Pahranagat Roundtail Chub

Gila robusta jordani

Description

The Pahranagat roundtail chub, Gila robusta jordani, also known as the Pahranagat bonytail chub, is a medium-sized fish, growing to about 10 in (25 cm). It has an elongated body with a narrow tail and a deeply cleft caudal fin. Its coloring is greenish with black blotches.

The Pahranagat roundtail chub is most similar in appearance to the common roundtail chub (G. r. robusta ), which is found in the Colorado River and its larger tributaries.

Behavior

The patchy distribution of Pahranagat roundtail chub in the Pahranagat Creek/Ditch suggests that this fish requires specific foraging habitat. Pahranagat roundtail chub typically congregate in pools below a portion of the river that is typically narrow and has increased water velocity. Fallen trees or branches are common to these areas, and increase water turbulence. The distinctive hydraulic conditions that Pahranagat roundtail chub occupy probably provide optimum opportunities for encountering food items with minimal energy expenditure. Pahranagat roundtail chub generally enter slightly faster water velocities when striking at a food item.

Pahranagat roundtail chub forage primarily on drifting invertebrates and secondarily, though infrequently, by pecking at substrate. The species rarely preys on other fish, although a Pahranagat roundtail chub was observed to successfully consume a mosquitofish. Rates of adult drift feeding vary, with more food consumed in the winter than in summer. The lower food consumption rate during the summer corresponds to a reduced availability of food items during the summer. The summer appears to be a period of austerity for adults, characterized by high metabolic demands due to wanner water temperatures and low food availability.

There is no relationship between feeding rate and relative food item abundance for two size classes of adult Pahranagat roundtail chub, although there is a relationship between feeding rate and water temperature for larger adults. Large Pahranagat round-tail chub may feed more selectively with increasing water temperatures, preferring bigger and energetically more efficient prey items. During winter, retrieval of smaller prey items in cooler water requires the expenditure of less metabolic energy.

Pahranagat roundtail chub have been observed spawning at three sites in the Pahranagat Creek, all approximately 2.0-2.2 mi (3.2-3.5 km) below Ash Springs. Adult Pahranagat roundtail chub begin to congregate in mid-January, although spawning generally does not start until late January. Peak daytime spawning activity generally occurs during early to mid-February, and although congregations persist through March, spawning usually does not occur after mid-February. In May 1988 spawning congregations appeared on two of the spawning sites, but no spawning activity was observed, and no larvae were produced.

Male and female Pahranagat roundtail chub are readily distinguishable by their reproductive behavior, which is similar to other cyprinids. The persistent and insistent behavior of a fish in the spawning congregation suggests that it is a male. Females are fewer in number and receive substantial attention in the form of male pursuit. When the female is ready to spawn, she swims down to the gravel bottom where she is attended by a group of two to 10 males. The spawning group vibrates violently for three to six seconds. The female generally swims away and is pursued by males until ready to spawn again. It is believed that females only appear on the spawning site when prepared to spawn, which occurs intermittently over several days.

Spawning occurs in relatively fast water in gravel-covered pool bottoms at water depths ranging from 1.9-3.4 ft (58-104 cm). Water temperatures during the spawning months range from 63-76°F (17-24.5°C).

Pahranagat roundtail chub eggs are broadcast over gravel substrates and apparently fall into the cracks. Convict cichlids and speckled dace have been observed picking at the spawning beds, presumably in search of eggs. Larvae reach "swim-up" stage approximately 28 days after eggs are deposited in the gravel bed. It takes 28-53 days for all larvae to leave the spawning beds, with peak emigration occurring on the 30th day. Larval emigration generally occurs between 6:00 p.m. and midnight, with the majority of emigration occurring between 7:00 and 8:00 p.m.

Habitat

This species lives in pools where water temperatures range from (81-86°F (27-30°C). Aquatic vegetation includes algae, Chara zeylania, Compsopogon coeruleus, Najas marina, and a variety of diatoms. It inhabits water with bottoms ranging from mud to firm sand.

During the winter of the study, Pahranagat roundtail chub congregate at the confluence of Crystal Springs and Pahranagat Creek to forage because ostracods (seed shrimps) and other invertebrates are abundant in the cooler water. During the summer, chub congregate in the occasional pockets of cool water created by irrigation runoff from adjacent pastures and forage on food items carded by the runoff. Because they reduce their active metabolism during the summer season, they may move into slower to reduce energy expenditures.

Distribution

The Pahranagat roundtail chub is endemic to the Pahranagat Valley in Lincoln County, Nevada. Precise limits of the historic range are not known because the valley waters were extensively altered before the fish was discovered. However, it is known to have occurred in Crystal, Hiko, and Ash Springs, and in the Pahranagat River. Two other Endangered fishes are found in the Pahranagat Valley-White River springfish (Crenichthys baileyi baileyi ) and Hiko White River springfish (C. b. grandis ). These fishes were historically the most abundant species found in Crystal, Hiko and Ash Springs.

The Pahranagat roundtail chub is considered one of the rarest fish in North America. Less than 75 adult chubs and, perhaps, 200 yearlings are thought to survive in approximately 7,590 ft (2,300 m) of an unmodified portion of the Pahranagat River downstream from Ash Springs on Burns Ranch. Fry and juveniles are sometimes found in irrigation ditches, where they usually do not survive.

