Acipenseriformes (Sturgeons and Paddlefishes)

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Acipenseriformes

(Sturgeons and paddlefishes)

Class Actinopterygii

Order Acipenseriformes

Number of families 2

Evolution and systematics

The order Acipenseriformes includes 25 species of sturgeons in four genera (Acipenser, Huso, Scaphirhynchus, and Pseudoscaphirhynchus) in the family Acipenseridae and two living species of paddlefishes in the family Polyodontidae. The Acipenseriformes are primitive fish; recognizable fossils date to the early Cretaceous (144–65 million years ago). The Acipenseridae and Polyodontidae probably diverged from each other during the Jurassic (208–146 million years ago).

Physical characteristics

Acipenseriformes are some of the largest freshwater fishes, with species ranging in maximum size from 2.5 ft (0.76 m) to nearly 28.2 ft (8.6 m). Their bodies are elongate with large heads, small eyes, and fins positioned towards the posterior. A lateral line and scales are absent. Sturgeons and paddlefishes are dark on the tops of their bodies, but pigmentation fades to much lighter ventral colors, and many have white bellies. Species of sturgeon take on a variety of dull colors: gray, brown, dark blue, olive-green, and nearly black. Paddlefishes may appear bluish gray, brown, or black on their dorsal surface.

All Acipenseriformes share relict characteristics, including a largely cartilaginous endoskeleton and heterocercal caudal fin. The only ossified bones are found in the skull, jaws, and pectoral girdle. Other common anatomical features include an elongated snout with sensory barbels, a ventral mouth, an unconstricted notochord, and a lack of scales covering their skin.

Although they share many similar characteristics, anatomical and ecological distinctions exist between sturgeons and paddlefishes. Sturgeons have four barbels used for detecting prey, and the ventral mouth is protrusible. Paddlefishes have only two small sensory barbels and nonprotrusible mouths. Another major anatomical difference between sturgeons and paddlefishes is in their body coverings. The skin of paddlefishes is largely naked, with patches of minute scales. In contrast, sturgeons are armored with five rows of bony shields along their bodies.

Distribution

Acipenseriformes are found throughout the Northern Hemisphere in North America, Europe, and Asia. Among the sturgeons, nine species inhabit North America, four are found in Europe, ten live in Asia, and four have Eurasian distributions. One species of paddlefish is found in North America; the other paddlefish species is endemic to China.

Habitat

Acipenseriformes inhabit seas, rivers, and lakes. Some species spend a large portion of their lives at sea but enter coastal rivers to spawn. Other species live strictly in freshwater rivers and lakes. Sturgeons are typically associated with sand, gravel, or rock substrates.

Behavior

Most sturgeons spend their lives in their native river or in nearshore areas of adjacent seas, but some individuals move long distances through coastal habitats. Sturgeons exhibit seasonal and spawning migrations. They may move from shallow to deep water in the summer and return to shallow areas in the winter. All sturgeons spawn in fresh water; thus, those that live in the sea migrate to fresh water for spawning. Paddlefishes swim constantly, both day and night, and migrate upstream to spawn.

Sturgeons are active primarily during the day, and many species congregate in discrete seasonal feeding areas.

Observations of lab-reared juveniles suggest that certain species may establish a dominance hierarchy based on size, with large fish acting aggressively towards smaller fish in disputes over limited foraging space. Although sturgeons and paddlefishes are solitary for most of their life, some aggregation has been observed in larvae, which migrate in unorganized groups.

Feeding ecology and diet

Sturgeons locate food by swimming close to the bottom with their sensory barbels dragging the substrate. They selectively ingest slow-moving benthic invertebrates, including insects, worms, crustaceans, and mollusks, and feed on other fishes to a limited extent. Paddlefishes feed by swimming through the water with their mouths open and filtering large amounts of water through their gill rakers. Paddlefishes primarily consume microcrustaceans and insect larvae in the plankton, but they occasionally eat benthic invertebrates and other fishes.

Because of their large size and protective bony scutes, adult sturgeons and paddlefish have few predators except humans. However, sturgeons may be attacked, and possibly killed, by the parasitic sea lamprey, Petromyzon marinus.

