Kendall Warm Springs Dace

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Kendall Warm Springs Dace

Rhinichthys osculus thermalis

ListedOctober 13, 1970
FamilyCyprinidae (Minnows)
DescriptionSmall fish; males are bright purple, females are olive green.
HabitatWarm springs and seeps.
FoodAquatic insects.
ReproductionSpawns several times a year.
ThreatsWater pollution, habitat destruction.


The Kendall Warm Springs dace, Rhinichthys osculus thermalis, is a small minnow about 2 in (5.4 cm) long. Breeding males are often a bright purple color; females are typically dull olive green.

This dace was first given the name Apocope osculus thermalis. The 1970 revision of Wyoming Fishes considered the Kendall Warm Springs dace and the Green River dace (R. o. yarrowi ) to make up a single species. However, further comparison showed that the Kendall dace differed by having fewer scales and fin rays, a larger head and fins, and a smaller body. Its taxonomic status remains unclear.


Although the Kendall Warm Springs dace has not been closely studied, spawning probably occurs several times a year, if not year round. It is thought that females lay several hundred eggs. Dace usually gather in small schools, due either to space limitations or to an inborn behavioral preference. A skit-tering flight to the nearest clump of plants is a typical reaction to danger, although some flee to the deeper, turbulent areas in the main current of its spring habitat.

Although the diet of the Kendall Warm Springs dace has not been confirmed, other R. osculus species are omnivorous with insects comprising an important food source.


Kendall Warm Springs in Wyoming consists of numerous seeps and springs scattered along the north face of a limestone ridge at an elevation of 7,840 ft (2,390 m) in the Bridger-Teton National Forest. The spring outflow flows southwest for 984 ft (300 m) before cascading into the Green River over an embankment formed by the water's mineral deposits. Water from the springs has a constant temperature of 85°F (29°C). It is slightly alkaline, mineralized, and high in dissolved solids.

Vegetation near the spring complex is limited to various grasses, forbs, and low-growing shrubs and trees, such as willow and sagebrush. Monkeyflower and moss are the dominant aquatic plants in the upper pool; below that area, sage pondweed, moss, and stonewart predominate. Overhanging vegetation is essential for the dace's cover.


The Kendall Warm Springs dace has probably always been restricted to the Kendall Warm Springs near the Green River drainage in Sublette County, Wyoming.

Because of the fish's small size and the inefficiency of survey techniques, population figures are uncertain. In 1934, biologists estimated the total population as between 200,000 and 500,000 individuals. Recent observations suggest that even the lower figure may have been exaggerated. It is thought that the current maximum population is only a few thousand individuals.


Over the course of many years, human activities have altered the Kendall Warm Springs dace's habitat. A road built across the creek built before 1934 is still the main access route to the upper Green River and the northern Bridger Wilderness. A culvert divides the upper half of the dace population from the lower. Several rock dams have been built over the years to provide small bathing and soaking pools, and people have used the pools to wash clothes. The presence of detergents in the water has probably had a detrimental effect on the fish populations.

Conservation and Recovery

To preserve water quality, the Forest Service closed the springs to bathing and prohibited the use of soaps, detergents, and bleaches. For many years, fishermen used Kendall dace as fish bait until prohibited by the Wyoming Game and Fish Department in the early 1960s. One hundred sixty acres (64 hectares) have been designated by the Forest Service as the Kendall Warm Springs Biological Management Unit. The boundaries include most of the small watershed and surrounding land. Because of this designation, mineral exploration, seining, and trapping are prohibited. The immediate area around the springs has been fenced and interpretive signs posted. To control traffic along the creek, vehicle access has been blocked.

The thermal spring and surrounding land may qualify as a research natural area. If so, a formal designation could provide more complete habitat protection.


Regional Office of Endangered Species
U.S. Fish and Wildlife Service
P.O. Box 25486
Denver Federal Center
Denver, Colorado 80225


Baxter, G. T., and J. R. Simon. 1970. "Wyoming Fishes." Bulletin 4. Game and Fish Department, Cheyenne.

Binns, N. A. 1978. "Habitat Structure of Kendall Warm Springs, with Reference to the Endangered Kendall Warm Springs Dace." Fisheries Technical Bulletin No. 3, Wyoming Game and Fish Department, Cheyenne.

U.S. Fish and Wildlife Service. 1982. "Kendall Warm Springs Dace Recovery Plan." U.S. Fish and Wildlife Service, Denver.