Big Bend Gambusia

Gambusia gaigei

Description

The Big Bend gambusia, Gambusia gaigei, grows to a length of about 1.2 in (3 cm). It is yellowish overall, with dark bars beneath the eye and chin and a faint lateral stripe. The male's anal fin is adapted as a sex organ for transferring sperm to the female.

Behavior

The gambusia is a livebearer. Eggs hatch inside the mother's body and are born alive. It is thought that females store sperm for several months after being impregnated. Big Bend gambusia's feeding habits are unknown but it is assumed that it feeds on insect larvae and small invertebrates such as zoo-plankton and worms.

Habitat

The Big Bend gambusia's natural habitat is clear, shallow streams and marshes fed by warm springs. This fish is most abundant amid shoreline vegetation, where there are overhanging trees.

Distribution

The Big Bend gambusia is known only from springs in Big Bend National Park in Texas. Biologists surmise that at least two separate populations originally existed—at Boquillas Spring, and at Spring 4, east of the present Rio Grande Village campground. It probably existed in other springs in the vicinity of Rio Grande Village. The Boquillas Spring population and the original Spring 4 population have been extirpated.

Currently, the Big Bend gambusia is restricted to the outflow stream of a single spring near Rio Grande Village in the Big Bend National Park (Brewster County), Texas. Descendants of the original Spring 4 population are being maintained in a natural holding pool nearby.

As of 1994, the gambusia population was stable in the refugium and in two other warm water spring ponds in the area.

Threats

Surface runoff and flooding of the Rio Grande River continually threaten the Big Bend gambusia's survival. Periods of high rainfall increase the amount of silt carried by surface runoff, increasing stream turbidity and bottom deposition. Floods provide an avenue for mosquitofish (G. affinis ) and other competitors to invade the Big Bend gambusia's pools. Over the years, federally sponsored projects, designed to enhance the "oasis image" of the Rio Grande Village area, have diverted spring out-flows into artificial ditches and ponds. Groundwater levels have been lowered, decreasing the flow from the gambusia's springs.

Conservation and Recovery

At one time, the gambusia population was reduced to one female and two males held in captivity. To ensure the species' future, the artificial refugium was modified to receive piped-in warm water on a day-to-day, year-round basis. Population stabilization has been achieved at the refugium and in two other warm water spring ponds. Park personnel regularly monitor the habitats. Continued threats to the species include proposed campground expansion, floods on the Rio Grande (which could allow invasion of the pond by competing or predatory fish species), and anglers transferring fish to the gambusia habitats. Captive populations of the Big Bend gambusia are maintained at the U. S. Fish and Wildlife Service's Dexter National Fish Hatch-ery in New Mexico and the University of Texas at Austin as a precaution against extinction of the wild population.

The Park Service's current management goals for recovery of the species include supplementing spring flows from wells in the dry season, and rehabilitating sections of two spring outflows to approximate pre-development conditions. Plans also call for eradication of mosquitofish from springs and streams in the campground area and the eventual establishment of this gambusia in other suitable locations.

Contact

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P.O. Box 1306
Albuquerque, New Mexico 87103
http://southwest.fws.gov/

References

Brune, G. 1981. Springs of Texas. Branch-Smith, Fort Worth.

Hubbs, C., and J. G. Williams. 1979. "A Review of Circumstances Affecting the Abundance of Gambusia gaigei, an Endangered Fish Endemic to Big Bend National Park." In R. Linn, ed., Proceedings of the First Conference on Scientific Research in the National Parks.

Minckley, W. L. 1962. "Two New Species of Fishes of the Genus Gambusia (Poeciliidae) from Northeastern Mexico." Copeia 1962:391-396.

U. S. Fish and Wildlife Service. 1984. "Big Bend Gambusia Recovery Plan." U.S. Fish and Wildlife Service, Albuquerque.