|Scientific Name:||Hybognathus amarus|
|Species Authority:||(Girard, 1856)|
Algoma amara Girard, 1856
|Red List Category & Criteria:||Endangered B1ab(iii,v)c(iv)+2ab(iii,v)c(iv) ver 3.1|
|Reviewer(s):||Smith, K. & Darwall, W.R.T.|
|Facilitator/Compiler(s):||Hammerson, G.A. & Ormes, M.|
This species is listed as Endangered because its extent of occurrence and area of occupancy are less than 300 sq km, number of locations is not more than 5, and habitat and population size are subject to declines and extreme fluctuations.
|Range Description:||Historical range included the Rio Grande and Pecos River systems, Texas, New Mexico, and Mexico; in the Rio Grande, this species occurred from Española, New Mexico, downstream to the Gulf of Mexico; in the Pecos River it occurred from the Rio Grande to Santa Rosa, New Mexico; it also occurred in the lower Rio Chama and the lower Jemez River, tributaries of the Rio Grande in New Mexico (USFWS 2010). It has never been found in any Mexican tributaries to the Rio Grande, despite extensive collection efforts (Edwards et al. 2003). Currently, the species is known to occur only in one reach of the Rio Grande in New Mexico, a 280 km stretch of river that runs from Cochiti Dam to the headwaters of Elephant Butte Reservoir; this includes a small portion of the lower Jemez River, a tributary to the Rio Grande north of Albuquerque (Sublette et al. 1990; Bestgen and Platania 1991; USFWS 1994, 1999, 2007; 2010; Bestgen and Propst 1996; Dudley et al. 2005). An estimated 90-95 percent of the population is believed to be in the 96 km portion of the Rio Grande downstream of the San Acacia Dam (Ikenson 2002).
Nonessential experimental population: Rio Grande, from Little Box Canyon (approximately 10.4 river miles downstream of Fort Quitman, Texas) to Amistad Dam, Texas; and Pecos River, from its confluence with Independence Creek to its confluence with the Rio Grande, Texas.
Native:Mexico; United States
|Range Map:||Click here to open the map viewer and explore range.|
This species is represented by 1-4 occurrences (subpopulations); it is known to occur only in one reach of the Rio Grande, which is split by three river-wide dams into four discrete sections (USFWS 2007). Analysis of microsatellite and mitochondrial DNA detected little spatial genetic structure over the current geographic range, consistent with high gene flow despite fragmentation by dams (Alò and Turner 2005).
In December 2008, Silvery Minnows were introduced into the Rio Grande River near Big Bend, Texas, as a nonessential, experimental population. Preliminary monitoring is being conducted to determine whether or not that reintroduction has been successful (USFWS 2010).
Analysis of temporal genetic data indicated that present-day effective population size of the largest extant population of this species (San Acacia reach) was 78 and the ratio of effective size to adult numbers was approximately 0.001 during the study period (1999 to 2001) (Alò and Turner 2005). Thus, the adult population size in this reach, which is estimated to include 90-95% of the total population, appeared to be roughly 78,000.
Extent of occurrence, area of occupancy, and population size have declined greatly. Formerly this was one of the most widespread and abundant species in the Rio Grande basin of New Mexico, Texas, and Mexico, but now the range has been reduced from an estimated 3,862 river-kilometers to only 280 river-kilometers (about 7 percent of the historical range) (USFWS 2010).
Abundance exhibits substantial (order of magnitude) fluctuations from year to year (Alò and Turner 2005).
Catch rates (CPUE) declined from the early and mid-1990s through 2003 (USFWS 2007). In 2004 and 2005, abundance of the remaining population (in the middle Rio Grande) increased. In 2004 and 2005, the Rio Grande experienced an increased stream discharge, compared to the extended low-flow conditions seen in 2002 and 2003. Relative abundance of the species declined from approximately 50 percent of the total fish community in 1995 to about 5 percent in 2004 (USFWS 2007).
|Habitat and Ecology:||Habitat includes pools and backwaters of low-gradient creeks and small to large rivers (Page and Burr 2011). This riverine minnow occurs in waters with slow to moderate flow in perennial sections of the Rio Grande and associated irrigation canals (Sublette et al. 1990). Most often it uses silt substrates (much less often sand) and typically occurs in pools, backwaters, or eddies formed by debris piles; larger individuals use a broad spectrum of habitats, including main and side channel runs, but this species rarely uses areas with high water velocities (USFWS 2007). The species most commonly occurs in depths of less than 20 centimetres in the summer and 31- 40 centimetres (median) in the winter; few use areas with depths greater than 50 centimetres (USFWS 2007). Winter habitat tends to be near instream debris piles (USFWS 2007). This is a pelagic spawner that produces thousands of semibuoyant, non-adhesive eggs that passively drift downstream while developing (see USFWS 2007). Developing eggs and larvae drift passively with river currents for about 3-5 days (Platania and Altenbach 1998). Drift distances may extend more than 100 km with elevated river flows during the spring-time spawning period.|
Throughout much of the historical range, the decline may be attributed in part to destruction and modification of habitat due to dewatering and diversion of water, water impoundment (several dams now intersect the historical range), and modification of the river (channelization). Competition and predation by introduced non-native species, water quality degradation, and other factors may also have contributed to the decline (USFWS 2010). Some of these threats have been reduced since the Rio Grande Silvery Minnow was listed as endangered, but none have been eliminated (USFWS 2010). The population has become fragmented and isolated and is vulnerable to natural and human-caused factors that could further reduce population size (USFWS 2010). Analyses by Alò and Turner (2005) suggest that the genetically effective size in the Rio Grande Silvery Minnow currently is "much smaller than expected based on adult census size; in fact, it is sufficiently small to warrant concern about extinction from genetic factors in the long term."
Survival of the species continues to be threatened by habitat and flow modifications (dewatering, channelization, regulation, and diversion of river flow to provide water for irrigation; diminished water quality caused by municipal, industrial, and agricultural discharges), and lack of suitable aquatic refugia during periods of low or no flow (Bestgen and Platania 1991; USFWS 1994, 2003, 2007, 2010). Drought exacerbates the problem of dewatering of habitat for human uses. Recruitment of young fish into breeding populations may be limited because eggs and larvae are entrained through water diversion structures and are transported downstream into unsuitable nursery habitats (e.g., Elephant Butte Reservoir) (Platania and Altenbach 1998). Additionally, dams may block upstream return to natal sites or prevent recolonization of areas where the species has been extirpated (e.g., Luttrell et al. 1999). It is unlikely that predation is a major factor in the decline of the Rio Grande Silvery Minnow, but it has probably played a minor role, with increasing importance as populations have come under greater stress from other factors (USFWS 2010). Competition and/or hybridization with non-native species in the future could potentially affect the remaining populations of Rio Grande Silvery Minnow (USFWS 2010). Small range makes the species vulnerable to catastrophic events (e.g., loss of surface water) that could wipe out the remaining population (USFWS 2003).
"Although various conservation efforts have been undertaken in the past and others are currently being carried out in the middle Rio Grande, and abundance in recent years is increasing, the threat of extinction of the Rio Grande Silvery Minnow continues because of the high probability of continued drought, the fragmented and isolated nature of currently occupied habitat, and the absence of Silvery Minnows in other parts of the historic range. Additional work needs to be done to conserve this species and the ecosystems upon which it depends." (USFWS 2010).
The recovery plan (USFWS 2007) summarized needed conservation measures as follows:
|Citation:||NatureServe 2013. Hybognathus amarus. The IUCN Red List of Threatened Species. Version 2015.2. <www.iucnredlist.org>. Downloaded on 01 September 2015.|
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