Reproductive biology

Sturgeons typically spawn during spring and summer months. Prespawning activities involve rolling near the bottom and leaping out of the water. Spawning takes place in groups of two to three fish, with one or two males per female. Female sturgeons produce large quantities of eggs (up to several million), which are deposited over shallow shoals or rocky areas and fertilized by males. No nests are constructed, but the eggs are adhesive and stick to the substrate. Sturgeons do not devote any parental care to their offspring. Adults of some species spawn every year, but most species allow longer intervals between spawning events.

Paddlefishes spawn in the early spring as water levels are rising. They migrate from lakes and rivers into streams to locate spawning sites in shallow water. Males and females broadcast eggs and sperm over gravel substrates while swimming in groups. No parental care is provided to the offspring. Female paddlefishes produce very large numbers of eggs (up to 600,000) and do not spawn annually.

Conservation status

Overexploitation and habitat alteration, particularly the construction of dams, threaten and limit populations of Acipenseriformes throughout their range. The commercial landings of sturgeons exceeded 3,000 tons (2,721 tonnes) in 1890, but landings declined by 99% over the next century. Overfishing threatened many populations with local extinction, and stock enhancement programs have been introduced to maintain many sturgeon fisheries. Dams limit access to spawning sites and isolate populations. Other human activities on the shores of rivers increase siltation and contaminate rock or gravel spawning areas.

All Acipenseriformes are cited on the IUCN Red List. While some species are considered Lower Risk/Near Threatened (2 species) by the IUCN, most species are at greater risk and are classified as either Critically Endangered (6 species), Endangered (11 species), or Vulnerable (8 species). The international trade of Acipenseriformes is regulated through the Convention on International Trade in Endangered Species of Wild Flora and Fauna (CITES). The short-nose sturgeon (Acipenser brevirostrum) and the common sturgeon (Acipenser sturio) are considered threatened with extinction and are listed on Appendix I of CITES. All other species of sturgeon and paddlefish are listed on Appendix II of CITES. The shortnose sturgeon is listed as an endangered species in the United States.

Significance to humans

Sturgeons have been valued for their caviar, the unfertilized eggs of the female, since the times of the ancient Persian, Greek, and Roman empires. The Chinese began trading caviar during the tenth century. It became popular as a luxury food in Europe during the seventeenth and eighteenth

centuries and remained prized as a culinary delicacy at the end of the twentieth century. The smoked meat of sturgeons also is highly valued, particularly in European and Asian markets. In the late 1800s, paddlefish eggs and flesh also were sought commercially.

Species accounts

List of Species

Shortnose sturgeon
Lake sturgeon
Atlantic sturgeon
White sturgeon
Beluga sturgeon
American paddlefish

Shortnose sturgeon

Acipenser brevirostrum

family

Acipenseridae

taxonomy

Acipenser brevirostrum LeSueur, 1818, Delaware River, United States.

other common names

English: Shortnosed sturgeon; French: Esturgeon à nez court; Spanish: Esturión hociquicorto.

physical characteristics

At approximately 3 ft (0.9 m) in length, the shortnose sturgeon is the smallest species in the genus Acipenser. It has a shorter snout than other sturgeons and a wide mouth. Its upper body is dark brown or black, with lighter colors on the ventral portion. The bony plates are light in color.

distribution

Shortnose sturgeons occur along the East Coast of North America, from St. John River in New Brunswick, Canada, to Indian River, Florida.

habitat

Shortnose sturgeons live in the ocean, estuaries, and large coastal rivers.

behavior

Shortnose sturgeons migrate upstream and downstream seasonally in coastal rivers. In southern portions of the range, these fishes spend longer periods of time at sea and migrate into rivers to spawn. Juvenile shortnose sturgeons may compete for limited foraging space, and larger individuals become aggressive to ward off encroaching individuals of smaller size.

feeding ecology and diet

Shortnose sturgeons are opportunistic benthic feeders. Young individuals eat insects and crustaceans. Adults consume mollusks, benthic crustaceans, polychaete worms, and insect larvae.

reproductive biology

Male shortnose sturgeons first spawn around three to four years of age, and females first spawn between six and fifteen years. Spawning takes place in the spring over gravel or rocky substrates. Females can produce 200,000 eggs per fish, and eggs hatch after approximately 13 days. Females spawn at intervals of three to five years, but males may spawn every year.

conservation status

The shortnose sturgeon is listed as Vulnerable by the IUCN and protected under Appendix I of CITES. It also is recognized as an endangered species under the U.S. Endangered Species Act and as a vulnerable species by the Committee on the Status of Endangered Wildlife in Canada.

significance to humans

The caviar and flesh of shortnose sturgeons were commercially important during the 1800s and 1900s. Populations began declining in the 1800s due to industrial pollution of rivers and overfishing. As of 2002, all fisheries for this species are closed.

