Rio Grande Silvery Minnow

Hybognathus amarus

Description

The Rio Grande silvery minnow is a stout fish with moderately small eyes and a small, slightly oblique mouth. Adults may reach 3.5 in (8.9 cm) in total length. The dorsal fin of the silvery minnow is distinctly pointed with the front located slightly closer to the tip of the snout than to the base of the tail. Life color is silver with emerald reflections. The belly is silver-white, fins are plain, and barbels absent.

Behavior

The silvery minnow spawns during the high spring flow, normally in June, as a result of snow runoff and reservoir release. The eggs are spawned in currents, then drift along the flow edge for about 24 hours before hatching. The drift time, however, is temperature dependent. After hatching, the fry seek slack water areas for safety; not being strong swimmers at birth, they attempt to find areas with no or low moving currents. Although there are no predators for the eggs, they are vulnerable to being sucked in by irrigation diversion.

The silvery minnow feeds on algae growing in shallow water in bright sunlight. Until recently, it was thought that the minnow did not reach sexual maturity until age two, but recent observations suggest that under ideal feeding conditions, the silvery minnow grows at a tremendous rate and can reach sexual maturity in one year.

Habitat

The silvery minnow prefers low velocity currents in slack backwater areas at least 18 in (45.7 cm) deep with shifting sandy substrates. However, construction of the Cochiti Dam altered the Rio Grande substrate to gravel, and water diversion sometimes leaves this reach of the river dry. The waters in the middle of the river are murky, partly as a result of natural siltation and partly from irrigation return flow.

In the past, during periods of extremely low flow, the species survived in areas where irrigation water returned to the river, in seepage and leakage pools located downstream of irrigation diversion dams, and, prior to the construction of Cochiti Dam, in the canyon of the Rio Grande upstream of Cochiti.

Distribution

This species was historically one of the most abundant and widespread fishes in the Rio Grande basin, occurring from Espanola, New Mexico, to the Gulf of Mexico. It was also found in the Pecos River, a major tributary of the Rio Grande, from Santa Rosa, New Mexico, downstream to its confluence with the Rio Grande in south Texas.

Collection data indicate that the species occupies about 5% of its historical range. It has been completely extirpated from the Pecos River and from the Rio Grande downstream of Elephant Butte Reservoir. Currently, it is found only in a 170-mi (274-km) reach of the middle Rio Grande, New Mexico, from Cochiti Dam, Sandoval County, to the headwaters of Elephant Butte Reservoir.

Thought to be extinct in 1969, the silvery minnow's population varies greatly from one year to the next, depending on food supply and water depths. In 50 fish collections made between Bernalillo and Elephant Butte Reservoir between 1987 and 1988, the silvery minnow was the second most abundant species, but by 1989-92 it had become the least abundant native fish species. If there is continual water flow year-round, the silvery minnow will become the dominant species within its reach, as was the case in 1994, but the U. S. Fish and Wildlife Service believes that without protection the species will likely become extinct.

Threats

Throughout much of its historic range, the decline of the silvery minnow may be attributed to modifications of stream desiccation by impoundments, water diversion for agriculture, stream channelization, and introduced exotic species. Its decline in the Pecos River, where it was replaced by a congener, the plains minnow (H. placitus ), occurred in 1968 when the plains minnow was introduced as bait. In less than a decade the silvery minnow was extirpated.

Decline in the Rio Grande probably began in 1916 when the gates at the Elephant Butte Reservoir were closed. Elephant Butte was one of five major dams constructed in the silvery minnow's Rio Grande habitat; it and the other dams were used to divert the flow of water for agriculture, and often the diversion caused reaches to dry up and destroy all fish life. Concurrent with building the dams was stocking the reservoirs with non-native fish, some of which made their way into streams and outcompeted native species.

Conservation and Recovery

The success of the silvery minnow is entirely dependent on water flow, which in the middle Rio Grande is controlled by the Rio Grande Compact Commission, established in 1929 to ensure the equitable apportionment of flows. The commission meets annually to review compliance during the preceding year and to consider water control implications. Federal agencies involved in determining water flows are the International Boundary and Water Commission, the Bureau of Reclamation, and the Corps of Engineers. All federal agencies are responsible for complying with the Endangered Species Act and as such must consider the survival of vulnerable fish in the Rio Grande. However, because water diversion is such a controversial issue, there has been much opposition from the agricultural sector. Under a 1906 charter with Mexico, the United States is required to deliver 600,000 acre feet annually to Mexico from the Rio Grande, which dictates certain water diversion decisions.

The growth of agriculture and cities along the Rio Grande may have adversely affected the quality of the river's water. During low flow periods, a large percentage of the river's flow consists of municipal and agricultural discharge.

Although New Mexico state law lists the silvery minnow as an endangered species, state law does not include provisions for acquisition of instream water rights for protection of fish and wildlife and their habitats. This has been a major factor affecting the survival of species dependent upon the presence of instream flow. Under the existing water rights administration, two native Rio Grande fish have become extinct and two others have been extirpated.

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/

Reference

U. S. Fish and Wildlife Service. 20 July 1994. "Final Rule to List the Rio Grande Silvery Minnow as an Endangered Species." Federal Register 59 (138).