Acipenser oxyrinchus

Sturgeons will migrate upriver to spawn. Sturgeons from the Gulf of Mexico will naturally exhibit spawning migration in the spring. Peak numbers have been observed in March and April, which is when the fish will migrate into the Suwannee River in Florida.[7] Sturgeon will migrate downstream for twelve days, peaking within the first six days. Atlantic sturgeons only need to move a short distance to reach rearing areas. Early sturgeon migrants tend to be nocturnal while later migrants are diurnal.[8] During summer months, sturgeon will remain in localized bottom areas of the rivers. In the late fall, the sturgeon migrate out of spawning rivers and into the Gulf of Mexico.[7]

For all populations and subspecies of sturgeon, there are spawning migrations into freshwater in early spring and movement into salt water in the fall. Timing and the unusual migratory behavior of sturgeon is a result of temporal water temperature changes.[7] Studies have also shown that amongst sturgeon in Gulf of Mexico and Suwannee River in Florida, fish gained 20% of their body weight while in Gulf of Mexico and lost 12% of their body weight during their time in the river.[9]

The maximum level of survival for eggs, embryos, and larvae is at 15 to 20 °C (59 to 68 °F). Studies have shown that high mortalities are seen at temperatures of 25 °C (77 °F) or higher.[7] In order for spawning to occur, water temperature should be above 17 °C (63 °F). Spawning normally lasts between nine and twenty-three days, but can continue past this as long as the water temperature remains below 22 °C (72 °F).[10]

Free sturgeon embryos (the first interval of sturgeon after hatching) hide under rocks and do not migrate. They are found in the freshwater spawning areas. Larvae and some juveniles start to migrate slowly for about five months downstream. This leads to a wide dispersal of the fish.[11] Typically, the entire freshwater reach of the river downstream from the actual spawning site is so filled with larva-juvenile individuals that it is considered to be a nursery habitat.[11]

Sturgeon populations will use the same spawning reefs from year to year. Habitat factors that can help determine spawning sites include the presence of gravel substrate, presence of eddy fields, slightly basic pH, and a range in calcium ion content. Eggs are usually deposited in a small area and scatter very little. It is not until larvae and juveniles start to migrate that the fish disperse widely.[10]

Some evidence has shown that developmental body color is related to migration style. Free embryos are light and are non-migratory, while migratory larvae and adults are dark. This is found to be consistent among many Acipenser species. The reason for this is unclear, but it may be adaptive to migration behavior and camouflage.[11]

The primary foods of sturgeon while in freshwater areas include soft-bodied annelids, arthropods, aquatic insects, and globular mollusks. Adults that have emigrated from estuaries and into the sea will usually feed on epibenthic and hyperbenthic amphipods, grass shrimp, isopods, and worms. Most adult sturgeon will also feed on detritus and biofilm.[12] Sturgeons are most generally known for feeding on crustaceans, worms, and molluscs.[13]

Larvae-juvenile feeding was at the bottom, benthic foraging. However, food in the benthic zone is scarce, so many adapt drift feeding, in which they have holding positions in the water column and wait for food.[11] Sturgeons may have dominance hierarchies with large fish being dominant when competing for limited foraging space.[8]

There are many wide-ranging subspecies along the Atlantic Coast of North America. Identification of distinct population segments (DPS) is problematic because of sturgeons' ability to disperse so widely. However, it is possible to do some characterization of genetic differentiation and estimate gene flow. This method has been used to determine possibility for listing under the U.S. Endangered Species Act.[14]

The sturgeon's characteristics and life history make it susceptible to anthropogenic disturbances and make population restoration particularly difficult. They have late sexual maturity, only moderate fecundity, and spawn at low frequencies. Females spawn once every three to five years, and males every one to five years. This is due to their ability to live for an extremely long time (various sub-species can have a lifespan ranging from ten years to sixty years).[14]

The population of Atlantic sturgeons has decreased dramatically due to overharvesting. The late 19th century saw a surge in demand for caviar, which led to overfishing of the Atlantic sturgeon. Today, only 22 out of its 38 original spawning rivers still have viable populations of the species.[15] They are particularly susceptible to bycatch mortality due to the many fisheries that exist within their natal estuaries. Their habitat range, which usually includes coastal spawning sites and coastal migrations, makes sturgeon well within contact of coastal fisheries.[14]

Hypoxia combined with high water temperatures in the summer has been shown to be consistent with decreased survival rates of young of the year sturgeon in Chesapeake Bay.[16]

Hypoxia is defined as low ambient oxygen levels, which may be very harmful to organisms living in the hypoxic body of water. Often, lower regions of the water column will be more hypoxic than upper levels, closer to the surface. When surface access is denied, the situation is lethal to sturgeon. Increased incidences of summertime hypoxia have led, in part, to degradation of many sturgeon nursery habitats in the United States.[16]