The Gila topminnow is a small (approximately two inches long), guppy-like fish. The body is generally tan to olive in color, with the back usually dark while the underbelly is often white. A fairly thick, dark band occurs along both sides of the fish. Breeding males are generally dark to jet black in color.1
Gila topminnow occupy headwater springs and vegetated margins and backwater areas of intermittent and perennial streams and rivers. Adults tend to congregate in areas of moderate current, below riffles and along the margins of flowing streams in accumulated algae mats.1 Topminnows can withstand water temperatures from near freezing to 100°F. They can also live in a fairly wide range of salinity, ranging from tap water to sea water.
Gila topminnow once occupied aquatic habitats below 5,000 feet elevation throughout the Gila River drainage in New Mexico, Arizona and Mexico. Presently, this species occurs in known ranges of Mexico and several localities in the Gila River drainage in Arizona. Some areas contain reintroduced populations.2
At one time, the Gila topminnow was the most common fish found in the Gila River basin. Its numbers have been greatly reduced due to the introduction of other fish species, especially the mosquitofish. With their high reproductive rates and long breeding season, Gila topminnow can rapidly expand into new habitats devoid of nonnative fish species.
Gila topminnow are live-bearers. The reproductive season normally occurs from April through November, but young may be produced year-round in thermally stable springs. During breeding, some males become dark and exhibit aggressive breeding behavior, while others become black but still attempt to mate inconspicuously with females. The typical brood consists of 1015 young, with larger broods produced during the summer. Young produced early in the breeding season may reach sexual maturity in a few weeks to several months. An individual's life span is approximately one year, but this appears to be dependent on the time of year in which it was born.1
Gila topminnows are omnivorous. They likely utilize a broad spectrum of food such as crustaceans and vegetable material but will feed voraciously on aquatic insect larvae, especially mosquitoes, when abundant.
In 1967, the Gila topminnow was listed by the U.S. Fish and Wildlife Service as "endangered" under the Endangered Species Act.3
Gila Topminnow in Pima County:
Historically, Gila topminnow occurred all along the Santa Cruz and San Pedro rivers. Records indicate that the species was eliminated from the Santa Cruz River near Tucson by 1904.4 Currently, this species is found in the upper reach of Cienega Creek within the Las Cienegas NCA, and in the spring of 2002 was found downstream in Cienega Creek Natural Preserve.
1. Minckley, W. L. 1973. Fishes of Arizona. Arizona Game and Fish Department, Phoenix. pp. 186-192
2. Arizona Game and Fish Department. Unpublished Abstract, Heritage Data Management System. April 1999
3. USDI, Fish and Wildlife Service. 1967. Federal Register 32:4001
4. Zander, M. and Jennings M. 1986. Site Specific Water Quality Criteria Study for the Santa Cruz River. Harding Lawson Associates. Job No. 17,144,010.01. p. 73
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