Snail Darter

Percina tanasi

Description

The snail darter, Percina tanasi, is a small, robust fish, rarely exceeding 3.4 in (9 cm) in length. Brown, with a faint trace of green above and white below, it has four dark brown, saddle-like patches across the back. The upper portion of its head is dark brown, the cheeks are mottled brown and yellow. A black bar extends vertically beneath the eyes, which have an orange-yellow iris. The median fins are mostly clear with black patterning tinged with yellow; the paired fins widely vary between individuals, from mostly clear to bright yellow-orange.

Behavior

Snail darters are relatively short-lived, reaching a maximum age of five or six years. The mortality rate between the first and second year is 75% to 80%. Snail darters spawn in the shallowest areas of river shoals between January and mid-March every year that they survive. Spawning is non-territorial with multiple, promiscuous mating after a courtship ritual in which the female visually solicits a single male, followed by touching and finally copulation. Spawning usually occurs on swift shallow shoals. About 600 eggs deposited in gravel or on rocks drift freely for 15-20 days before hatching. The larvae probably feed on tiny crustaceans and invertebrates. During this period the larvae are extremely vulnerable to predation by many species, including adult snail darters. Newly hatched darters drift downstream. The benthic juveniles are transformed into adults 15-30 days after hatching. As they grow, they move upstream to the shoal areas where they were spawned.

The snail dater feeds exclusively on gastropods, especially aquatic snails, during the fall and winter, and overall gastropods make up about 60% of the darter's food, although its diet tends to vary seasonally. Insects provide the other major food source during the spring and summer, and caddisfly may be the primary food source, supplemented by other insects and fish eggs. The diet of juveniles is the same as adults.

Habitat

Most snail darters prefer moderately flowing, vegetated streams with sandy bottoms and wide shoals for spawning, but because of the expansive range of the darter in three states, the habitats are associated with a wide variety of vegetation. Originally, oak-pine and oak-hickory forests surrounded the large streams and rivers, but these have largely been replaced by farm land.

The snail darter requires clean gravel-sand shoals for feeding and shallow, slow moving water for spawning. Spawning is delayed during times of flooding, suggesting that both water depth and water clarity are necessary for the mating ritual and egg deposit. Survival of eggs and larvae require slackwater areas, such as deep pools, downstream from the spawning site. Large, low-gradient, undisturbed rivers and streams with alternating pools and riffles provide the best spawning conditions.

Distribution

The snail darter was first collected in 1973 in the lower reaches of the Little Tennessee River in Loudon County, Tennessee, an area that was eventually inundated by completion of the Tellico Dam, a project of the Tennessee Valley Authority (TVA). According to the U. S. Fish and Wildlife Service (FWS), it is difficult to determine the range of the snail darter before construction of the dam. Snail darters were probably confined to the upper portions of the Tennessee River upstream from north-central Alabama, and the lower portions of the Hiwassee, Clinch, Little Tennessee, French Broad, and Holston Rivers.

The snail darter is now found in the main channel of the Tennessee River and in six of its tributaries. Darters have been found in small numbers in three Tennessee reservoirs—Watts Bar (Loudon County), Nickajack (Hamilton County), and Guntersville (Marion County). Only adult darters have been found and researchers think these fish migrated from tributary spawning grounds. No reproduction has been documented in these reservoirs.

In 1975 and 1976, snail darters were successfully transplanted to the Hiwassee River (Polk County). Surveys have indicated the population is thriving.

Additional snail darter populations have been located since the fish was initially described. In 1980 the first new naturally occurring population of snail darters was discovered in South Chickamauga Creek, which straddles the Tennessee-Georgia border (Hamilton County, Tennessee, and Catoosa County, Georgia). In 1981 small snail darter populations were discovered in the Sequatchee River (Marion County) and Sewee Creek (Meigs County) in Tennessee. An additional population was found in September 1981 in the Paint Rock River (Jackson and Madison counties) in Alabama.

Threats

The snail darter declined because of habitat destruction resulting from impoundments throughout the Tennessee River drainage system. Siltation, channelization, and dredging, along with pollution from industrial and urban waste, and pesticides from agricultural practices, further compounded the adverse conditions of the river. Logging and mining activities, along with the conversion of forests to farm land, increased turbulence in the river system, and flood control and navigation management finally destroyed most of the darter's habitat.

Unknown to anyone before 1973, the snail darter became the focus of a major political controversy during the late 1970s when its existence halted the completion of the TVA's Tellico Dam on the Little Tennessee River. It was listed as an Endangered Species in 1975 with the Little Tennessee River designated as habitat critical to its survival. At the time of listing the only known population was threatened by the flooding of its habitat by the Tellico Dam.

In 1977 a federal appeals court ruled that the dam could not be completed since it would likely eliminate the snail darter. The following year, the U. S. Supreme Court upheld that decision, maintaining that the Endangered Species Act of 1973 was clear on the matter and that exceptions to the law must be made by the U. S. Congress, not the court.

In response to the Supreme Court's decision, Congress amended the Endangered Species Act in 1978, creating an Endangered Species Committee. The committee was given the responsibility for considering exemptions to the Act for resource development projects which had an unresolvable conflict with the Act. Since the committee was given the power to approve projects that would likely cause the extinction of a species, it soon became known as the God Committee.

In 1979, however, contrary to many expectations, the committee voted unanimously not to exempt the Tellico Dam from compliance with the Endangered Species Act. Congress responded by passing legislation, which was signed into law, exempting Tellico Dam from the Act and mandating its completion. This act went on record as the first official U. S. government decision to extirpate a species.

Conservation and Recovery

Prior to the climax of the political controversy in 1979, the FWS attempted a number of transplants of the snail darter into other Tennessee waters. Only one transplant has proven successful. In 1975 and 1976, 710 snail darters were introduced into the Hiwassee River in Polk County. Regular surveys have confirmed reproduction, and the darter appears to be thriving there.

In 1980, following the exemption awarded the Tellico Dam by Congress, a new snail darter population was discovered in South Chickamauga Creek, in Tennessee and Georgia. Other small populations were subsequently discovered in Tennessee and Alabama.

In light of the discovery of additional snail darter populations, the FWS downlisted the snail darter from Endangered to Threatened in 1984. If substantial new snail darter populations are discovered or if current populations remain stable or increase over a 10-year monitoring period, the FWS Recovery Plan states the agency will consider removing the snail darter from the federal Endangered Species list.

Contact

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
1875 Century Blvd., Suite 200
Atlanta, Georgia 30345
http://southeast.fws.gov/

References

Biggins, R. G. 1984. "Proposal to Reclassify the Snail Darter from an Endangered Species to a Threatened Species and Rescind Critical Habitat Designation." Federal Register 49 (35):6388-6389.

Etnier, D.A. 1976. "Percina tanasi, a New Percid Fish from the Little Tennessee River, Tennessee." Proceeds of the Biological Society 88 (44):469-645.

Hickman, G. D. and R. B. Fitz. 1978. "A Report on the Ecology and Conservation of the Snail Darter from 1875-1977." Tennessee Valley Authority Technical Note B28. 129 pp.

Starnes, W. C. 1977. "The Ecology and Life History of the Endangered Snail Darter." Ph.D. Dissertation, University of Tennessee, Knoxville. 143 pp.

U.S. Fish and Wildlife Service. 1983. "Snail Darter Recovery Plan." U.S. Fish and Wildlife Service, Atlanta.