Rock sturgeon
The lake sturgeon (Acipenser fulvescens), also known as the rock sturgeon, is a North American temperate freshwater fish, one of about 25 species of sturgeon. Like other sturgeons, this species is a bottom feeder and has a partly cartilaginous skeleton, an overall streamlined shape, and skin bearing rows of bony plates on the sides and back.
The lake sturgeon uses its elongated, spade-like snout to stir up the substrate and sediments on the beds of rivers and lakes to feed. Four sensory organs (barbels) hang near its mouth to help the sturgeon locate bottom-dwelling prey. Lake sturgeons can grow to a large size for freshwater fish, topping 7.25 ft (2.2 m) long and 240 lb (108 kg).
The lake sturgeon has taste buds on and around its barbels near its rubbery, prehensile lips. It extends its lips to vacuum up soft live food, which it swallows whole due to its lack of teeth. Its diet consists of insect larvae, worms (including leeches), and other small organisms (primarily metazoan) it finds in the mud. Some populations consume fish as a significant component of their diet, particularly since the introduction in the early 1990s of the invasive round goby. Given that it is a large species surviving by feeding on very small species, its feeding ecology has been compared to that of large marine animals, like some whales, which survive by filter-feeding.
This species occurs in the Mississippi River drainage basin south to Alabama and Mississippi. It occurs in the Great Lakes and the Detroit River, east down the St. Lawrence River to the limits of fresh water. In the west, it reaches Lake Winnipeg and the North Saskatchewan and South Saskatchewan Rivers. In the north, it is found in the Hudson Bay Lowland. In the east, the species lives in Lake Champlain and in some Vermont rivers, including the Winooski, Lamoille and Missisquoi rivers, and Otter Creek. This distribution makes sense in that all these areas were linked by the large lakes that formed as the glaciers retreated from North America at the end of the last ice age (e.g., Lake Agassiz, Lake Iroquois).
These sturgeon often migrate in search of food or suitable spawning locations, or in response to seasonal environmental conditions. Juveniles typically inhabit pools greater than about 6 feet in depth, and adults typically live deep in large lakes. They are not often far from suitable spawning locations. The abundance of prey also plays a large factor in finding a suitable habitat.
Lake sturgeon have a very long lifespan. Males typically live for 55 years and females can live for 80 to 150 years. They grow quickly during a lengthy juvenile stage.
Lake sturgeon eggs begin yellowed and are attached to a fatty ovarian mass. When the eggs are mature, they become olive green, grey or black. The eggs typically hatch after 8 to 14 days. Observations suggest lake sturgeon and other fish and invertebrates likely consume some fertilized eggs while on the spawning grounds.
At hatching, the larvae are barely discernible and are about 10 mm long. The larvae soon become pelagic, remaining far from the surface and bed, and negatively phototactic, or attracted to darkness, while searching for rocky places to hide. About two weeks after hatching, they disperse downstream with the current for several miles until settling back down upon the river bottom.
As juveniles, all definitive adult structures, except for gonads, form. They are thought to feed on benthic invertebrates like adults. It is thought that during late summer, yearlings gather in large schools in shallow river mouths and bays. The juveniles can be found in the same habitats as adults after a year.
Male lake sturgeon typically reach sexual maturity at 8 to 12 years, but may take up to 22 years. Females reach sexual maturity at 14 to 33 years, most often from 24 to 26 years of age. These sturgeon spawn on clean, gravel shoals and stream rapids, usually from April to June. They prefer to spawn in temperatures between 55 and 64 degrees Fahrenheit. Lake sturgeon reproduce by swimming around each other in circles and shaking violently; the male stops circling when he has released his fertilizer and the female then lays her eggs. Males spawn every 2 to 7 years while females spawn every 4 to 9 years. Only 10 to 20 percent of adult lake sturgeon are sexually active during a season
Lake sturgeon are polygamous, maximizing genetic diversity.
These fish were once killed as a nuisance by catch because they damaged fishing gear. When their meat and eggs became prized, commercial fishermen targeted them. Between 1879 and 1900, the Great Lakes commercial sturgeon fishery brought in an average of 4 million pounds (1800 metric tons) per year. Such unsustainable catch rates were coupled with environmental challenges such as pollution and the construction of dams and other flood control measures. Sturgeon, which return each spring to spawn in the streams and rivers in which they were born, found tributaries blocked and spawning shoals destroyed by silt from agriculture and lumbering. In the 20th century, drastic drops in sturgeon catches, increased regulations, and the closure of viable fisheries occurred. Currently, 19 of the 20 states within the fish's original U.S. range list it as either threatened or endangered. It is considered "Vulnerable" by NatureServe.
This sturgeon is a valuable gourmet food fish, as well as a source of specialty products including caviar and isinglass. "In 1860, this species, taken on incidental catches of other fishes, was killed and dumped back in the lake, piled up on shore to dry and be burned, fed to pigs, or dug into the earth as fertilizer." It was even stacked like cordwood and used to fuel steamboats. Once its value was realized, "They were taken by every available means from spearing and jigging to set lines of baited or unbaited hooks laid on the bottom to trap nets, pound nets and gillnets." Over 5 million lb were taken from Lake Erie in a single year. The fishery collapsed, largely by 1900. It has never recovered. Like most sturgeons, the lake sturgeon is rare now and is protected in many areas.
In addition to overharvesting, it has also been negatively affected by pollution and loss of migratory waterways. It is vulnerable to population declines through overfishing due to its extremely slow reproductive cycle; most individuals caught before 20 years of age have never bred and females spawn only once every four or five years. The specific harvesting of breeding females for their roe is also damaging to population size. Few individuals ever reach the extreme old age or large size that those of previous generations often did.
