily eat aquatic invertebrates, whereas subadults may also consume mollusks, fish, and crayfish. Some species, such as white sturgeon, are good predators and willingly prey on other fish. Migrating adults of Acipenser, except white and lake sturgeon, typically do not feed while in freshwater.

Sturgeon are most often found on or near the bottom. They are typically concentrated in deep pools that occur in river bends. During migration (spring and fall), juveniles and adults inhabit deep pools that occur in brackish water along the freshwater-saltwater interface of coastal rivers.

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