Pantosteus plebeius

Unmack et al. (2014) elevated the subgenus Pantosteus to generic status based on a comparison of molecular, morphological, and paleontological data. Page et al. (2023) accepted this elevation, but others (e.g., Bangs et al. 2018) retain as a subgenus pending additional morphological and molecular analyses.

Genes have introgressed into C. discobolus at Nutria Creek, New Mexico (Smith et al. 1983, Crabtree and Buth 1987). At the southern end of the range in Durango and Zacatecas, Mexico, the population in the Pacific slope Rio Mesquital drainage was markedly genetically divergent from populations in the Rio Nazas and Rio Aguanaval drainages; possibly two different taxa are represented (Ferris et al. 1982, Echelle 1991). Miller (2005) recognized the Rio Nazas population as distinct (C. nebuliferus). Populations in the rios Fuerte, Mezquital, and Piaxtla (Pacific Slope of Mexico) may constitute one or more distinct taxa and (pending further study) were not identified to species by Miller (2005).

See Smith (1992) for a study of the phylogeny and biogeography of the Catostomidae.

Wide ranging from Colorado and New Mexico south in Mexico to Chihuahua, Durango, and Zacatecas; introduced in areas of Arizona and New Mexico; common in some parts of the range; stable in some areas, has declined in north, trend in Mexico uncertain; habitat loss, degradation (e.g., reduced stream flows, increased sedimentation), and fragmentation have contributed to declines; some populations have been detrimentally affected by interactions with white suckers and other non-native fish species.

Wide native range extends from the Rio Grande drainage of Colorado and New Mexico and Mimbres River of New Mexico southward through the endorheic Guzman basin (rios Casas Grandes, Santa Maria, and del Carmen), thence into the upper Rio Conchos in Chihuahua and Durango; this fish occupies only a single tributary of the Rio Bavispe (Pacific Slope, Rio Yaqui basin) in Chihuahua) (Miller 2005).

Populations in the rios Fuerte, Mezquital, and Piaxtla (Pacific Slope of Mexico) may constitute one or more distinct taxa and (pending further study) were not identified to species by Miller (2005).

This species has been introduced in the Rio Hondo of the Pecos drainage, New Mexico Sublette et al. 1990) and San Francisco drainage, Arizona and New Mexico (Minckley 1973, Lee et al. 1980). Populations established in Sapello Creek in the upper Gila River basin (New Mexico and Arizona) may have been introduced (Buth and Crabtree 1985) or perhaps originated naturally via a stream capture from the Mimbres River (Sublette et al. 1990). Page and Burr (2011) implied that this species is native in the upper Gila River system, Arizona and New Mexico.

This species is represented by a fairly large number of distinct occurrences (subpopulations). TNHC (1996) mapped the following collection sites: 4 in Colorado, 26 in New Mexico, 21 in Mexico, and 2 introduced populations on the borders of Arizona and New Mexico. A number of sizable populations occur in New Mexico; only one population currently remains in Colorado (see New Mexico Department of Game and Fish 1996). In Colorado, historically found throughout much of the Rio Grande drainage; currently, known from only one location (Swift-Miller et al. 1999).

Rees and Miller (2005) summarized primary threats as follows: "Primary threats to this species generally result from anthropogenic activities that alter the physical or biological characteristics of the Rio Grande sucker's habitat. Most of the historic physical changes to the aquatic environment and the majority of future threats are related to water management and flow modifications. These include the construction of migration barriers, which can result in habitat fragmentation and dewatering, and land use practices or landscape scale changes that result in degraded aquatic conditions. Specific threats to the Rio Grande sucker include the modification of stream channels (including channelization, diversions, rerouting and straightening). The primary human-induced biological threat to Rio Grande suckers is the introduction of non-native predators and competitors."

Factors that have caused declines include depleted flows that result in increased temperatures, dewatering, etc.; habitat alteration from siltation, channelization, etc.; habitat destruction, including pollution, transbasin diversions, etc.; and interactions with non-native fish (Zuckerman and Langlois 1990).

Hybridization with the introduced white sucker (Catostomus commersoni) has been reported as the primary reason for the decline in northern New Mexico and southern Colorado, but Swift-Miller et al. (1999) found that hybridization with the nonnative white sucker does not appear to be a major factor in decline of Rio Grande sucker in Colorado, whereas other biotic interactions with that species may be important. Similarly, Calamusso et al. (2002) reported that white suckers simply "replaced" native Rio Grande suckers in New Mexico, and they stated that further research is needed to determine the mechanisms by which white suckers and other non-native species limit the distribution and abundance of C. plebeius. Based on genetic data, McPhee and Turner (2004) concluded that hybridization between the two species occurs rarely, if ever.

Miller (2005) did not mention any threats or conservation concerns in Mexico, except that competition from other suckers may have reduced the distribution of this species in the Rio Yaqui basin