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1 4.0 DISCUSSION This study was designed to test three hypothesis associated with apparent overwinter decreases in age-0 Colorado squawfish. Theses hypotheses are: Ho 1: Age-0 squawfish use habitats unavailable to wnventional seining methods. Ho 2: Age-0 squawfish move or are transported into and from the region. Ho 3: Overwinter mortality contributes to decreases of age-0 squawfish. In addition to addressing these hypotheses the data collected for this study were consistent with ISMP criteria and can be used to assess cohort strength and long term trends in age-0 squawfish numbers. 4.1 HYPOTHESIS The hypotheses addressed by this study were developed to account for apparent decreases in age-0 Colorado squawhish catch rates observed over the 6 month winter period--September to March. Except for the 1990 and 1992 year classes, overwinter decreases in catch rates are quite wnsistent; 1987 (77%), 1988 (62%}, 1989 (56%), and 1991 (62%), even though catch rates from year to year varied dramatically (48.40/100 m2 in fall 1988 compared to 2.23/100 m2 in fall 1991). Consistent ovetwinter decreases in age-0 squawfish suggest that the factors influencing this decrease are likely the same from year to year. 4.1.1 Ho 1: A,ge-0 sguawtish use of habitats unavailable to conventional seining methods. Hypothesis 1 addresses the idea that overwinter decreases in age-0 squawftsh are more a function of sampling bias than actual decreases in numbers. This hypothesis implies a shift in habitat shift use from fall to spring. Habitat preferences of age-0 Colorado squawfish are well documented. Holden and Stalnaker (1976) found age-0 squawfish in shallow backwaters and juveniles in backwaters and eddies 2 - 3 ft deep. Kidd (1976) indicated that squawfish used backwaters almost exclusively in the Colorado River near Grand Junction (times of capture were not noted). Recent research in Reach 3 (Valdez 1990) found that age-0 squawfish were most often found in backwaters and isolated pools, whereas juvenile fish were found to move between shorelines and backwaters. These studies all show that age-0 squawfish prefer backwater habitats, shifting to mainchannel habitats asjuveniles--or about 80 mm TL. It is unlikely that the overwinter decreases observed in this study were caused by a habitat shift in age-0 squawfish from fall to spring. The change from one habitat type to another is typically related to a subsequent change in size--as shown in the studies above. Overwinter growth ofage-0 squawfish in the 5 years of sampling on the Green River was minimal, averaging oNy 4 mm TL. Thus, the same size modes were being sampled (Figure 2), and the fish were still well below juvenile size, where habitat shifts are usually observed. Age-0 squawfish have been observed using habitats other than backwaters. Valdez et al. (1987) found age-0 ' squawfish from the Colorado, Green and Yampa rivers in eddies, runs and along shorelines, however 85%'0 were captured in backwaters. Valdez (1990) sampled 95%ofage-0 Squawfish in backwaters and isolated pools. Tyus and Haines (1991) reported similar results with the majority (84°!0) of postlarval Colorado squawfish found in backwaters. ISMP sampling criteria was designed to sample habitats most commonly used by age-0 squawfish. While these habitats are not used exclusively, research indicates the majority of age-0 Colorado squawfish use these areas. Similarly there is no evidence to suggest that backwaters are used more in fall than in spring. ' 17