Lake sturgeon

Acipenser fulvescens

family

Acipenseridae

taxonomy

Acipenser fulvescens Rafinesque, 1917, Lake Erie, North America.

other common names

English: Freshwater sturgeon, Great Lakes sturgeon; French: Esturgeon jaune; Spanish: Esturión lacustre.

physical characteristics

The back and sides of large lake sturgeon are olive-brown to dull gray in color; juveniles are light brown with dark blotches. Most lake sturgeons today are 3–5 ft (0.9–1.5 m) long and weigh 10–80 lb (4.5–36.3 kg), but a female of nearly 8 ft (2.4 m) and 310 lb (140.6 kg) has been documented.

distribution

Lake sturgeons occur in the following North American drainages: Great Lakes, St. Lawrence River, Hudson Bay, and Mississippi River.

habitat

Lake sturgeons inhabit large rivers and lakes.

behavior

Lake sturgeons migrate seasonally between shallow and deeper waters, particularly in the northern extent of their range. They also undertake extensive migrations, typically of around 80 mi (128.7 km), to find suitable spawning grounds in rivers.

feeding ecology and diet

Lake sturgeons primarily consume insects, as well as other benthic invertebrates, such as snails, clams, and crayfishes. They occasionally feed on fish eggs, algae, and small fishes.

reproductive biology

Lake sturgeons first spawn at 14–23 years for females and 12–20 years for males. Spawning intervals range from two to seven years in males and four to nine years in females. In the spring when ice clears, lake sturgeons migrate into smaller rivers and streams for spawning. Spawning typically takes place in swift-moving water 2–15 ft (0.6–4.6 m) deep. In the Great Lakes, lake sturgeons spawn along rocky shores in groups of two to three individuals. Females shed eggs in batches over a period of days. The eggs adhere to rocks for five to eight days before hatching.

conservation status

Populations of lake sturgeons are threatened because of human exploitation, as well as habitat alteration and fragmentation that is caused by the construction of dams and roads. Lake sturgeons are listed as Vulnerable by the IUCN. They are protected by state and provincial fishing regulations and habitat restoration efforts in the United States and Canada.

significance to humans

Lake sturgeons were harvested for food by Native Americans before Europeans settled in North America, and commercial markets developed for the eggs and smoked flesh in the mid-1800s. Isinglass, a gelatin obtained from the swim bladder, was used to make jam and jellies, as a pottery cement, and as a waterproofing agent. Recreational fishing for lake sturgeons remains popular.

Atlantic sturgeon

Acipenser oxyrinchus

family

Acipenseridae

taxonomy

Acipenser oxyrinchus oxyrinchus Mitchill, 1815, New York, United States. Two subspecies are recognized.

other common names

English: Sea sturgeon, common sturgeon.

physical characteristics

The Atlantic sturgeon is a large species that often grows to over 10 ft (3 m) long. Individuals are blue-black in color, with lighter shades on the sides. The head, ventral portion of the body, and fin edges are typically white.

distribution

Atlantic sturgeons are found along the Atlantic coast of North America from Ungava Bay, Quebec, to the St. John's River in Florida.

habitat

This species lives in the ocean and in bays, estuaries, and rivers.

behavior

Atlantic sturgeons migrate between the sea and freshwater. Juveniles spend several years in freshwater before first entering the sea. Most individuals remain near their native river, but some travel long distances over the continental shelf. The migratory behavior of this species is typically associated with spawning activities, but some individuals move into freshwater and do not spawn. Some evidence suggests that Atlantic sturgeons establish priority for foraging areas based on body size, with larger individuals dominant over smaller ones for feeding space.

feeding ecology and diet

Atlantic sturgeons consume bottom-dwelling plants and animals, such as insects, crustaceans, and mollusks. As adults, they also eat small fish.

