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SPAWNING CUES FOR COLORADO SQUAWFISH Wyoming River Green ,q~ ~ ~a ~e~ g Yampa ~m Dinosaur ~ Canyon ~: National v Craig Steamboat Monument ~ Springs Yampa River Deerlodge Maybell Gage Gage to N : m ~ 0 ~: o I~ km -~ Wyoming ~ 0 60 120 i Utah Colorado 69 FIGURE 1.-Yampa River basin in Colorado. Hatched area denotes Colorado squawfish spawning azea. Arrows denote migration routes of adult spawners from upper Yampa River, Colorado, and Green River, Utah. measured from the river's mouth (Tyus et al. 1987). This river reach is considered the primary spawning ground in the Yampa River. Some Col- orado squawfish have migrated 100-350 km round-trip via two or three rivers to use the spawning grounds on the Yampa and Green rivers (Wick et al. 1983; Tyus et al. 1987). Adjunct to these studies on adult spawners, intensive sam- pling for larval Colorado squawfish was con- ducted, especially downstream of suspected spawning areas and during the period of suspected spawning dates. In most cases, collections of Colorado squawfish larvae supported observa- tions of presumed adult spawning behavior. Documentation that Colorado squawfish larvae drift downstream was first achieved with the capture of two protolarvae by drift-net sampling in the Colorado River at Black Rocks in Ruby Canyon in 1982 (Haynes et al. 1985). The occur- rence of Colorado squawfish as drifting larvae in the Yampa River (Yampa Canyon) was subse- quently documented in 1983 (Haynes et al. 1985). The 1983 data indicated that the hatched larvae drifted out of the Yampa River and into the Green River. During 1983-1985, drift-net sampling was developed as a technique for sampling Colorado squawfish larvae (Haynes et al. 1985). Capture data and total length of Colorado squaw- fish larvae collected by both drift net and seine have been used to estimate spawning dates for Colorado squawfish in the Yampa River (Haynes et al. 1984; Haynes et al. 1985; Nesler 1986). In this paper, these dates are related to Yampa River flow regimes and temperature data to identify the ranges of these environmental variables that occur during the esti- mated spawning period of wild Colorado squawfish in this relatively unregulated river. Methods From 1980 through 1984, habitats with low- velocity water flows (e.g., shallow shorelines, backwaters, and embayments) throughout the lower 33 km of the Yampa River were sampled with seines and dip nets for Colorado squawfish larvae (Table 1). Seine collections were made during post runoff periods during July and August; the seines had dimensions of 3.0 x 1.2 m and 1.0 x 1.2 m and 1.6-mm-square mesh. From 1983 through 1986, drift nets were used during July and August at a site on the Yampa River, 3.1 km above the Green River confluence, to collect larvae drifting from the primary spawning area 21 km further upstream. The nets were attached to fence posts, anchored in the substrate near shore at wading depth (1.4 m), and fished just below the water surface. These nets were 0.5-m-diameter, conical plankton nets mounted on 0.5 x 0.3-m rectangular steel frames, and fitted with 33-cm- long, removable polyvinyl chloride collection buckets with a threaded cod end (10-cm diame- ter). Each net had a 560-µm Nytex nylon mesh, a length of 4.0 m and an open-mesh-to-mouth area ratio of 11:1. Samples were taken at sunrise minus 0.5 h, noon, sunset plus 0.5 h, and midnight. Midnight samples were not taken in 1986. Sam- pling duration was 30 min to 2 h depending upon