Distribution and Notes on the Biology of Phoxinus eos (Cyprinidae) in Colorado

Abstract

Distribution of the northern redbelly dace, Phoxinus eos, in the South Platte River Basin, Colorado, was historically restricted to a narrow band of stream habitat in the transition zone along the Front Range of the Rocky Mountains. Phoxinus eos has been taken only in the West Plum Creek drainage since 1951, where it mainly occupies off-channel entic habitats that are cool, clear, vegetated, and without large predaceous fishes. Age and growth, reproduction, and diet characteristics are similar to those known for populations of P. eos in the center of the species' range. The naturally limited range and habitat of P. eos in Colorado has been much reduced by stream channelization, reductions in discharge, and changes in water quality. Continued urban development threatens remaining populations. The northern redbelly dace, Phoxinus eos, inhabits cool, boggy lakes, ponds, and streams in the northern United States and Canada (Scott and Crossman, 1973; Stasiak, 1980). The southern-most population known for the species occurs in the South Platte River Basin, Colorado (Fig. 1). Collections of P. eos in Colorado are uncommon, both historically and at present, and the species is considered rare (Propst and Carlson, 1986). The range of P. eos in Colorado apparently has declined in the period of historic collecting (since 1891), and little is known of the habitat requirements and biology of the species there. The purpose of this paper is to describe the past and present distribution patterns of the species and suggest probable reasons for the decline of P. eos in Colorado. Notes on aspects of the life history and ecology of this species are also presented. MATERIALS AND METHODS-Lotic systems in the South Platte River Basin ranged from cold, highgradient montane streams with large substrate particles to low gradient, seasonally warm, sandybottomed streams on the plains. Streams flowing through a short transition zone between the montane and plains-types had intermediate gradient, substrate, and temperature characteristics. Discharge was maintained primarily by melting snowpack and groundwater discharge. Plum Creek, a south-to-north flowing transition zone stream, emptied into Chatfield Lake, a mainstream reservoir on the South Platte River. The drainage basin encompassed 782 km2 above the U.S. Geological Survey gage near Louviers, Colorado (gage no. 06709500). Mean discharge was 0.96 m3/s; high and low values of 4,360 m3/s and 0.00 m3/s, respectively, have been recorded (United States Geological Survey, 1985). Streams were shallow and braided extensively over a predominantly shifting sand substrate. Runs were the dominant habitat type; pools and riffles were uncommon. Woody riparian vegetation was composed primarily of willow (Salix sp.), and various grasses and sedges grew extensively along stream banks. Instream vegetation was nearly non-existent due to shifting sand and scouring streamflows. Abundant submergent and emergent vegetation grew in or near standing water habitats that were adjacent to mainstream channels. Distributional data were derived from field collections made in conjunction with monitoring studies on transition zone stream reaches in Colorado (T. Chart and S. Gillihan, in litt.; K. D. Fausch, pers. comm.; Department of Fishery and Wildlife Biology, Colorado State University, personal collections), museum records, and the literature. Fishes collected by the author in a survey of Plum Creek were fixed in 10% formalin. Adult fishes were preserved in 45% isopropanol, and age-0 specimens in 3% buffered formalin. Notes of physical habitat parameters, microhabitats, and species associates were HWESTERN URALIST 34(2):225-231 JUNE 1989 The Southwestern Naturalist