reproductive biology

Male Atlantic sturgeons typically reach sexual maturity around 12–24 years of age, and females are capable of spawning at 18–28 years. It is believed that females spawn in approximately four-year intervals, whereas males may spawn every year. The spawning season extends from late spring to early summer. Eggs are demersal and adhere to substrates near the spawning area.

conservation status

Although populations have declined due to habitat alteration and fishing activities, Atlantic sturgeons are not considered threatened or endangered in the United States or Canada. They are listed as Lower Risk/Near Threatened by the IUCN.

significance to humans

Atlantic sturgeons are valuable for their flesh and roe, with colonial fisheries extending back to the 1600s. In the United States, commercial fisheries for Atlantic sturgeons were closed in 1998, although fishing had ceased in many states before that date. Commercial fishing continues in the St. Lawrence and St. John Rivers of Canada.

White sturgeon

Acipenser transmontanus

family

Acipenseridae

taxonomy

Acipenser transmontanus Richardson, 1836, Vancouver, Washington, United States.

other common names

English: Pacific sturgeon, Columbia sturgeon, Oregon sturgeon; French: Esturgeon blanc.

physical characteristics

The white sturgeon is the largest North American sturgeon, attaining a maximum length of 20 ft (6.1 m). The upper body is gray, olive, or gray-brown, and its lower body is light gray to white.

distribution

Native distribution of the white sturgeon is along the Pacific coast of North America from the Aleutian Islands, Alaska, to Monterey, California. Landlocked populations occur in Montana and California. The species has also been introduced in the Colorado River in Arizona.

habitat

White sturgeons populate the ocean, estuaries, rivers, and lakes.

behavior

White sturgeons spend most of their lives at sea but enter large rivers to spawn. Some individuals move long distances in coastal migrations.

feeding ecology and diet

Juvenile white sturgeons feed on benthic invertebrates, such as chironomids, mollusks, and crustaceans. Adults primarily consume other fishes, shellfish, and aquatic invertebrates.

reproductive biology

White sturgeons usually spawn in May or June in swift waters over rocky substrates. Males spawn initially between 11 and 22 years of age; females do not spawn until they are between 26 and 34 years. Younger females spawn every four years, while the interval increases to nine to eleven years for older females. The largest female spawners may produce three to four million eggs.

conservation status

White sturgeons are classified as Lower Risk/Near Threatened by the IUCN. This species has been particularly affected by the damming of rivers. Populations were also severely overfished, but successful stocking programs and fishing regulations have enabled recovery.

significance to humans

White sturgeons have been used by Native Americans in the northwest region of the United States since long before the arrival and settlement of Europeans in the area. A commercial fishery for white sturgeons began on the Columbia River in the late 1800s, but the stock was depleted within a decade. Strict regulations put in place during the 1950s led to a population recovery by the late 1990s. By the early twenty-first century, commercial, recreational, and tribal fisheries actively targeted white sturgeons throughout their range.

Beluga sturgeon

Huso huso

family

Acipenseridae

taxonomy

Huso huso Linnaeus, 1758, Danube and rivers of Russia.

other common names

English: European sturgeon, great sturgeon.

physical characteristics

The beluga sturgeon is the largest sturgeon species. It has been recorded to attain a length of 28.2 ft (8.6 m) and weight of 2,866 lb (1,300 kg), although such large specimens are rare. The body is gray or dark green in color with lighter sides and a white belly.

distribution

Beluga sturgeons occur in the Black, Caspian, and Adriatic Seas and in most of their tributaries.

habitat

This species inhabits nearshore areas of seas and large channels of rivers.

behavior

Adult beluga sturgeons live at sea for most of the year but migrate up large river tributaries for spawning. The fry, or young fish, move downstream from rivers to the sea immediately after hatching.

feeding ecology and diet

Juvenile beluga sturgeons feed on benthic invertebrates, such as mollusks, worms, and crustaceans; adults eat other fishes.

reproductive biology

Beluga sturgeons mature slowly and are extremely long lived (up to 150 years). Sexual maturity occurs around 14 years of age for males and 18 years for females. Females may produce over seven million eggs, but reproduction only occurs once every five to seven years. Beluga sturgeons spawn in late spring by scattering eggs and sperm in the water over rocky substrates.

conservation status

The beluga sturgeon is listed as Endangered on the IUCN Red List. It may be extinct in the Adriatic Sea, and populations have declined throughout its range. The Caspian population is made up largely of fish from stocking programs.

significance to humans

Beluga sturgeons are valued throughout the world as the source of superior caviar. The caviar commands high prices, and the market demand has driven fisheries in eastern Europe to continue exploitation despite severe population declines.

