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500 400 cn ~`- 300 LL.. O t.~ ~ 200 z X00 020 O FALL SAMPLING ~ SPRING SAMPLING 25 30 35 40 45 50 55 60 65 70 r_~ TOTAL LENGTH (mm) Figure 2. Length frequency histograms for all fall and spring age-0 Colorado squawfish sampling of Reach 3, on the Green River, 1987-1993. 4.1.2 Ho 2: AQe-0 spuawtish move or are transported into and from the region. Hypothesis 2 addresses overwinter change in age-0 catch rates as a function of immigration or emigration of squawfish. This hypothesis suggesu that changes in age-0 catch rates from fall to spring are due to movement of fish into or out of the reach. It is generally accepted that larval Colorado squawfish are dispersed downstream by river currents. Nesler et al. (1985) proposed that movement of larval fish shortly after hatching maybe mostly involuntary since only protolarvae and early mesolarvae (size range 7.2 - 9.3 mm TL) were collected in drib nets, while larger individuals were captured in seines (size range 8.1- 29.0 mm TL). This suggests that following the larval stage, squawfish probably remain in or near nursery backwaters, as long as these habitats persist. Fish captured in this study averaged 42 mm TL--substantially larger than those reported drihing. If fish were moving during the winter period it was likely due to abandonment of backwaters due to factors such as; (1) ice build up, (2) water quality changes (temperature, dissolved oxygen, etc.) and/or (3) habitat loss. If this were the case, fish were probably using shorelines or moving downstream until suitable habitat was found. Distribution data from this study indicate that extensive downstream movement during the winter period was minimal. Figure 3 shows average catch rates from fall and spring sampling for ten-mile river sections sampled 18 i 1 i~