9549 - Laserfiche WebLink

Home Browse Search

occurred in the Yampa River, when Colorado pikeminnow (and other taxa) abundance in drift samples typically increased when turbidity events occurred (Bestgen et al. 1998). Because such events in the Green River happened after native fish reproduction occurred, no additional larvae were available for re-colonization and their abundance remained low through autumn. Non- native fish were likely similarly reduced by turbidity events (e.g., red shiner and sand shiner in Whirlpool Canyon, summer 2003) but, because many of those taxa spawn into late-summer, populations could recover by autumn to achieve high population levels we observed. Fish transport may be particularly high from canyon reaches, because few shallow, channel margin backwaters occurred where fish could find refuge. Abundance patterns for red shiner and sand shiner increased downstream and, although highly variable and in contrast to patterns for several native fishes, often increased from spring through summer and autumn. Reasons for this pattern were discussed above for pooled fish sample statistical analyses. Relatively low summer abundance of some small-bodied non-native cyprinids may be related to turbidity events; for example, sand shiner and red shiner in Whirlpool Canyon in 2003. Green sunfish abundance that was highest in Lodore Canyon was likely a result of consistent sampling of a few permanent backwaters that usually held green sunfish. Spring smallmouth bass abundance was low, relatively high in summer, and lower in autumn. Age-0 smallmouth bass were typically observed in low-velocity backwaters or slow, sandy main channel margins, but age-1 bass moved to rocky main channel areas that had higher current velocity and cobble substrate. Abundance was lowest in 2002 and higher in 2003 and 2004, reflecting an expanding population. Trammel net sampling.-A total of 535.5 net h (one 23 m-long trammel net set for one h = one net h) of trammel net sampling was conducted during 2002 to 2004. Trammel nets were effective at capturing native suckers and chubs. The most common native species captured was flannelmouth sucker (0.54 fish/h), followed by roundtail chub (0.51 fish/h), and bluehead sucker (0.23 fish/h; Table 3). Humpback chub capture rate was 0.016 fish/h, or about one fish for every 60 h of trammel net sets. The few net sets in autumn 2004 (n = 16 net h) captured 0.313 45