American paddlefish

Polyodon spathula

family

Polyodontidae

taxonomy

Polyodon spathula Walbaum, 1792, Louisiana, Mississippi River, United States.

other common names

English: North American paddlefish, Mississippi paddlefish, spoonbill cat; French: Poisson spatule.

physical characteristics

A defining characteristic of the American paddlefish is its large paddle-shaped rostrum, or snout. The paddle is covered with electroreceptors that enable paddlefish to sense objects and concentrations of planktonic prey. American paddlefish live up to 30 years and may attain lengths of 6.6 ft (2 m) and weights of 190 lb (86.2 kg).

distribution

American paddlefishes currently occur within the Mississippi River and Mobile Basin drainages in the United States, although the historical distribution included the Laurentian Great Lakes of Canada.

habitat

This species is found in large rivers and lakes.

behavior

American paddlefishes swim continuously, often moving long distances. They typically are found near the water surface and frequently leap from the water.

feeding ecology and diet

American paddlefishes swim through the water with their mouths open and feed passively by filtering zooplankton and larvae of aquatic insects. Other fishes are occasionally found in stomach samples, indicating that paddlefishes are not strictly filter feeders.

reproductive biology

Male paddlefishes mature between seven and nine years of age; females, between 10 and 12 years. Females may produce up to 600,000 eggs. Paddlefishes spawn in fast-flowing waters with clean gravel bottoms at intervals of two to five years. Spawning takes place in early spring in water depths of approximately 10 ft (3 m). Eggs and sperm are broadcast into the water column; eggs stick to the substrate and hatch within about seven days.

conservation status

American paddlefishes are listed as Vulnerable by the IUCN. This species once occurred throughout the Mississippi River system, but habitats were fragmented by damming of the main stem of the Mississippi and its tributaries. Paddlefishes have been overfished, but state regulations and stocking programs are attempting to restore populations. Although fishing for paddlefishes is prohibited in most states, a few states allow commercial and recreational fisheries that target this species.

significance to humans

Like sturgeons, paddlefishes are valued for their flesh and roe. An important commercial fishery existed for paddlefishes in the Mississippi Valley following the decline of the sturgeon fishery in 1895, but this fishery reached its peak in 1900.

Resources

Books

Birstein, Vadim J., John R. Waldman, and William E. Bemis, eds. Sturgeon Biodiversity and Conservation. Dordrecht, The Netherlands: Kluwer Academic Publishers, 1997.

Periodicals

Billard, Roland, and Guillaume Lecointre. "Biology and Conservation of Sturgeon and Paddlefish." Reviews in Fish Biology and Fisheries 10 (2000): 355–392.

Jennings, Cecil A., and Steven J. Zigler. "Ecology and Biology of Paddlefish in North America: Historical Perspectives, Management Approaches, and Research Priorities." Reviews in Fish Biology and Fisheries 10 (2000): 167–181.

Kynard, B., and M. Horgan. "Ontogenetic Behavior and Migration of Atlantic Sturgeon, Acipenser oxyrinchus oxyrinchus, and Shortnose Sturgeon, A. bervirostrum, with Notes on Social Behavior." Environmental Biology of Fishes 63 (2002): 137–150.

Kynard, B., E. Henyey, and M. Horgan. "Ontogenetic Behavior, Migration, and Social Behavior of Pallid Sturgeon, Scaphirhynchus albus, and Shovelnose Sturgeon, S. platorynchus, with Notes on the Adaptive Significance of Body Color." Environmental Biology of Fishes 63 (2002): 389–403.

Peterson, Douglas L., Mark B. Bain, and Nancy Haley. "Evidence of Declining Recruitment of Atlantic Sturgeon in the Hudson River." North American Journal of Fisheries Management 20, no.1 (2000): 231–238.

Wilkens, L. A., D. F. Russell, X. Pei, and C. Gurgens. "The Paddlefish Rostrum Functions as an Electrosensory Antenna in Plankton Feeding." Proceedings of the Royal Society of London B 264 (1997): 1723–1729.