Gila intermedia

Three species in the Gila robusta complex, robusta, nigra, and intermedia, have a controversial taxonomic history. Some studies found that these are morphologically similar and genetically indistinguishable at the species level (Carter et al. 2018, Copus et al. 2018, Page et al. 2016, 2017). Other studies found them to be distinct (Minckley and Marsh 2009, Dowling et al. 2015, Marsh et al. 2017, Chafin et al. 2021). Analyses by Suchocki et al. (2023) support recognition of Gila robusta as a single, polytypic species with isolated geographic populations. They found that within a watershed the three species are more similar to one another than they are to the same species in other watersheds.

Extirpated or much reduced in numbers and distribution in majority of historical range in the upper Gila River basin in Arizona, New Mexico, and adjacent Sonora, Mexico; has been detrimentally affected by habitat degradation and interactions with exotic fishes.

Historically, this species occurred in springs and small streams in the upper Gila River basin in southern Arizona, southwestern New Mexico, and northeastern Sonora, Mexico (Miller and Lowe 1964, Minckley 1973, USFWS 2002, Page and Burr 2011). The vast majority of the range is in Arizona.

In Arizona, Gila chubs are known to have occupied portions of the Salt, Verde, Santa Cruz, San Pedro, San Carlos, San Simon, San Francisco, and Agua Fria drainages and smaller tributaries of the mainstem Gila River. Small remnant populations remain in most of these drainages with the exception of the Salt and San Simon Rivers, where all known populations have been extirpated.

An observation of a Gila chub in Turkey Creek in the upper Gila River Basin in New Mexico was made in 2001 (Telles, pers. comm., 2001, cited by USFWS 2002).

The current known distribution in Mexico has been reduced to two small spring areas, Cienega los Fresnos and Cienega la Cienegita, adjacent to the Arroyo los Fresnos (tributary of the San Pedro River), within 2 km (1.2 mi) of the Arizona-Mexico border (Varela-Romero et al. 1992). No Gila chubs remain in the Mexican portion of the Santa Cruz River basin (Weedman et al. 1996).

Historically, 47 populations were recorded in approximately 43 rivers, streams, and spring-fed tributaries (Miller and Lowe 1967, Rinne and Minckley 1970, Minckley 1973, Rinne 1976, DeMarais 1986, Bestgen and Propst 1989, Weedman 1996, USFWS 2005). Of the 47 known populations, 29 are regarded as extant (USFWS 2005).

Where still present, populations are often small, scattered, and at risk from known and potential threats and from random events. Threats include: predation by and competition with nonnative organisms, including fish in the family Centrarchidae (Micropterus spp., Lepomis spp.), other fish species, bullfrogs (Rana catesbeiana), and crayfish (Orconectes virilis); and habitat alteration, destruction, and fragmentation resulting from water diversions, dredging, recreation, roads, livestock grazing, changes in the natural flow pattern, extirpation of beavers and loss of habitats they generated, mining, degraded water quality (including contaminants from mining activities and excessive sedimentation), and groundwater pumping (see USFWS 2002 and 2005 for further details; see also Hubbs 1954, Miller 1961, Minckley and Deacon 1968, and Meffe 1985).

Chubs in and adjacent to the San Carlos Reservation have been recorded with various skin lesions, likely due to water contaminants (Weedman et al. 1996). Watershed changes and the introduction of non-native fishes have occurred concurrently and it would be difficult to separate out one factor as a primary cause for the decline; most likely, multiple factors are involved. Destruction of cienegas and associated habitats undoubtedly has had an adverse impact (Hastings 1959, Hendrickson and Minckley 1984). A population in Monkey Spring was decimated from predation by largemouth bass following the introduction of this gamefish (Minckley 1973). Increasing green sunfish abundance in the San Carlos River was correlated with the decline or disappearance of Gila chub (Minckley 1985, Propst et al. 1985). Unfortunately, dietary data documenting predation by exotic fishes on the Gila chub are lacking. In addition to predatory and competitive impacts, exotic fishes also may spread exotic parasites. Of the 24 populations extant in the mid-1990s, at least 14 were subject to grazing at the site or upstream, at least 10 contained exotic fishes, 6 had limited habitat, and 6 had water diversions or impoundments; many were subject to multiple impacts from these or other factors (Weedman et al. 1996).