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"1"'7' ~."':'I species, is it even appropriate to apply statistical tests of significance on any data other than that signifying the peak in drift densities? The number of larvae sampled may thus be a factor. In 1983 and 1984, 102 and 89 larvae, respectively, were collected by drift nets, while 455 were collected in 1985= Large numbers of larvae collected in a distinct peak over a three day period in 1985 may be indicating a true diel drift pattern that was not diluted by the river trip to Box Elder or simply an overwhelming numbers bias. Another complication posed by the separation of origin and sample sites is whether or not the indicated preference by larval Colorado squawfish for dawn drift is real, or a function of average drift rate between the two sites. For the three other species that use larval drift as part of their reproductive dispersal strategy, it is assumed that spawning occurs more ubiquitously throughout the lower Yampa, since the long-range migratory spal-.Jn i. ng bc:>h21.vi or and si te fidel it Y Df t:i:i.f:hQ.f;.hei 1 u~ ~ uc ius; documented by Tyus et a1. (1981), Tyus (1983), l1Ji ck et a1. (1983), Archer and Tyus (1984), and Tyus and McAda (1984), is not characteristic for these ather species. However, the lack of consistency displayed by the diel drift density data for all three of these species emphasizes the need to monitor environmental variables considered to influence larval fish drift behavior, and limits any generalization of diel drift periodicity for any of the Yampa River fish species. It is apparent that representative sampling Qf drifting larval fish in the Upper Basin will still require considerable effort to account far changing environmental conditions. It is interesting to note the sequential pattern among the four species comprising the 1985 drift collections. The two sucker species were present in abundance in early July; Colorado squawfish were present in mid-June; and channel catfish in late July - early August. Re 1 at i Cl n ':?Ju.J2._-9 e t1.'L~ n_-'l? t i f!1.5TI..Q.fL.2.R..:~:.~!J..:UJ.9.~~"L(t...Ll9.~~_r.::.gq i mg. - !~n a ly s e s 0 f Colorado squawfish spawning with seasonal flow regime in the Yampa River is quite simplistic relative to the complex interaction of external and internal cues that may initiate and regulate gonadal development, migratory behavior and actual spawning/egg deposition. Bye (1984) summarized the role of environmental factors in the timing of fishes' reproductive cycles. Within his summary he cites a paper by Scott (1979), indicating that the relationship between endogenous and e:<ogenous rhythms is subtle and the timing of a species' reproductive cycle is certainly a compromise involving many environmental considerations. Timing of annual spawning has evolved to ensure that young hatch and commence feeding at the optimum season for survival (Bye 1984). Stacey (1984) suggested that a temporally appropriate response to exogenous cues ma:<imizes reproductive success by achieving an optimal balance between survival of adult spawners and progeny. These postulates on reproductive strategy certainly apply to our increasing knowledge on the spawning ecology of the Colorado squawfish in the Green - Yampa sub-basin. Past work by Haynes et ala (1984, 1985) and others have adequately documented the background environment of temperature and flows in which this species attempts to spawn. Mutually supporting evidence from Vanicek and Kramer (1969), Tyus and McAda (1984) and Haynes et al. (1984,1985) indicate that Colorado squawfish conduct spawning on the descending limb of the hydrograph, and that the timing may be largely influenced by water temperatures. Threshold temperatures of 20-22 C and the number of