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10 BIOLOGICAL REPORT 24 channel at baseflows. They may feed in these environments (Valdez and Wick 1983) or simply move into low velocity habitats to avoid the higher flow of the main channel (Doug Osmundson, U.S. Fish and Wildlife Service, Grand Junction, Colo- rado, personal communication). Growth is opti- mum at 25° C, based on experimental studies; Kaeding and Osmundson (1988) showed that growth in the 15-mile reach of the Colorado River was reduced because maximum temperatures were less than optimum for maximum growth year round. Warmer temperatures in backwater environments could offset the coldwater ef- fect (Wick et al. 1983), assuming food supply is adequate and small squawfish can avoid preda- tion. Long-term monitoring data strongly indicate to researchers in the recovery program that num- bers of larval and YOY squawfish and subsequent year classes are highest when intermediate (about the long-term average) peak flows occur during spring runoff. Numbers ofYOY were substantially lower on years of very high spring flows (e.g., flow peaks of record in 1983 and 1984 at the state line 0.2 E ! 0.15 0.1 4) E G 0.05 E 0 c? 85 as T r 87 1 88 82 83 94 10 20 30 40 50 60 70 80 Maximum-annual discharge (c1s X 1,000) Fig. 4. Catch per effort of postlarval squawfish as related to maximum annual discharge for the Colorado River. Data are geometric means ±1 standard error for fish collected in backwaters using standardized sampling protocol (U.S. Fish and Wildlife Service 1987b) during October between the Westwater Canyon (km 177) and confluence with the Green River (km 0). Thus, these data are a relative measure of recruitment from spawning that occurred during the high flow periods each year (from McAda and Kaeding 1989, also included in Osmundson and Kaeding 1991). Data collected in 1989-1991, which were low to average water years, are consistent with this relationship (C. McAda, U.S. Fish and Wildlife Service, Grand Junction, Colorado, unpublished data). gauge, Fig. 4; Osmundson and Kaeding 1991), owing either to poor spawning conditions or mor- tality associated with flushing effects of high run- off. However, Tyus and Haines (1991) observed higher recruitment of YOY on low flow years in the Green River. Low recruitment of YOY on low flow years in the Colorado River may be related to lack of suitable habitat, either for spawning or rearing or both. An alternate interpretation of Fig. 4 is that the extremely high flows of 1983-84 created or rejuvenated substantial spawning habitat that was available but gradually deterio- rating during 1985-88. Regardless of how the relationship is interpreted with respect to the peak (1983-84) and low (1982) flow events, pro- duction of young squawfish can occur over a very wide range of spring flows (i.e., the recruitment threshold of YOY is very wide). Like McAda and Kaeding (1991). I conclude that squawfish spawn- ing may be much less site-specific than is sug- gested by the literature, or a very wide range of preferred spawning conditions exists on the spawning bars where squawfish are routinely found (e.g., Cleopatra's Couch Bar on the Yampa; Three Fords on the Green). The life history strategy of squawfish seems to be strongly influenced by the propensity of the larvae and juveniles to drift far downstream from the spawning site; survivors subsequently move back upstream as they mature. Adults, especially large fish (Fig. 5), are most commonly found at or near the potamon-rhithron2 transition zone in the Yampa and Colorado rivers. Recruitment of adults presumably is lower for cohorts spawned in low- flow years, owing to reduced spawning success to start with and increased predation pressure per fish during each subsequent life history stage. Predation on YOY and juveniles may be more intense in low-flow years, when habitats are con- fined. The positive relation between year-class strength and peak discharge generally seems to hold for the Green and Colorado rivers (Tyus and Karp 1989, 1991; Osmundson and Kaeding 1991) and also applies to humpback chub in the Grand Canyon (R. Valdez, personal communication). Re- cruitment seems weak in very high- and low-flow years and relatively good in years of long-term average flows. 2 Headwater reaches of a river continuum characterized by cold, clear water, bedded gravel and cobble substrata on the river bottom, and alternating canyons (constrained) and intermontane floodplains (less constrained).