Gila Topminnow Yaqui Topminnow

Poeciliopsis occidentalis occidentalis Poeciliopsis occidentalis sonoriensis

Description

The Gila topminnow, Poeciliopsis occidentalis occidentalis, is a guppy-like livebearer that averages 1.2-1.6 in (3-4 cm) in length; it is tan to olive above and white below. Breeding males darken to jet black and develop bright yellow fins and golden tints along the midline.The Gila topminnow species consists of two subspecies—the Gila topminnow (P. o. occidentalis ) and the Yaqui topminnow (P. o. sonoriensis ). Although the two subspecies are visually very similar, the Yaqui topminnow has a longer snout and its mouth is positioned higher on its head. Both sub-species are federally listed as endangered. The Yaqui topminnow has a slightly curved, elongated body with a rounded to almost square caudal fin. The males are small, rarely exceeding 1 in (2.5 cm) but the females may be twice as large. The anal fin of the male is elongated into a copulatory organ (gonopodium), extending forward past the tip of the snout when in copulatory position. The eggs are fertilized internally and the young develop within the female's body and are born alive. Gravid females show distended abdomens and darkened urogenital areas. The body color is tan to olivaceous, darker above, often white on belly. The scales on the dorsum are darkly outlined by melanophores, extending as specks to upper belly and pre-pectoral area. The dark lateral is continuous along the sides posterior, and the fin-rays are outlined with melanophores. Breeding males are black, with some gold on the predorsal midline and orange at base of the gonopodium and sometimes on the bases of dorsal and pelvic fins.

Behavior

Topminnow lifespan appears to be about one year. Onset of breeding is affected by water temperature, daylight, and food availability. Gestation varies from 24 to 28 days for the Gila topminnow and 12-14 days for the Yaqui subspecies. Young are born alive from the mother. Topminnows feed on a wide variety of plant and animal material and bottom detritus.

Habitat

Topminnows can live in a broad range of habitats. They prefer shallow, warm, fairly quiet waters, but can also be found in moderate currents and depths up to 3.3 ft (1 m). They inhabit permanent and intermittent streams, marshes, and may be found close to the banks of larger rivers. Preferred habitat contains dense mats of algae and debris, usually along stream margins or below riffles, with sandy substrates sometimes covered with mud and debris. They become most abundant in marshes, especially those fed by thermal springs or artesian outflows.

Distribution

The Gila topminnow was historically abundant throughout the Gila River system in Arizona, New Mexico, and northern Mexico. The species was first described in 1853 from a specimen collected from the Santa Cruz River near Tucson. The Yaqui sub-species was formerly abundant throughout the Rio Yaqui drainage in southeastern Arizona and in Sonora and Chihuahua, Mexico. Each subspecies now occupies only a remnant of its historic U.S. range. Populations of this once abundant species are so small and suitable habitat so fragmented that there is a definite concern for the survival of the species in the United States.

Surveys conducted in 1989 showed the status of the Gila topminnow to be declining. In Arizona, the fish apparently is extirpated from two of the 11 sites where it once naturally occurred. One disappearance was due to an invasion of competing mosquitofish (Gambusia affinis ) and the other was the result of unknown factors. Mosquitofish also reinvaded a third site from which they had been removed several years before, and their return threatened the native topminnow population there as well. The 1989 surveys also found that 14 reintroduced Gila top-minnow populations had failed since the last survey in 1987. There remained approximately 50 top-minnow populations, many of which were located in aquaria and other captive facilities. Several of the wild populations were introduced in 1989, including the first in the topminnow's historic range in New Mexico. The status of the topminnow in the Mexican portion of its range is believed to be stable, but information is sketchy.

Threats

Water projects have transformed all free-flowing southwestern rivers into intermittent, deeply cut streams or broad, sandy washes subject to flooding. As a result the Gila and Yaqui topminnows have been reduced to a fraction of their pre-1860s range. Beginning in the late 1800s, exotic fish species were introduced into the habitat. The aggressive and predatory mosquitofish (Gambusia affinis ), introduced in 1926, has been the cause of much of the topminnow decline. The mosquitofish harasses adult topminnows and eats juveniles. Only when the habitat is sufficiently large and complex, can the two species coexist.

Conservation and Recovery

The Cottonwood Springs Partners for Wildlife project, located in southern Arizona on Sonoita Creek (a major tributary of the Santa Cruz River), continues to serve as an excellent opportunity for biodiversity restoration. This effort has improved habitat for the Gila topminnow, and the Huachuca water umbel (Lilaeopsis schaffneriana var. recirva ), a plant proposed in 1995 for listing as endangered. The U.S. Fish and Wildlife Service's (FWS) Arizona Ecological Services State Office hopes to use this Partners for Wildlife partnership and others nearby as examples to promote similar restoration efforts along the Santa Cruz River in Mexico.

Topminnow populations on the San Bernardino National Wildlife Refuge are well protected. The land for the refuge was purchased by the Nature Conservancy and donated to the FWS. The top-minnow has been successfully reared at the Dexter National Fish Hatchery in New Mexico, and this stock will be used for reintroduction into the wild.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
P.O. Box 1306
Albuquerque, New Mexico 87103-1306
Telephone: (505) 248-6911
Fax: (505) 248-6915
http://southwest.fws.gov/

References

Minckley, W. L. 1969. "Native Arizona Fishes: Live-bearers." Wildlife Views 16:6-8.

U.S. Fish and Wildlife Service. 1984. "Sonoran Top-minnow (Gila and Yaqui) Recovery Plan." U.S. Fish and Wildlife Service, Albuquerque.