Rio Grande Cutthroat

Oncorhynchus clarki virginalis

Native Trout Fly Fishing

A large spotted Rio Grande cutthroat from a Pecos River tributary in New Mexico.

Introduction

The Rio Grande cutthroat was the first trout to be discovered in North America by Europeans and was first described in 1541 by Spaniards crossing the southwest in search of gold. This beautiful trout is considered a minor subspecies of the Yellowstone cutthroat and is native to the Rio Grande, Canadian and Pecos River drainages of New Mexico and Colorado. Rio Grande cutthroat have the distinction of being the southern most subspecies of cutthroat trout. However due to the loss of populations across their native range and reports of Rio Grande cutthroat in Mexico and Texas, it is unclear how far south this trout once occurred. The Mexican reports have been all but dismissed but Garrett and Matlock (1991) provided evidence indicating that Rio Grande cutthroat were likely native to Texas. Today the southern most known populations are found on the eastern flanks of the Gila Mountains in Southern New Mexico.

Life History Information

The Rio Grande cutthroat historically occurred as stream resident, fluvial and lacustrine populations, although they are confined to fragmented stream resident populations today. As there are no large lakes within the native range of the Rio Grande cutthroat, the lacustrine life history is relatively rare within the subspecies and the stream resident and fluvial forms predominated. The stream resident form of this trout is the only form remaining today, however there have been no formal life history studies conducted on the Rio Grande cutthroat, so little is known about variations in their habits in comparison to other species. However some facts about the life history of these fish can be discerned from various other studies done on Rio Grande cutthroat. According to Behnke (2002) these trout are thought to have a life span of between five and eight years, with stream dwelling populations generally reaching a maximum size of around 15" or 16". As with other subspecies of cutthroat, the Rio Grande cutthroat a spring spawning fish, with spawning occurring between April and July depending on the elevation (Rinne 1995). Males typically will spawn for the first time at age two, while females generally spawn at three years old at a size of between 4" and 10" long (Trotter 2008). Rio Grande cutthroat are known to be generalists in their feeding habits and incorporate both aquatic and terrestrial insects into their diet. However a study done by Shemai et al. (2007) indicated that small larval caddis flies tend to make up the bulk of their diet. Like other trout, Rio Grande cutthroat require streams with cool water and ample riparian cover and in-stream structure (boulders and woody debris). Harig and Fausch (2002) also indicated that stream gradients of less then 10% are required in Rio Grande cutthroat habitat and they also require overwintering pools that are deeper than 6".

Status

his trout like other interior cutthroat subspecies has experienced major declines across its native range over the past century. These declines led to the Rio Grande cutthroat being petitioned to be listed as threatened under the Endangered Species Act in 1998 (SCBD 1998). While this petition was originally declined, in 2008 the USFWS determined that the Rio Grande cutthroat is still declining and should be listed as threatened under the ESA. Like other interior cutthroat subspecies, the same basic group of problems have also plagued the Rio Grande cutthroat and led to its demise. These include habitat destruction, over-harvest and most notably the introduction of nonnative fish. While rainbow trout are the most problematic nonnative trout and have played a large role in loss of pure Rio Grande cutthroat populations due to hybridization, brown trout are the most common nonnative trout throughout the Rio Grande basin and have also had a substantial impact on some populations. Brown trout are fall spawning fish and do not hybridize with Rio Grande cutthroat, but they do exert competitive and predatory pressures on native cutthroat. Shemai et al. (2007) showed that when Rio Grande cutthroat are found in the same stream as brown trout, they had a lower body fat content and also had a much different diet than those found in streams without brown trout. The cutthroat in these studies also showed fin damage when in with brown trout suggesting agonistic behavior by the brown trout. Brown trout are also known to grow to larger sizes than the Rio Grande cutthroat and become piscivorous once the get large enough and as such predation by browns on cutthroat has also been an issue. There are some instances of coexistance between browns and Rio Grande cutthroat, typically with the brown trout utilizing the warm lower reaches of the rivers and the cutthroat confined to the colder headwaters. An issue with the coexistence of brown trout and Rio Grande cutthroat has to deal with fishing related harvest. Since brown trout tend to be much more difficult to catch than the native cutthroat, anglers are much more likely to catch the cutthroat. If harvest of cutthroat is allowed in these waters it can have a significant effect on the cutthroat population and allow the brown trout to dominate the watershed (Behnke 1992). Overall brown trout remain a major threat to the continued existence of these cutthroat across much of their native range.

Although the introduction of nonnative trout is the single largest threat to the Rio Grande cutthroat, it is not the only one and historically habitat destruction has also had a major impact on these trout as well. Mining, timber harvest, irrigation and overgrazing by cattle can all be credited with contributing to the degradation of the Rio Grande cutthroat's habitat. Today 43.3% of streams containing Rio Grande cutthroat are considered to have habitat conditions rated as fair, poor or unknown (Alves et al. 2008). Overgrazing by cattle has been a significant issue due to the destruction of crucial riparian vegetation and undercut banks that the Rio Grande cutthroat depend on for cover and and to rest. The loss of riparian vegetation also leads to increased sedimentation and higher summertime stream temperatures, which can lead to spawning failures and mortality in these cutthroat. At high altitudes loss of riparian vegetation also leads to the formation of stream anchor ice in the winter, which can greatly reduce the available winter habitat and lead to increased spring scour (Rinne 1995). While is native range has been reduced by 90 to 95 percent today, there are still 120 known populations of Rio Grande cutthroat (Alves et al. 2008) and restoration efforts are underway. It can be hoped that with more enlightened management practices the Rio Grande cutthroat will begin staging a comeback across its native range in the future.

Description

Like its northern cousins the Colorado River and greenback cutthroat the Rio Grande cutthroat is often brilliantly colored, although a more somber coloration does tend to be more common. The overall coloration of these fish is typically a brassy-yellow or bronze along the sides and generally a yellow or greenish-bronze tone over the back. Sexually mature fish often display much more brilliant colors with some fish displaying bright red or orange along their lower flanks. Some individuals also have a pink or orange color along their lateral line and the gill plates are generally a rosy or bright orange color. There are two major spotting variations across the native range of the Rio Grande cutthroat, with fish from the Pecos River watershed displaying much large spots, similar to those of the Greenback cutthroat. Typical Rio Grande cutthroat have moderate sized round spots that tend to be concentrated towards the caudal peduncle and above the lateral line.

Stream Resident Form

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Native Trout Fly Fishing
 

Stream Resident Form - Pecos Drainage

 

Native Range

Native Trout Fly Fishing

A map of the original native range of the Rio Grande cutthroat. Data Source: Behnke (2002)and Trotter (2008).