Linda Benedict | 5/5/2005 7:10:59 PM
Sturgeons have inhabited lakes, streams and rivers for millions of years. Worldwide there are 24 species of sturgeon, including the beluga sturgeon of the Caspian Sea, which can reach a length of more than 16 feet and weigh more than 1,000 pounds. The largest sturgeon in North America, the Pacific white sturgeon, can reach lengths of more than 20 feet, weigh more than 400 pounds and has been reported to live more than 100 years.
Although some sturgeons are distributed only in rivers, others are anadromous, moving from marine systems into fresh water to reproduce. There are four anadromous and four freshwater sturgeons in North America, with the anadromous Gulf sturgeon and freshwater shovelnose and pallid sturgeons found in Louisiana.
Sturgeons have been described as living fossils because of their ancient origins and primitive characteristics. These bizarre-looking fishes have several length-wise rows of bony scutes rather than scales, an asymmetrical tail and a cartilaginous skeleton. These primitive characteristics along with their shovel-shaped snout and protrusible toothless mouths with four sensory “whiskers” make the sturgeon family easy to identify.
Historically, sturgeons have been harvested commercially for their flesh, but they are usually sought out for their valuable eggs, from which the most prized caviar is made. These fishes are typically long-lived, with delayed maturity and infrequent spawning resulting in low numbers of young produced annually. Because of these unique life history characteristics, historic commercial harvesting practices and ongoing alterations to large riverine habitats, most sturgeons are either threatened or endangered. The Gulf sturgeon, listed as a federally threatened species, now inhabits the Pearl River in eastern Louisiana but is thought to have once lived in the Mississippi River. The pallid sturgeon was federally listed as an endangered species in 1990 and is found in the Mississippi, Atchafalaya and Red rivers. Interestingly, the endangered pallid sturgeon co-occurs and even hybridizes throughout its distribution with smaller and more abundant shovelnose sturgeon.
Pallid Sturgeon Decline
Pallid sturgeon were once widely distributed throughout the Mississippi River. In the last 50 years, there has been a drastic decline in pallid sturgeon abundance over much of their former range. This decline has been coincidental with reservoir construction on the Missouri River for flood control and the development of a series of 24 locks and dams on the upper Mississippi River to improve commercial navigation. These construction activities have greatly altered the river by lowering flow velocities and greatly reducing turbidity levels.
LSU AgCenter fisheries biologists, with funding from the U.S. Army Corps of Engineers, designed a study to help describe poorly understood habitat-use patterns for pallid sturgeon in the Atchafalaya River. To monitor sturgeon movements, fish were fitted with internal ultrasonic transmitters. Biologists surveyed the river by boat and were able to use a receiver to locate each fish by detecting the characteristic pinging from each implanted transmitter. Understanding biological requirements of any endangered species helps natural resource managers make recommendations that may result in the ultimate recovery and delisting of the species. Tracking fishes is labor intensive, but it allows biologists to monitor fish locations electronically and record habitat and water quality characteristics at each location over an extended period.
With the help of Atchafalaya River commercial fishermen, pallid, shovelnose and pallid-shovelnose hybrid sturgeon were collected with gill and hoop nets. From these collections, 26 pallid sturgeon were surgically implanted with transmitters and released after a short recovery period. After a one-week acclimation period, telemetry surveys were conducted to locate each tagged sturgeon. Each survey consisted of drifting over the deepest part of the river channel, stopping every 300 feet to 1,000 feet and submerging a directional hydrophone to detect the ultrasonic signal from each transmitter. Surveys of about 30 miles of Atchafalaya and Red river habitats took a minimum of five days to complete and were generally conducted at two-week intervals in a 28-month period in 1995-97.
When a pallid sturgeon was located, a bottom profile of the channel was taken 300 feet upstream and downstream, and these data were used to determine cross-sectional area and bottom slope. At each fish location, data were recorded on the river mile, river stage, water quality and habitat. Additionally, along each bottom profile, invertebrates were collected to determine the abundance of potential pallid sturgeon prey.
Movements of tagged pallid sturgeon showed that 44 percent remained in the same location on consecutive trips and exhibited extended periods of relative inactivity for up to six weeks. Pallid sturgeon movements were greatest when water temperatures ranged from 57.2-69.8 degrees F, which typically occurred in the fall and spring. All pallid sturgeon movement stopped when winter river temperature dropped below 45 degrees F. Movement studies of pallid sturgeon in northern rivers have reported significant associations between fish movements and moderate river temperatures, increasing river stages and low water clarity; however, these relationships were absent in our study because of the characteristic high summer water temperatures (69.8-89.6 degrees F) in the Atchafalaya River.
Pallid sturgeon preferred deep river areas (mean 49.9 feet; range 23-69 feet) with low bottom slopes (0-0.33 feet/foot) and sand substrates. Fish typically avoided shallow water habitats (less than 23 feet) and areas with high bottom slopes and greater flow velocities (faster than 0.98 feet/foot). Atchafalaya River bottom contours characteristically have a dune-like profile with 3-feet to 16-feet spacing between troughs and peaks. Pallid sturgeon typically select the low-slope, sandy-bottomed trough areas characterized by low current velocities. These low current troughs support low densities of invertebrates, so sturgeon typically select locations near areas with silt-clay substrates, higher current velocities and higher prey availability. This behavior allows pallid sturgeon to benefit from reduced energy costs associated with low flow velocity areas but remain close to an abundant source of food.
Humans Cause Change
Like most endangered species, pallid sturgeon populations suffer from human-induced habitat alterations. Because pallid sturgeon are adapted to large turbid rivers, free-flowing systems like the Atchafalaya River and, to a lesser extent, the lower Mississippi and Red rivers still maintain viable populations. Although we have no estimates of the number of pallid sturgeon in Louisiana, we do know that surveys of abundance throughout their historic range have shown great reductions at many locations and complete elimination at others. Movement patterns and habitat preferences described for pallid sturgeon in this study can provide natural resource managers with information to use to recreate or restore habitat loss throughout the historic distribution of the pallid sturgeon.
Scientists tend to focus efforts to save threatened and endangered organisms on large, high profile species like bald eagles and grizzly bears. But thousands of other plants and animals depend on the same habitats as listed species; habitat alteration and loss can be devastating to the biodiversity of any terrestrial or aquatic system. Species’ extinctions have occurred throughout geologic time, but biologists are increasingly alarmed at the high rate of extinction associated with the pervasive effects of an ever-increasing human population. In this light, species like pallid sturgeon serve as bioindicators of environmental health, and restoration of productive populations of threatened and endangered species through protection and enhancement of degraded habitats will enable us to maintain the long-term productivity of Earth’s ecosystems.