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70 N °0 60 U 50 ~_ tit 40 U ~ 30 v 20 1= 1 O 0 a~ ~~ O ,y0 ,~o'~;• ® FALL SAMPLING ~ SPRING SAMPLING 1 Ten-Mile River Sections Figure 4. Cumulative geometric mean CPE for ten-mile river sections sampled in Green River Reach 3 during fall and spring sampling (1987 through 1993 combined). , 4.1.3 Ho 3: Overwinter mortality contributes to decreases of age-0 sguawtish. Hypothesis 3 suggests that decreases in age-0 squawfish catch rates observed between fall and spring sampling represent actual loss of fish during the winter period. There is considerable evidence that winter riverine ' conditions significantly influence cohort strength of Colorado squawfish. This impact may be most significant in the first year of life, when age-0 fish are exposed to winter conditions at less than 4 months of age, and often less than SO mm in length. Kaeding and Osmundson (1988) stated that the unusually small size of age-0 ' fish going into the winter might be an important factor affecting recruitment to adult stock. Thompson (1989) determined that age-0 Colorado squawfish entering the winter period at lengths greater than 3S mm had a higher fat content and presumably have a greater chance of overwinter survival Low survival of age-0 squawfish during the six month winter period is attributed to: (1) competition and ' predation by large numbers of non-native species, (2) diseases and parasites, and (3) wintertime icing of backwaters and exclusion of low-velocity habitats. Factors 1 and 2 were not addressed in this report, but are , recognized as potentially significant to cohort strength. Harsh and changing river conditions faced by age-0 squawfish probably account for a substantial portion of the mortality observed during the winter months. Ice formation, cold temperatures and habitat loss may force age-0 squawfish out of backwaters into less suitable habitat in the mainchannel. Movements and habitat use of squawfish during the winter period is unknown. However it was hypothesized that if backwaters provided sheltering habitat during the winter, forced ' abandonment would increase mortality of age-0 squawfish. m ~ ~,~o moo ~o ~o ,~o ~o ~ t,,o ~o 00 ~o ,moo ~, ~, oo. moo. oo. oo. ao. moo. moo. moo. o. ~~. ~.