Threats

When the Pahranagat roundtail chub was listed as Endangered in 1970, a published summary of the factors affecting the species and the reasons for its listing was not required. However, it is probable that the species was granted Endangered status because it had been extirpated from two of three historically occupied spring systems, and was considered to be extremely rare. Degradation of the riparian habitat due to grazing, crop production in adjacent habitat, and loss of riverine canopy was believed to be contributing to the declining Pahranagat roundtail chub population. Though these activities may have contributed to decline of the fishes in the past, recent field visits suggest that habitat conditions have improved. However, improvements in the current habitat conditions, while maintaining current land use practices, will be needed before the fish can be recovered.

The introduction of non-native fishes to the watershed are primarily responsible for the decline of the Pahranagat roundtail chub. Streams in the Pahranagat Valley have been extensively altered to accommodate irrigation. The present restricted habitat of the Pahranagat roundtail chub is one of the few stream reaches that has not been lined with concrete. Non-native fishes competing with the chub include the convict cichlid, carp, mosquitofish, and the shortfin molly.

In the Pahranagat Valley habitat overlap between Pahranagat roundtail chub and shortfin molly occurs primarily during the Pahranagat roundtail chub larval stage. Convict cichlids were believed to be the more formidable threat to larval Pahranagat roundtail chub based on gut analysis and observations of them picking at gravel spawning beds of adult Pahranagat roundtail chub. In laboratory experiments using a castostomid (sucker) larvae as a substitute for the Endangered Pahranagat roundtail chub, shortfin mollies were discovered to be extremely effective larval predators. Mollies are now considered a greater threat to larval Pahranagat roundtail chub than cichlids because of their tendency for greater spatial overlap. Fortunately, Pahranagat roundtail chub reproduction occurs in late winter when populations of non-natives are depressed and in reaches of river with the smallest non-native populations.

Conservation and Recovery

In the years immediately following publication of the 1985 Recovery Plan for the Pahranagat round-tail chub, much effort was directed toward establishing the captive population at Dexter National Fish Hatchery and identifying the ecology of the Pahranagat roundtail chub. The U. S. Fish and Wildlife Service's (FWS) National Fisheries Research Center-Reno completed many research tasks specified in the 1985 plan, including Pahranagat roundtail chub life history, abundance and distribution, food habits, habitat use, movement patterns, population dynamics, and inter-and intraspecific interactions. This research also provided information on White River and Hiko White River spring fishes.

Twenty chubs relocated to the Endangered Fish Facility at Shoshone Ponds in Nevada failed to reproduce, but in 1985 the FWS successfully established a captive population of the Pahranagat round-tail chub at the Dexter National Fish Hatchery. Alerted to severe drying of an inhabited irrigation ditch, personnel from the Nevada Department of Wildlife and the Great Basin Complex netted about 50 juveniles, held them overnight in a live trap in a stream, and transferred them the next day in plastic sacks filled with stream water to the airport at Las Vegas. From there the fish were flown to Roswell, New Mexico, and handed over to hatchery personnel. The facility is located south of Roswell near Dexter in Chaves County. Previous attempts to translocate the chub to the hatchery had failed.

It is hoped that this captive stock can be used as the basis for a reintroduction effort when suitable habitat can be located or significant stretches of the Pahranagat River rehabilitated.

The Pahranagat roundtail chub may be considered for reclassification from endangered to threatened when:

1. Pahranagat Creek/Ditch contains adequate cool water pools, for chub to persist through the summer months;
2. a self-sustaining Pahranagat roundtail chub population (comprising three or more age-classes, a stable or increasing population size, and documented reproduction and recruitment) is present in a combined total of approximately 75% of either 4.7 mi (6.8 km) of the Crystal Spring outflow stream through its confluence during the winter months with the Ash Springs outflow stream, or 6.2 (10 km) of Pahranagat Creek/Ditch below the confluence for three complete generations (or a minimum of 15 consecutive years); and
3. impacts to the species and its habitat have been reduced or modified to a point where they no longer represent a threat of extinction or irreversible population decline.

The Pahranagat roundtail chub may be considered for delisting provided that all reclassification criteria have been met and when:

1. a minimum year round in-stream flow of 1.75 cu ft (49.5 l) per second is present, at the point where Pahranagat Ditch starts, to sustain a Pahranagat roundtail chub population;
2. the riparian corridor along the outflow stream of Crystal Spring has been enhanced;
3. all impacts to its habitat have been neutralized or reduced sufficiently for both the species and land uses to coexist; and
4. a Pahranagat roundtail chub population as defined in the downlisting criteria inhabits both approximately 75% of both the Crystal Spring outflow stream through its confluence during the winter months with the Ash Springs outflow stream, and approximately 75% of Pahranagat Creek/Ditch from the beginning of Crystal and Ash Springs outflows to Upper Pahranagat Lake.

Contact

Regional Office of Endangered Species
U.S. Fish and Wildlife Service
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232
http://pacific.fws.gov/

References

Courtenay, W. R., Jr., et al. 1985. "Comparative Status of Fishes Along the Course of the Pluvial White River, Nevada." Southwestern Naturalist 30:503-524.

Deacon, J., C. Hubbs, and B. Zahuranec. 1964. "Some Effects of Introduced Fishes on the Native Fish Fauna of Southern Nevada." Copeia 2:384-388.

U.S. Fish and Wildlife Service. 1985. "Recovery Plan for the Pahranagat Roundtail Chub, Gila robusta jordani. " U.S. Fish and Wildlife Service. Portland.

U.S. Fish and Wildlife Service. 1998. "Recovery Plan for the Aquatic and Riparian Species of Pahranagat Valley." U.S. Fish and Wildlife Service, Portland. 92 pp.