Laserfiche WebLink
<br />the former habitat (Tyus et al. 1987), behavior similar to that of spawning <br />northern squawfish (Beamesderfer and Congleton 1981). Turbid riverine <br />conditions have precluded direct observations of egg deposition; however, <br />cobbles removed from the substrate during this time of year are clean of <br />sediment and algae (Archer and Tyus 1984; FWS, unpublished data). There is <br />substantial field and laboratory data showin9 that Colorado squawfish and <br />other squawfish species require cleaned cobble surfaces for successful egg <br />adhesion (Burns 1966; Patten and Rodman 1969; Hamman 1981). Hamman (1981) also <br />noted hatching of Colorado squawfish larvae from cobble surfaces. The need for <br />cleaned cobble and boulder substrates is supported by spawning of Colorado <br />squawfish following peak flows and peak sediment transport (Tyus and Karp <br />1989). Spring scouring, a gradual decrease in summer flows, and a concomitant <br />decrease in sediment load aids in preventing siltation of cobble bars. Thus, <br />magnitude, timing and duration of spring flows are considered potential <br />limiting factors for successful reproduction by Colorado squawfish. <br /> <br />larvae and Postlarvae <br /> <br />larval Colorado squawfish emerge as sac-fry from cobble bars and drift <br />downstream with declining flows (Tyus et al. 1982b; Haynes et al. 1984; Tyus <br />and Haines 1991) to concentrate in shallow backwater habitats in the Green <br />River (Tyus et al. 1982b, 1987). About 6 days are required for transport of <br />newly emerged Colorado squawfish fry to the mouth of the Yampa River from the <br />midpoint of the spawning grounds, km 26.4 - 29.1 (Tyus and Haines 1991). <br />Nesler et al. (1988) also noted rapid downstream transport of larvae (3-15 <br />days) following hatching, From 1979 to 1988, peaks of abundance of young <br />Colorado squawfish were noted about 160 km downstream of the Yampa River <br />spawning reach (Tyus et al. 1982b, 1987; Tyus and Haines 1991). Young fish <br />presumably use river transport for dispersal from upstream spawning grounds to <br />downstream nursery habitats (Tyus and McAda 1984; Tyus 1986; Nesler et al. <br />1988; Tyus and Haines 1991; Paulin et al., in prep). These productive nursery <br />habitats are created with gradually decreasing flows following spring runoff, <br />and persist with summer-winter baseflow conditions. Availability (quality and <br />quantity) of these habitats in the Green River are considered important to <br />successful recruitment of the species. <br /> <br />Mortality of drifting larvae is presumably related to flow, river <br />temperature, availability of backwater habitat, and predator load. Berry <br />(1988) noted that larval Colorado squawfish acclimated to about 220C were <br />adversely affected by cold water of 100C and 150C. Young Colorado squawfish <br />are routinely collected in isolated pools in the Green River system (USFWS, <br />unpublished data). These pools form when decreasing flows strand bodies of <br />water from the main channel. Natural fluctuations in river level usually make <br />this a gradual process and allow entrapped fish an escape route. However, <br />abrupt fluctuations in river level, as is characteristic of some regulated <br />systems, could increase mortality of small fishes by increasing the potential <br />for competitive interactions with other species and exposure to aquatic and <br />terrestrial predation. Although difficult to detect in nature, predation on <br />larval Colorado squawfish by common non-native fishes has been observed in the <br />lab (R. Muth, personal communication). Herons (Ardeidae), raccoons (Procyon <br />lotor), garter snakes (ThamnoDhis spp.), and other terrestrial animals have <br /> <br />19 <br />