Gila nigrescens

The taxonomic status and relationships among isolated chub populations in the Laguna Bustillos and Guzmán basins and the Mimbres River need further study (Propst and Stefferud 1994).

Small range in Chihuahua and New Mexico; has declined dramatically in both range and abundance; declines in Mexico probably are continuing; threatened by habitat loss/degradation (e.g., dewatering, channelization, pollution of streams) and effects of non-native fishes.

This species is restricted to tributaries of the endorheic Guzmán basin, including the Mimbres River (Sublette et al. 1990) in New Mexico and the Guzmán and Laguna Bustillos basins in Chihuahua, Mexico (Propst and Stefferud 1994, Page and Burr 2011).

Surveys throughout the historical range by Propst and Stefferud (1994) yielded the following results:

LAGUNA BUSTILLOS BASIN (CHIHUAHUA): At 10 sampled sites, Chihuahua chub was common at only two sites.

GUZMAN BASIN (CHIHUAHUA): At nine sampled sites in the Río Santa Clara, Chihuahua chub was moderately common in the upper portion of the river but uncommon or rare downstream. In the Río de Santa Maria, Chihuahua chub was present at all riverine sites upstream of Presa el Tintero, and moderately common at 2 of 12 sampled sites. In the Río Casas Grande, G. nigrescens was present at three stream sites and a spring system (of a total of 8 sampled sites), and moderately common near Zaragoza and El Rusio. In the Río Piedras Verdes, the species was present at two of four sampled sites (common only near Hernández Jovales). In the Río San Pedro, Chihuahua chub was present at three of four sampled sites (moderately common at one site).

MIMBRES RIVER (NEW MEXICO): Sampling in 21 locations in the Mimbres River and four tributaries yielded Chihuahua chubs in the Mimbres River from the confluence of Allie Canyon downstream for about 12 km and in Archuleta/Moreno Spring; the species was not collected at all sites in the occupied reach nor was it common where found; Archuleta/Moreno Spring supported the greatest number of individuals.

This species is represented by one occurrence (subpopulation) in New Mexico and several in Chihuahua (Propst and Stefferud 1994).

In a range-wide survey, Propst and Stefferud (1994) found Chihuahua chubs at 28 of 40 stream sites having fish, in one of nine spring sites, and at neither of two lakes sampled in Chihuahua. It was moderately common at 12 of the stream sites, but fewer than 30 chubs were collected at six of these. In the Mimbres Drainage, the Chihuahua chub was regularly found only in Archuleta/Moreno Spring and was not common there. [Note: a distinct occurrence or subpopulation may comprise multiple "sites."]

Development of water resources for agricultural and municipal uses has dewatered the downstream ends of streams, and surface flow is now seasonally diminished where water was historically permanent (Brand 1937). By the 1950s, upland streams in the range of the Chihuahua chub were severely damaged by the effects of uncontrolled logging and pollution from sawmills; harvest of fish with dynamite depleted surviving fish populations (Needham and Gard 1959).

Currently, the Chihuahua chub is limited to stream reaches where modification (e.g., channelization and dewatering) and human-induced habitat degradation (e.g., livestock overgrazing, municipal and agricultural pollution) are minimal; such areas are uncommon (Miller and Chernoff 1979, Propst and Stefferud 1994). The presence and abundance of pools associated with root masses of uprooted or standing large trees appeared to be the best predictor of the occurrence and abundance of Chihuahua chubs (Propst and Stefferud 1994). Chubs were moderately common only in remote and relatively unmodified portions of the rios Santa Clara, Santa Maria, and Casas Grandes where mature stands of cottonwood were fairly common. Where cultivated fields bordered the stream or where towns were nearby, large riparian trees were uncommon and no snags or root masses were in the stream; in these areas Chihuahua chubs were uncommon or absent and individuals were small (Propst and Stefferud 1994). Additionally, Chihuahua chubs were also uncommon where non-native fishes were present, even if habitat was not degraded.

Reasons for its decline in New Mexico include modification of habitat by agricultural and flood control practices, and establishment of non-native fish species (USFWS 1983, Propst and Stefferud 1994). Severe modification and destruction of riverine habitats (dewatering, channelizing, and removal of woody riparian vegetation) are the greatest threats and impediments to recovery in the Mimbres River. Sections of the Mimbres River that historically contained pools and flowing water are now usually dry. Severe flooding caused by degradation of the watershed (e.g., deforestation and consequent erosion, siltation, and water temperature alteration), and loss of riparian vegetation have contributed to the decline. Nonnative fishes such as rainbow trout usurp Chihuahua chub habitat and may prey upon them. Longfin dace (Agosia chrysogaster) may displace young Chihuahua chub from shoreline habitats. The yellow grub infestation of chubs in Moreno Spring presents a severe health threat to this population (New Mexico Department of Game and Fish, September 2000 Biennial Review and Recommendations).

In Chihuahua, water quality degradation (pollution), surface water diversion for irrigation, groundwater pumping (and subsequent drying up of springs), stream channelization, and introduction of non-native fishes have contributed to the decline (Miller and Chernoff 1979, Propst and Stefferud 1994). Miller and Chernoff (1979) reported habitat conditions in Chihuahua, Mexico, as follows. The Rio Casas Grandes about 21 km southwest of Ascension was badly polluted; the pools were stagnant and choked with Spirogyra and other algae; flow was minimal and riffles were absent (this leads to reduced dissolved oxygen). In the Rio Casas Grandes just east of Casas Grandes proper, the river was sluggish, murky, strewn with trash, and choked with Spirogyra and narrow-leafed Potamogeton. In the Rio San Miguel at Ignacio Zaragoza, the river channel has been dredged and levees built. The water at this site was murky, less than 5 cm deep, and without noticeable current. The Rio Santa Maria at Bachiniva was, in places, used as a garbage dump. Water in the Rio del Carmen near Ahumada has been diverted for irrigation and no longer flows in the river channel. The Rio del Carmen near Ricardo Flores Magon was dry due to the presence, about ten kilometers upstream, of an earthen dam and hydroelectric power plant. In the Laguna Bustillos Basin, Arroya Miguel Chiquito was dry and the waters in Arroyo San Antonio were polluted.

A number of exotic fishes have been introduced within the range of Gila nigrescens, including Agosia chrysogaster and Oncorhynchus mykiss in the Mimbres River (USFWS 1986), Cyprinus carpio and Ictalurus melas in Rio Casas Grandes, Ictalurus melas and Ambloplites rupestris in Rio Piedras, Gambusia affinis in Rio Santa Maria near Buenaventura, and Cyprinus carpio in Rio Santa Maria at Santa Ana de Bavicora (Miller and Chernoff 1979). Specific effects of these fishes are not well documented but likely they are detrimental.