Atlantic & Shortnose Sturgeon. The Atlantic sturgeon (Acipenseroxyrhynchus) and the shortnose sturgeon (Acipenserbrevirostrum) belong to the family Acipenseridae, one of the most primitive families of the bony fishes. Sturgeon originated over 300 million years ago and have remained relatively unchanged for over 40 million years. Although their ancestors had a bony skeleton, the present day sturgeon have a cartilaginous skeleton. Sturgeon have a protrusible, toothless mouth, with bulbous lips on the underside of the head with two pair of barbels preceding the mouth. They are armored with five rows of plates called "scutes," and have a heterocercal (sickle-shaped) tail.
The Atlantic sturgeon are an anadromous species which attain most of their growth in the marine environment but return to freshwater to spawn. Shortnose sturgeon are also considered an anadromous species. Although they are not known to leave the influences of the river systems in Maine, they are found in brackish water during part of their life cycle.
Both species of sturgeon are found mainly in the larger river systems of Maine. Shortnose sturgeon are known only to occur in the estuarial complex of the Kennebec and Sheepscot Rivers and in the Penobscot River. Atlantic sturgeon are known to occur in the Kennebec, Penobscot, and Piscataqua Rivers and may occur in the St. Croix River and other smaller drainages. The Atlantic sturgeon is distributed from Labrador to the northern coast of South America. The shortnose sturgeon is distributed from the St. John River in New Brunswick, Canada, to the St. Johns River in Florida.
Atlantic sturgeon enter the river in the early summer at water temperatures from 55-70oF. Ripe Atlantic sturgeon have been found in the Kennebec River from mid-July through early August. Spawning habitat consists of small rubble, gravel, or hard bottom in running water or in pools below waterfalls. Historical records indicate that the major spawning area for Atlantic sturgeon in the Kennebec River was between Augusta and Waterville. The construction of the Augusta dam in the early 1800s was believed to have caused the commercial catch to decline over 50%. A female Atlantic sturgeon may spawn from 1-4,000,000 eggs depending on the size of the fish. The adhesive eggs vary in diameter from 2-2.9mm and attach to rocks, sticks, shells, etc. in strung clusters of ribbons. The eggs hatch in 3-7 days, depending on water temperature.
The larvae grow rapidly and are 4-5 1/2" long at a month old. At this size, the young sturgeon bear teeth and have sharp, closely spaced spine-tipped scutes. As growth continues, they lose their teeth, the scutes separate and loose their sharpness. The young spend up to six years in the Kennebec River system and reach a length of 3' before migrating to sea.
Atlantic sturgeon feed on molluscs, polychaeta worms, gastropods, shrimps, amphipods, isopods, and small fishes in the marine environment. The sturgeon "roots" in the sand or mud with its snout, like a pig, to dislodge worms and molluscs which it sucks into its protrusible mouth, along with considerable amounts of mud. The Atlantic sturgeon has a stomach with very thick, muscular walls that resemble the gizzard of a bird. This gizzard enables it to grind such food items as molluscs and gastropods.
The age at which the Atlantic sturgeon returns to the river system to spawn varies between sexes and increases with latitude. The youngest ripe male observed in the Kennebec River was 17 years old and the smallest was 57", fork length. The youngest ripe female was 25 years old and 67", fork length. Dovell50 found that spawning male Atlantic sturgeon in the Hudson River were at least 12 years old and ranged in length from 3 1/2 to 6 1/2'. The youngest female was 19 years old and 6 1/2' in length.
The age of sturgeon is usually determined by counting growth rings (annuli) in a basal cross section of the first pectoral ray. Atlantic sturgeon have been found to attain an age of 60 years in the St. Lawrence River. The oldest sturgeon aged in the Kennebec River was 40 years old. The largest Atlantic sturgeon observed in the Kennebec River to date was 7'2" in length. The largest Atlantic sturgeon on record was a 14' female, 811 pounds, caught off the mouth of the St. John River, New Brunswick, Canada, in July 1924.51 The shortnose sturgeon is a much smaller fish and slower growing than the Atlantic sturgeon. A 3' long shortnose sturgeon from the Kennebec River would be approximately 28 years old, whereas a 3' long Atlantic sturgeon would be only six years old.
To distinguish an adult shortnose sturgeon from a juvenile Atlantic sturgeon, one has to compare the ratios of the mouth width/interorbital width (bony width between the eyes). As a general rule, if the mouth width/interorbital width x 100 exceeds 60%, it is a shortnose sturgeon. In addition, all juvenile Atlantic sturgeon checked to date in the Kennebec River have had small, bony plates (supra-anal scutes) present between the anal fin and the lateral scutes. No supra-anal scutes have been found on any of the shortnose sturgeon checked from the Kennebec River.
The shortnose sturgeon also differs from the Atlantic sturgeon in its life cycle. The shortnose sturgeon spawns at lower temperatures, thus, earlier in the season than does the Atlantic sturgeon. In the Kennebec River, the shortnose spawns in late April and early May at temperatures of 10-15oC. The spawning sites on the Kennebec River (including the tidal portion of the Androscoggin River) are characterized by a substrate of gravel, rubble, and large boulders adjacent to deep, turbulent areas. The eggs of the shortnose sturgeon are slightly larger than those of the Atlantic sturgeon. The average diameter of fully matured eggs is 3.10mm. The number of eggs per female averaged 5,250 eggs/lb. for St. John River fish.52 Juvenile shortnose sturgeon remain in the upper freshwater portion of estuaries where they feed mainly in deep channels over sandy mud or gravel/mud bottoms. The adult shortnose sturgeon, at least in the northern part of their range, are confined to the river systems. The migratory movements of the adult shortnose sturgeon in the river system involves movements between the spawning, feeding, and wintering areas. The spawning areas in the Kennebec/Sheepscot River estuarial complex are located close to head-of-tide in the Kennebec River and in the Androscoggin River, and possibly in the tributaries of Merrymeeting Bay. Although the shortnose sturgeon feed throughout the estuarial complex, it appears that the greatest concentration is in the mid-estuary around Bath. It is believed that the majority of the shortnose sturgeon overwinter in the lower estuary, although occasionally one is caught in the upper estuary during the winter smelt hook and line fishery.