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 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.
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Reproduction
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 °C, although successful spawning can occur from 6.5-15 °C; 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 cm/second. The eggs hatch after 13 days, into 7- to 11-mm-long hatchlings 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 20 mm 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 lives. Juveniles, up to 18 in 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 cm (18 to 22 in) 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 ft 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 in which 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. Maximum ages are thought to be lowest in the south and highest in the north.
Ecology
The maximum salinity in which the species has been found is 30-31 ppt, slightly below the salinity of sea water. In three locations (the Connecticut River in Massachusetts, the Santee River in South Carolina, and the Saint John River in New Brunswick) the shortnose sturgeon was able to survive as a landlocked population following construction of river dams. This indicates the species does not require salt water in its lifecycle. Hatchery-raised sturgeon appear to do best in zero-salinity fresh water. 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% nonfood 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 to be at least 60,000 adults in 2007, 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 nonhuman predation is documented. Yellow perch have been caught with the current year's young in their stomachs, and sharks and seals may occasionally eat adults. Parasites are not believed to be harmful. No incidents of diseases among wild shortnose sturgeon have been reported, although one hatchery population has suffered a disease outbreak. The waste of the shortnose sturgeon helps to fertilize the area in which they live.
Genetically, shortnose sturgeon are hexaploid, having chromosomes in groups of six, rather than the pairs that most vertebrates have. It may have been one of the last sturgeon species to evolve.
Conservation status
It is an endangered species in the United States, having been recognized as such in 1967.
Source of the article : Wikipedia
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