Shortnose sturgeon
The shortnose sturgeon (Acipenser brevirostrum) is a small North American sturgeon which can be found in 16 to 19 large river and estuary systems along the Atlantic seaboard from the Saint John River in New Brunswick, Canada, to the St. Johns River in Florida, United States.  Populations may be disjunct, evidenced by lack of records in the sea outside the influence of their home river and minimal captures of tagged individuals outside the river in which they were tagged.  The maximum salinity in which the species has been found is 30-31 ppt, slightly below the salinity of sea water.  It is an endangered species in the United States, having been recognized as such in 1967, and is a vulnerable species in Canada.
The species is sometimes mistaken for juvenile Atlantic sturgeon, as adults of this species are similar in size to juveniles of that species.  Prior to 1973, U.S. commercial fishing records did not differentiate between the two species, both were reported as "common sturgeon", although it is believed based on sizes that the bulk of the catch was Atlantic sturgeon.
They spawn in fresh water, above the head of the tide, in moving water over rubble or gravel bottoms with little silt or organic material.  Time of spawning varies by latitude and is likely based on water temperatures in the range from 9-12 degrees celsius although successful spawning can occur from 6.5-15 degrees celsius; this may be as early as January in South Carolina or Georgia or as late as May in Maine and New Brunswick.  Other spawning requirements include a day length of 13.9-14.9 hours, and water velocity at the bottom of 30-120 centimeters per second.  The eggs hatch after 13 days, into 7-11 mm long hatclings with a large yolk-sac, minimal sight, minimal swimming ability and a strong tendency to seek cover.  After another 9-12 days they mature to a swimming larval stage at about 15 mm in length, resembling a miniature adult by the time they reach 20mm in length and begin feeding.  They then drift downstream in the deep channels of the river, remaining in fresh water for the first year of their life.  Juveniles, up to 18 inches long, generally move to the area where fresh and salt water come together, and move with it through the tidal cycle.  Adults can be found in either fresh or salt water.  Adults mature sexually at 45 to 55 centimeters (18 to 22 inches) in length, at an age varying with latitude.  Males mature after 2-3 years in Georgia or 10-14 years in New Brunswick, and females mature between 6 and 17 years of age (again, earlier in southern rivers).  First spawning occurs after sexual maturity; 1-2 years later for males and up to 5 years later for females.  Adults continue to grow to between 3 and 4 feet in length.  A male may breed every year or every other year, and seldom lives beyond age 30.  Females usually breed every third to fifth year, laying between 40,000 and 200,000 eggs in those years that they breed, and can live to age 67.  Females spend multiple years with reduced feeding and growth while they are producing the gonadal material needed for spawning.  It is believed, but not yet well proven, that maximum ages are lowest in the south and highest in the north.
In three locations (the Connecticut River in Massachusetts, the Santee River in South Carolina, and the Saint John River in New Brunswick) the shortnosed sturgeon was able to survive as a landlocked population following construction of river dams.  This indicates that the species does not require salt water in its life cycle.  Northern populations generally spend more time in salt water than southern populations do, to the extent of being anadromous instead of amphidromous.
Sturgeon are bottom feeders eating primarily insects and small crustaceans.  Juveniles have been observed with stomach contents with as much as 90% non-food items leading to a belief that they randomly vacuum the bottom.  Adults in fresh water primarily eat mollusks, supplemented by polychaetes and small benthic fish in estuaries or crustaceans and insects in fresh water.
The largest population, estimated as at least 38,000 adults, is found in the Hudson River.  The second largest, 18,000 adults and roughly 100,000 of all ages, is in the Saint John River.
Little non-human predation is documented.  Yellow perch have been caught with the current year's young in their stomach, and it is believed that sharks and seals may occasionally eat adults.  Parasites are not believed to be harmful.  There are no reported incidents of diseases among wild shortnosed sturgeon, although one hatchery population has suffered a disease outbreak.
Genetically, shortnose sturgeons are 16-ploid, having chromosones in groups of 16, rather than the pairs that most vertebrates have.  It may have been one of the last sturgeon species to evolve.