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review). This generally occurred about 26 days (range: 17 - 33 d) following <br />migration. Peak discharge preceding spawn, and mean minimum temperatures <br />during spawn were highly correlated with the spawning period (r = 0.84, r = <br />0.88, respectively; P < 0.05), ostensibly because discharge, temperature, and <br />spawning period are correlated. Spawning of Colorado squawfish was considered <br />a result of complex environmental and biological influences and is not <br />triggered by a single flow or temperature event (Tyus, in review). As an <br />example, flow spikes from rainstorms during spring runoff may also influence <br />ovulation and spawning in Colorado squawfish, as recently hypothesized by <br />Nesler et al. (1988). Radiotracking data suggests that all adult Colorado <br />squawfish do not spawn each year, and male and female fish may require <br />different stimuli for gonadal maturation (Tyus, in~ review), some of which are <br />presumably flow-related. <br />Breeding adults were most often concentrated in river reaches containing <br />deep pools, eddies, and submerged cobble/boulder bars. Radio-tagged fish <br />moved from pools or eddies to presumably spawn on bars and then returned to <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sediment and algae (Archer and Tyus 1984; FWS, unpublished data).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 showing 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). A gradual decrease in summer flows following spring scouring and a <br />concomitant decrease in sediment load, aids in preventing siltation of cobble <br />bars. Thus, magnitude, timing and duration of spring flows are potential <br />limiting factors for successful reproduction by Colorado squawfish. <br />Larvae and Postlarvae <br />Larval Colorado squawfish emerge as sac-fry from cobble bars with <br />declining flows and drift downstream (Tyus et al. 1982b; Haynes et al. 1984; <br />Tyus and Haines, in review) to concentrate in shallow backwater habitats in <br />the Green River (Tyus et al. 1982b, 1987). About 16 d are required for <br />transport of newly emerged Colorado squawfish fry to the mouth of the Yampa <br />River from the midpoint of the spawning grounds, km 26.4 - 29.1 (Tyus and <br />Haines, in review). From 1979 to 1981, peaks of abundance of young Colorado <br />squawfish were noted about 160 km downstream of the Yampa River spawning reach <br />(Tyus et al. 1982b, 1987; Tyus and Haines, in review). Young fish presumably <br />use river transport for dispersal from upstream spawning grounds to downstream <br />nursery habitats (Tyus and McAda 1984; Tyus 1986; Nesler et al. 1988; Tyus and <br />Haines, in review; Paulin et al., in review). 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 is considered important to <br />successful recruitment of the species. <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 22°C were <br />18 <br />