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250 '" 200 "" I '. 10 ~ n_2.4 ..175 , 5150 ~ 125 < is 100 is 7S ,,:1 ,"-'.:- . " : , , , , , , , , " 25 APRil ~, - M' MONTH Fig. 8. Relation between average discharge, temperature, and initiation of spawning migration for Colorado squawlish, Yampa River, 1981--S3 and 1988 combined. Solid vertical lines = average dates first and last radio-tagged fish exhibited movement to spawning ground; dashed vertical line = average date all radio-tagged fish exhibited migratory movements; n = number of radio-tagged fish. certain times due to intrinsic biological rhythms, as discussed by McKeown (1984). Homing behavior in Colorado squawfish is indicated by long-distance movement patterns and repeated recaptures of the same fish on the Yampa River spawning grounds in subsequent years (Wick et al. 1983; Tyus 1985). Of four fish radio-tracked to the Yampa River spawning reach for more than 1 year, two migrated to the same location in consecutive years, indicating a fidelity to this spawning site. Recaptures of fish on the Yampa River spawning grounds also support the concept of fidelity in Colorado squawfish. Five Colorado squawfish in breeding condition were tagged and recaptured in the Yampa River between RK 17.6 and 28.8 for intervals of 1 + years (three fish for 2 consecutive years, one fish after a 2-year interval, and one fish after a 3-year interval). Adult Colorado squawfish using the Yampa River spawning grounds have not been found to use any other spawning site in the Green River system. This suggests that these spawning areas are unique and critical to the conservation of the species. The Groundwater Seepage Hypothesis, proposed for other species by Harden-Jones (1981), may be implicated as a possible homing mechanism for Colorado squawfish (Tyus 1985). Migrating adult Colorado squawfish pass through miles of potentially good spawning habitat (i.e., canyon-bound cobble bars in Split and Whirlpool canyons in the Green River for downstream fish; upper Yampa Canyon, aoss Mountain Canyon, and Juniper Mountain Canyon in the upper Yampa River for upstream fish) to reach specific spawning grounds in the Yampa Canyon. Although no experimental evidence to date confirms or disproves the 25 existence of an olfactory imprinting mechanism for Colorado squawfish, observations at the two confirmed spawning grounds in upper Green River basin indicated that Colorado squawflSh that migrate to these areas may be orienting to them because of freshwater inflow from spring-fed tributaries (e.g., Florence Creek, Green River; Warm Springs Creek, Yampa River) and sandstone-limestone seeps (e.g., at Coal Creek, Green River: at Cleopatra's Couch, Yampa River). There is good agreement between the arrival of migrating fish on spawning grounds, collections of ripe fISh, and estimated dates of egg deposition (Fig. 9). These data were used to estimate total and optimal spawning periods (Table 2). Total spawning period included widest range of activities associated with 20 YAMPA RIVER 1979 1980 1981 -----c:::.=:J-- 1982. ------! 1983 --c:=::=J-- 1984 }-- 1986 --=- 1986 1987 --c=:::J- 1988 ~ APR MA Y JUN JUL AUG SEP OCT Fig. 9. Migration of radio-tagged Colorado squawfish C _); collections of ripe Colorado squawfish C-); and dates of estimated egg deposition C. -), Yampa River. 1981-88. 0 indicates presence of radio-tagged fish on spawning grounds. 9