Fish and Aquatic Conservation



illustration of a Rio Grande Silvery minnow

Rio Grande Silvery minnow

Hybognathus amarus, (Girard, 1856)

Cool Facts

Rio Grande silvery minnows travel in schools. The abundance of the Rio Grande silvery minnow varies from season to season and from year to year. If stream or river flows do not significantly increase during the spring, this species is less likely to spawn that year.

SIZE: The maximum size for the Rio Grande silvery minnow is 8.9 cm (3.5 in).

RANGE: The silvery minnow was once abundant throughout the Rio Grande and Pecos basins, but now is limited to just a few locations of the Rio Grande in New Mexico. Within North America, the Rio Grande silvery minnow inhabits the Rio Grande River. The Rio Grande silvery minnow currently occupies less than 10% of its historic range, and is now only found in the Rio Grande River from Cochiti Pueblo, downstream to the in-stream flow of Elephant Butte Reservoir. Apparently this species is now extinct in Texas.

HABITAT: Silvery minnows prefer large streams with slow to moderate current flowing over a mud, gravel substrate, or shifting sand-silt substrate bottom. Silvery minnows typically occupy stream habitats where water depths are moderate 0.2 to 0.8 m (8 in. to 31.5 in.) and have velocity from 0 to 30 cm (0 to I ft./sec). During the winter, these minnows are most commonly found in nearly still water with debris cover. However during low flows, they are found in isolated pools and in watered reaches immediately down stream of diversion structures. Rio Grande silvery minnows have also been found in irrigation ditches and canals.

DIET: Silvery minnows tend to skim the bottom of rivers and streams. These fish are herbivores, whose diet consists of river plants and benthic macroinvertabrates.

Natural History

Rio Grande silvery minnows are pelagic broadcast spawners. Reproduction normally consists of several spawning episodes with approximately 10 minute intervals in between spawning sessions.

Spawning behavior includes male silvery minnows pursuing a single female. If a female minnow is ready to spawn, a single male will nudge her abdominal region and then wrap himself around the female. Usually, around this time, eggs and milt are extruded simultaneously by the female and male silvery minnows.

Silvery minnow eggs hatch within 24 hours of being fertilized. Silvery minnow larvae can begin to swim in just three to four days after hatching.

Conservation

The Rio Grande silvery minnow was listed as an Endangered Species under the delineation of the Endangered Species Act by the U.S. Fish and Wildlife Service in 1994 and now exists in less than 5% of its native range. This species is also listed as Endangered under New Mexico state law. The Rio Grande silvery minnow became extinct in the Pecos River around 1968 which coincides with the introductions of the Plains minnow and the Arkansas River Shiner in these same waters.

The decrease in Rio Grande silvery minnow populations is directly correlated with man-made modifications and alterations to the Rio Grande over the past century. These modifications and alterations include water diversions for municipal and agricultural uses, alterations of the natural hydrological cycle of the Rio Grande, habitat degradation, and the construction of dams which prevent migration of the silvery minnow within its native range.

In 2000, The U.S. Fish and Wildlife Service (FWS) began a Rio Grande Silvery Minnow egg Salvage Pilot Project. The purpose of this joint project between the FWS, the U.S. Bureau of Reclamation, and the University of New Mexico is to rear Rio Grande silvery minnows to adult size and then transplant them back into their native range or hold them for captive propagation.