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530 The SO'ltthweslern Naturalist vol, 53, no, 4 Yampa River at Echo Park (river km 1) was just upstream of the confluence of the Green River and sampling was conducted with drift nets (4 m long, 0.5 m2, 560-llm mesh). The sampling site on the Green River, 68 river km downstream of the confluence of Yampa River near Jensen, Utah (backwater of Cliff Creek confluence, river km 487 of Green River, and other backwaters downstream of there), wa~ with light traps (quatrefoil design, slits were 4 or 6 mm wide) set overnight. Results reported here are from portions of several studies with differing research objectives, which is why different types of gear were used at most locations. In 2003, we collected one adult Iowa darter in the Yampa River, in Little Yampa Canyon, from a low-velocity, shallow, channel margin (Table 1), Abundance of the Iowa darter in Little Yampa Canyon increased in subsequent years; we collected 40, 148, 166, and 108 specimens in 2004, 2005, 2006, and 2007, respectively. Relative abundance of Iowa darters in those same electric-seine samples increased in a similar fashion and represented ca. 1 % of fishes collected dm;ng 2004-2007, We also collected two Iowa darters from Milk Creek in 2007, a tributary of the Yanlpa River in Little Yampa Canyon. We identified juvenile and adult Iowa darters using presence of a frenum, two anal-fin spines, an incomplete lateral line (usually ex- tending posterior to between the origin of the soft dorsal and tlle midpoint of soft-dorsal base), . dark saddles or other pigment on the lateral surface, and a terminal mouth and snout. These are characteristics that distinguish tllem from other darters, including those native to eastern Colorado, We identified early life stages of the Iowa darter using characteristics from Auer (1982), No other species of darter was collected. We preserved voucher specimens in 10% forma- lin and those are deposited in the collection at the Larval Fish Laboratory, Colorado State University, Iowa darters were captured in the Yampa River at Lily Park in 2005 and ,2006. No Iowa darter was collected at Lily Park in 2007, but effort was comparatively low and turbid water inhibited efficiency of sampling ,~i.th an electric seine. In 2005, we also captured one juvenile Iowa darter in the Yampa River at Echo Park. No Iowa darter was captured there in 2006 and 2007, but drift nets set in summer and designed for capture of lanJae are a relatively inefficient gear for Iowa darters that are in juvenile life stage or larger by then. Additional Iowa darters were captured in spring in the Green River near Jensen, Utah, at Cliff Creek; 11 in 2005, 42 in 2006, and 214 in 2007. Only early life stages, 3-14 mm total length, were captured with this size-selective gear (K R. Bestgen, unpublished data), Extensive sampling of backwater by seine in the Green River upstream and dOl-ltTIstream of Cliff Creek and light-trap sampling dov.'Ilstream from there in 2003-2007 did not yield the Iowa darter, which suggested a localized distribution (K R. Bestgen, unpublished data; T. Hedrick, pers. comm,). Specimens from the Yampa River and Green River that were captured in our study area during 2003-2007, must have resulted from a relatively recent introduction or colonization from up- stream because extensive sampling in ,Little Yampa Canyon and Lily Park during 1999-2001 (R. Anderson, pers. comm,) , and sampling at Echo Park and Cliff Creek since the early 1990s (Bestgen et aI., 2006; K R. Bestgen, unpublished data) failed to detect the species. Iowa darters spawn in spring at water temperatures of 12- l60C Oaffa, 1917; Becker, 1983), so specimens in early life stage captured in Green River initially might have dispersed as larvae from Little Yampa Canyon dUling high spring-snowmelt-runoff in late May and early June. Persistence of the Iowa darter at Little Yampa Canyon and Lily Park and colonization of tributary Milk Creek suggested the species is established in that area. Presence of early life stages of the Iowa darter in 3 consecutive years at Cliff Creek, and increasing abundance over time, suggested it also was established there, although no adult specimen has been reported in seine sampling upstream and downstream of that vicinity during 2005- 2007, Iowa darters in samples collected using electric seines from Little Yampa Canyon and Lily Park were from relatively shallow near-shore runs, pools, or backwaters over sand, gravel, and cobble substrate" Rooted macrophytes, a com- mon habitat attribute of the Iowa darter. were uncommon in the Yampa River, where flows are fluctuating and erosive. However, Iowa darters were found commonly among algae-covered substrate. The most common species associates of the Iowa darter in our samples from Yanlpa River were, in descending order of abundance, smallmouth bass Mic-ropterus dolomieu, sand shiner