Pecos Gambusia

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Pecos Gambusia

Gambusia nobilis

Status	Endangered
Listed	October 13, 1970
Family	Poeciliidae (Livebearer)
Description	Robust, silvery yellow fish with an arched back.
Habitat	Springs and outflow streams.
Food	Insects and other invertebrates.
Reproduction	Bears live young.
Threats	Fragmentation of habitat, dying springs.
Range	New Mexico, Texas

Description

The Pecos gambusia, Gambusia nobilis, also known as the Texas gambusia, is closely related to the common mosquitofish (G. affinis ). It is a small, robust silvery yellow fish, and in profile, shows an arched back. Females have a black area on the abdomen that surrounds the anal fin. The anal fin of the male is modified as an organ to transfer milt during copulation.

Behavior

The Pecos gambusia gives birth to live young. It feeds on the surface, taking a wide variety of insects and small invertebrates. It shelters from predators in beds of aquatic vegetation.

Habitat

The Pecos gambusia is found in springs and out-flow streams over a large geographic range in the Pecos River basin—from an elevation of 3,870 ft (1,180 m) in the uplands of New Mexico to about 2,690 ft (820 m) in Texas. It is found in springheads, limestone sinks, spring-fed creeks, and sedge-covered marshes. Common factors among these habitats are: clear, clean water, stable flows, and fairly constant temperatures. The Pecos gambusia is intolerant of water temperatures above about 100°F (38°C).

Distribution

The Pecos gambusia is endemic to the Pecos River basin in southeastern New Mexico and western Texas, occurring from Fort Sumner in De Baca County, New Mexico, as far south as Fort Stockton in Pecos County, Texas. New Mexican populations, known from the Pecos River south of Fort Sumner and the North Spring River near Roswell, have been extirpated. Texas populations in Leon and Comanche Springs were eliminated when the springs dried up.

In New Mexico, populations occur in Ink Pot within the Salt Creek Wilderness Area, in a group of springs, sinkholes, and outflows at Bitter Lake National Wildlife Refuge (Chaves County), at Living Desert State Park, and in Blue Spring and its outflows (Eddy County). In Texas, populations are found near Balmorhea in East Sandia, Phantom Lake, and Giffin springs (Reeves County), and near Fort Stockton in Leon Creek and Diamond-Y Spring outflows (Pecos County).

In 1980, the total population of the Pecos gambusia was estimated at about two million fish, divided fairly equally among sites in New Mexico and Texas. The largest concentrations occurred in Pecos County, Texas. In spite of the seemingly large numbers, all populations are considered vulnerable because of alterations to the Pecos River.

Threats

Water diversion from the Pecos River for irrigation intensified after five major and three lesser dams were constructed along the main river channel. A new dam (Brantley Dam) to replace the existing McMillan Dam was recently proposed for construction.

Although the mainstream never provided preferred habitat for the Pecos gambusia, it enabled fish to migrate from dry springs to habitable ones during drought years. Dam construction and water diversion have reduced natural stream flows, increased salinity and turbidity, and altered water temperatures, prohibiting migration. The isolated gambusia populations now depend on uninterrupted spring discharges for survival. Introduction of non-native gambusia into the river system has been an additional factor in limiting the Pecos gambusia's range. Although it is better adapted to springs and outflows, its competitors are better suited to existing downstream conditions.

Conservation and Recovery

During the 1970s, U. S. Fish and Wildlife Service personnel attempted to transplant the Pecos gambusia to more than 20 springs and sinkholes within the Bitter Lake National Wildlife Refuge in an effort to expand its range and improve its chances of survival. Most of these attempts were unsuccessful, but small populations were established at Bitter Creek, Sago, and two smaller springs. In 1987 Bitter Creek and Sago Spring supported a population of about 500 gambusia.

In the past, the Dexter National Fish Hatchery in New Mexico successfully raised a captive population of the Pecos gambusia. Although this population was eventually released, the hatchery's success demonstrates that a captive stock could be used to replenish drought-depleted populations.